On the seasonal variability and the spatial distribution of lightning activity over the broader Greek area and their connection to atmospheric circulation.

The spatio-temporal regime of lightning strokes over the broader Greek area is studied using 15-day ZEUS cloud-to-ground lightning data for a 10-year period (2005-14). The multivariate statistical method Factor Analysis (S-mode and T-mode) is applied on the mean intra-annual variation of lightning in order to reveal: i) the main modes of intra-annual variation of lighting activity that correspond to specific sub-regions of the Greek area and ii) the main characteristic patterns of strokes prevailing during specific periods of the year (“seasons”). According to the results there are three main modes of intra-annual variation: a) the continental one with maximum lightning activity in early summer, b) the Ionian Sea one with maximum in early autumn and c) the Aegean Sea one with maxima in middle autumn and late May. Also, according to the patterns of strokes, three main “seasons” are found: a) the summer one (early May-middle August) with highest lightning activity over the continental regions of the country due to the atmospheric instability associated with the intense land heating and the persistence of cool upper air masses, b) the winter one (middle December-middle February) with the maximum of lightning activity over the sea where instability is high due to the frequent prevalence of cold air masses over the warm sea-surface and the associated passages of Mediterranean depressions and c) the autumn one (early September-middle October) with maximum over northwestern Greece associated with the early cold season low pressure systems, the warm sea body of Ionian Sea and the essential role of topography




























On the intra-annual variation of cloudiness over the Mediterranean region.

The seasonal variability of cloudiness in the Mediterranean region is studied, through the examination of the mean day-to-day intra-annual variation of gridded NCEP/NCAR total cloud cover for the period 1948-2014. A dimensionality reduction process is followed by using a multivariate statistical methodology and three main modes of seasonal variation are found. The first mode is characterized by a winter maximum and a summer minimum and prevails mainly over the Mediterranean and Black Sea areas; a weak see-saw teleconnection over the Alps represents the opposite intra-annual variation. The second mode presents maxima in early autumn and late spring and minima in late summer and winter and prevails over the SW Europe and NW Africa inland regions. The third mode shows a maximum in June and a minimum in October and prevails over the eastern part of central Europe. The main characteristics of these modes are attributed to the seasonal variation of: i) the temperature and humidity of the lower and the middle troposphere affecting static stability and ii) the intensity and the position of the main circulation systems affecting the region, as for example the Azores subtropical anticyclone and the Mediterranean depressions. The examination of inter-decadal changes in the seasonal variation of cloud cover in the above core regions reveals a long-term decrease of cloudiness during the 66-year period 1948-2014. A comparison with contemporary satellite MODerate resolution Imaging Spectroradiometer (MODIS) and surface based European Climate Assessment & Dataset (ECA&D) station cloud cover data is carried out for the above core regions, evidencing a similarity among the data sets as to their seasonal variability, which supports the validity of the NCEP/NCAR reanalysis cloud cover results





























Assessment of changes in atmospheric dynamics and dust activity over southwest Asia using the Caspian Sea - Hindu Kush Index.

This study examines the influence of the Caspian Sea - Hindu Kush Index (CasHKI) on local and synoptic meteorology as well as on dust emissions over southwest (SW) Asia by means of NCEP-NCAR re-analyzed mean sea-level pressure (MSLP), geopotential height at 700 hPa and surface meridional wind, along with meteorological data at Zabol, east Iran and Meteosat/IDDI (Infrared Difference Dust Index) retrievals. The analysis focuses on the summer period (June - September) of 2000 - 2014 and the winter period (November - March) of 1963 - 2014. The CasHKI values are mostly controlled by the MSLP anomalies over the Caspian Sea domain, varying from ~ -25 to +35 hPa in winter and from ~ -10 to +14 hPa in summer, but without a clear annual pattern. The CasHKI values are classified into four modes for each month depending on their intensity. In the summer months, the high CasHKI mode is associated with enhanced MSLP over central Asia and deepening of the Indo-Pakistan thermal low associated with the Indian summer monsoon. At 700-hPa level, the high CasHKI mode shows an enhancement of the Arabian ridge, expanding it to the north over Iran and the Caspian Sea, with a concurrent strengthening of the Indian trough, leading to intensification of northerly winds along east Iran. This results in significant increase in dust activity over SW Asia, which is also apparent in the winter months. Furthermore, the intensification of the northerly flow associated with the high CasHKI modes drops the temperature and increases the relative humidity over Zabol, especially during winter. The SPRINTARS-model simulations also show increased dust emissions and concentrations for the high CasHKI values, confirming that the CasHKI variations modulate the dust activity over SW Asia throughout the year.






























Impact of meteorological factors on the emergence of bronchiolitis in Northwestern Greece.

Objective: To evaluate the relationship between meteorological factors in Northwestern Greece and the incidence of bronchiolitis. Methods: Meteorological data (air temperature and rainfall) for Ioannina city in Northwestern Greece and medical data from hospitalized patients at University Hospital of Ioannina were collected between January 2002 and December 2013. The association between meteorological factors and rate of hospitalization due to bronchiolitis was investigated. The data processing was done using the Pearson product-moment correlation coefficient and applying the chi-square test at contingency tables of the parameters. Results: Of the 792 hospitalized cases, 670 related to infants (< 1 year) and 122 concern patients aged 1-2 years old. The disease is more common among boys (59.5%) than girls (40.55). The disease course through the year has a double variation with a main maximum in March and a main minimum in August. The statistical study showed statistically significant correlation of bronchiolitis with: a) the temperature parameters on annual basis, b) precipitation in autumn and dryness in spring and c) with sudden changes in diurnal temperature range on annual basis. Conclusion: A peak incidence of bronchiolitis was noticed in cold and wet seasons during the five days preceding hospitalization.






























Observational and modelling study of a mesoscale convective system during the HYMEX – SOP1.

An intense and fast moving convective line that crossed Massif Central/Cevennes - Vivarais area (south France) during the field campaign of Hydrological Cycle in Mediterranean Experiment (HyMeX) - Special Observing Period 1 (SOP1) is examined. The mesoscale analysis demonstrates a complex convective system with a V-shape in the Infrared (IR) satellite imagery and a squall line pattern on the radar imagery. Ground stations observed up to 60 mm h-1 of rain accumulation, while the lightning activity, as observed by 4 detection networks, was also exceptionally high. The Weather Research and Forecasting (WRF) model was used to simulate this convective episode and sensitivity tests were performed with various microphysics and convective parameterization schemes. Satellite data from Meteosat SEVIRI Rapid Scanning Service were used in conjunction with radar, lightning and rain gauge data to conclude on the best simulation for which WRF model exhibits a rather precise and realistic distribution and evolution of the precipitation patterns. Finally, a study of the microphysics was performed indicating the interconnection of graupel with lightning activity, confirming recent results, compared against a sophisticated hydrometeor classification radar algorithm and lightning data.






























The Caspian Sea– Hindu Kush Index (CasHKI) – a regulatory factor for atmospheric dynamics and dust activity in southwest Asia.

This paper introduces a new climatology index, the Caspian Sea – Hindu Kush Index (CasHKI), which consists of changes in the mean sea level pressure (MSLP) gradient between the Caspian Sea (CS, 40-50°N, 50-55°E) and Hindu Kush (HK, 35-40°N, 70-75°E) which modulate the atmospheric dynamics and dust activity over southwest (SW) Asia. The CasHKI is defined as the difference between MSLP anomalies over the CS and HK (MSLPanom.CS – MSLPanom.HK) and is initially examined during the summer months (June-September, JJAS) of 2000-2014. The NCEP/NCAR data for MSLP and surface meridional wind, along with IDDI (Infrared Difference Dust Index) retrievals from Meteosat, OMI-AI and TRMM rainfall products and SPRINTARS model simulations of dust emissions are analyzed as a function of CasHKI. The CasHKI variations modulate the at-mospheric dynamics over SW Asia. Increase in CasHKI intensity strengthens the northerly flow along the eastern Iranian borders (Levar wind), resulting in activa-tion of the dust sources over the region and in significantly larger dust outflows over the northern Arabian Sea. Extending the research to the whole year, during the enlarged period 1963-2014, verifies the role of the CasHKI as a regulatory factor for dust activity over SW Asia, even during the winter period






























Modes of diurnal variation of illuminance in Bratislava, Slovakia, during winter.

Ôhe diurnal variation of illuminance during winter is studied in Bratislava, Slovakia. The database consists of mean 5-min values of global horizontal illuminance (CIE IDMP station 48.17°N, 17.08°W, alt. 195 m) around winter solstice (26 November - 6 February, up to 110 % of solstice day length) for the 14-year period 1994-2007. Two separate sets of data are determined: (i) sunrise- noon and (ii) noon-sunset. They consist of the semi-diurnal courses of illuminance anomalies from the mean semi-diurnal course of each calendar day of the year. By using an appropriate statistical methodology, i.e. Factor Analysis, to reduce the dimensionality of the data set, and then Cluster Analysis, to group objectively days with similar semi-diurnal courses, the most significant modes of illuminance, are revealed. Six typical semi-diurnal courses for sunrise-noon and seven for noon-sunset are found. Some of these courses present the almost smooth ascending/descending diurnal variation of illuminance, while others show significant disturbances during various daytime periods, due to the development of clouds. Finally, the most representative day of each Cluster is selected and presented.






























Seasonal variability of total cloud cover in the Mediterranean region for the period 1948-2014.

The seasonal variability of total cloud cover in the Mediterranean region is studied, using NCEP/NCAR daily grid point data. The mean day to day intra-annual variations of total cloud cover during the hydrological year (1 October-30 September) are extracted for the 66-year period 1948-2014. A dimensionality reduction method is applied on the 324 time series and three main modes of seasonal variation of cloud cover over the Mediterranean region are found. The first one is simple, characterized by a maximum in winter and a minimum in summer, prevailing mainly over the sea. The second one is double showing maxima in early autumn and late spring and minima in late summer and winter and prevails over SW Europe and NW Africa inland regions. Finally, the third one shows a maximum in June and a minimum in October and prevails over the eastern part of central Europe. The main characteristics of these modes are attributed to the seasonal variation of the various thermal and dynamical factors affecting the region. An examination of the seasonal variability of cloud cover in the above core regions during sub-periods of the total period 1948-2014 reveals some long-term changes during the last 66 years.






























A Study on the intra-annual variation and the spatial distribution of lightning activity over Greece.

The intra-annual variation and the spatial distribution of lightning activity over Greece is studied for the 10-year period 2005-14, on a 15-day basis, using Zeus 0.5x0.5 degrees data. Factor Analysis (S-mode and T-mode) is applied to the average lightning activity for each fortnight of the year and for each grid box, for the studied period. (i) For the intra-annual variation of lightning activity, 3 sub-regions are defined, presenting characteristic modes of variation: In the first one, over the mainland, a broad maximum appears during late spring-early summer; in the second, mainly over the Ionian Sea, a maximum is seen during early autumn; while in the third, mainly over the central and south Aegean Sea, two maxima are found, one in middle autumn and another in late May. (ii) For the spatial distribution of lightning activity, 4 main patterns are revealed: the "summer" pattern (1/5-15/8), characterized by a maximum over the mainland; the "autumn" pattern (1/9-15/10), with maxima over the north Ionian Sea and the north Aegean Sea; the "winter" pattern (15/12-15/2), with maxima over the south Ionian Sea and Dodecanese Islands; and a low variance "transitional" pattern (1/4-30/4 and 1/12- 15/12), with maximum over the central Aegean Sea.






























A 50-year climatology of sea-surface response to atmospheric circulation anomalies in the Mediterranean and the Black Seas during summer.

The influence of the atmospheric circulation over Europe, the east Atlantic and north Africa on the sea surface of the Mediterranean and the Black Seas is studied for the 50-year period 1958–2007, during summer. The dataset consists of monthly (May-September) anomalies of: (i) sea surface temperature, sensible and latent heat flux, short and long wave radiation and total cloud cover for the sea surface and (ii) 500 and 1000 hPa geopotential height for the atmosphere. At first, Factor Analysis is applied in the data matrix of each field, in order to reduce its dimensionality. Then, the reduced matrices for the sea surface as well as for the atmosphere are unified in two final ones. Canonical Correlation Analysis is applied on these two matrices leading to five statistically significant canonical pairs. The correlation coefficients between the canonical variates and the original data indicate the centers of action for the connection between air and sea surface. The position and the intensity of a low/high pressure system and the corresponding trough/ridge over Europe, along with the physical geography of the Mediterranean and the Black Seas, regulate the anomalies of the sea-surface parameters.






























Merominia. An investigation of their scientific basis.

In this work, the ability of a long-range weather forecast via the folklore "Merominia" is investigated in detail. Merominia are called either the first twelve days of August or the days from the 14th to the 25th of August. According to local traditions and beliefs, by observing the weather of these days, someone can forecast the weather of a whole year, as the weather of each of these twelve days corresponds to the mean weather of a future month. For a scientific investigation of this belief, 42 year daily values mean temperature and precipitation were used for Ioannina and Thessaloniki. The values of these parameters during the above twelve days periods were correlated with the corresponding average values of the next 12 months or 30-day periods. Furthermore, it was investigated whether there is another 12-day period during the year, which could be used for a successful weather forecast for the next twelve 30-day periods. In all cases it was proved that the weather of a single day is not able to forecast the average weather conditions during a future month.






























Atmospheric circulation characteristics favoring extreme minimum air temperature values over the greater Mediterranean region.

The main characteristics of atmospheric circulation favoring extreme daily minimum surface air temperatures over the Mediterranean region are studied. NCEP/NCAR Reanalysis grid point daily values of minimum surface air temperature (Tmin), 500 hPa and 1000 hPa geopotential heights (Z500 and Z1000) and 850 hPa air temperature (T850) over the Mediterranean region are used for the 50-year period 1963-2012. Factor Analysis is applied on the Tmin data set, revealing 12 main sub-regions of Tmin covariability. For each of these sub-regions, the days with the highest and the lowest 1% values of factor scores are identified. Then, Factor Analysis and Cluster Analysis are applied on the Ć500, Ć1000 and T850 data sets for the above extreme cases and the mean Ć500, Ć1000 and T850 patterns are constructed for the days classified to each one of the clusters found. These patterns show the main atmospheric circulation characteristics associated with extreme Tmin values in the various Mediterranean sub-regions. Generally, extreme high Tmin values are favored by warm advection due to a southerly flow and/or high cloud cover minimizing nocturnal radiative cooling. On the other hand, extreme low Tmin values are favored by cold advection due to a northerly flow or anticyclonic conditions allowing intense nighttime radiative cooling.






























Deucalion: modern educational tools for precaution measures during extreme weather events.

The prevention and confrontation of natural hazards during extreme weather conditions constitute an educational subject usually provided by bodies such as the meteorological service or environmental education, health education and educational mobility programs. In such actions, Information and Communication Technologies (ICT) have a dominant role, since students use them widely, not only in the educational process but also in their daily lives. A modern approach to the pedagogical use of ICT is the use of digital learning objects. Learning objects are described as self-standing resources of learning content that include or indicate learning objectives. The Deucalion project proposes the use of digital learning objects for natural disaster education and especially for disasters caused by atmospheric phenomena. The learning objects include interactive virtual and multimedia environments, as well as educational games. The learning objects refer to atmospheric phenomena such as rain, snow, hail, frost, wind, etc. Their usage is carried out through meaningful teaching interventions and educational scenarios. Their inclusion in the educational process is proposed either by incorporating them into the curriculum, or in environmental education and health education programs.






























Numerical simulations and observational study of a Mesoscale Convective System in France, during the HyMeX - SOP1 using the WRF model.

An intense and fast moving convective line that crossed Massif Central/Cevennes-Vivarais area (south France) during the field campaign of HYdrological cycle in Mediterranean EXperiment (HyMeX)-Special Observing Period 1 (SOP1) is examined. The mesoscale analysis demonstrates a complex convective system with a V-shape in the Infrared (IR) satellite imagery, while ground stations observed up to 100 mm of rain accumulation. The Weather Research and Forecasting (WRF) non-hydrostatic model was used to simulate this convective episode and sensitivity tests were performed on explicit microphysics, as well as on convective parameterizations in high resolution domains. Satellite data from SEVIRI Rapid Scan Service were used in conjunction with rain gauge data to conclude at the best simulation at which the WRF model exhibits a rather precise and realistic distribution and evolution of the precipitation patterns.






























On processing extreme data.

Extreme Data is an incarnation of Big Data concept distinguished by the massive amounts of data that must be queried, communicated and analyzed in near real-time by using a very large number of memory or storage elements and exascale computing systems. Immediate examples are the scientific data produced at a rate of hundreds of gigabits-per-second that must be stored, filtered and analyzed, the millions of images per day that must be analyzed in parallel, the one billion of social data posts queried in real-time on an in-memory components database. Traditional disks or commercial storage nowadays cannot handle the extreme scale of such application data. Following the need of improvement of current concepts and technologies, we focus in this paper on the needs of data intensive applications running on systems composed of up to millions of computing elements (exascale systems). We propose in this paper a methodology to advance the state-of-the-art. The starting point is the definition of new programming paradigms, APIs, runtime tools and methodologies for expressing data-intensive tasks on exascale systems. This will pave the way for the exploitation of massive parallelism over a simplified model of the system architecture, thus promoting high performance and efficiency, offering powerful operations and mechanisms for processing extreme data sources at high speed and/or real time.






























Spatial and temporal variability of precipitation over the Mediterranean Basin based on 32-year satellite Global Precipitation Climatology Project data. Part-II: Inter-annual variability and trends.

Monthly mean satellite data from the Global Precipitation Climatology Project (GPCPv2) are used to examine the year by year variability of precipitation over the Mediterranean basin and its changes over the period 1979-2010. The results show that the mean annual precipitation averaged over the study area has slightly increased from 1979 to 2010 by 1.28 mm or by 0.2% (trend not statistically significant at the 95% confidence level). Nevertheless, spatial and temporal patterns are revealed examining the trends at local scale with opposite trends in adjacent areas and increasing precipitation in summer and autumn, against almost unchanged or decreasing precipitation in winter and spring, respectively. Inter-decadal changes of precipitation appear, with precipitation decreasing in the 1980s, then increasing through the late 1990s, and finally declining in the 2000s, before levelling off since 2007. These changes are significantly anticorrelated (R = -0.57, up to -0.66 in winter) with the North Atlantic Oscillation (NAO) Index, thus confirming the critical role of this large scale teleconnection for the regional precipitation over the basin.






























Spatial and temporal variability of precipitation over the Mediterranean Basin based on 32-year satellite Global Precipitation Climatology Project data. Part-I: Evaluation and climatological patterns.

The precipitation regime over the Mediterranean basin is investigated for the period 1979-2010 using monthly mean satellite data from the Global Precipitation Climatology Project (GPCPv2). The results show that a clear contrast exists between the more rainy northern part of the study region (Southern Europe) and the drier southern area (North Africa, Iberian Peninsula), and between the western sides (rainsides) of the Iberian, Italian and Balkan peninsulas and their eastern sides (rainshadows). The mean annual precipitation averaged over the study area is P=593±203 mm/year, but it has a strong spatial variability ranging from 20 mm/year (North Africa) to 1500 mm/year (Alps). A significant seasonal variability exists, with the early winter and late autumn months (November and December) being the wettest with precipitation amounts larger than 60 mm/month. The GPCPv2 satellite precipitation data are satisfactorily correlated with rain gauge measurements from 433 stations within the study area (correlation coefficient R=0.78 for all stations on a yearly basis, with values ranging between 0.72 and 0.82 depending on the season) with a slight overestimation. They also compare well with the higher spatial and temporal resolution Tropical Rainfall Measuring Mission (TRMM) data, which supports the validity of the present study.






























The Caspian Sea - Hindu Kush Index (CasHKI): a regulatory factor for dust activity over southwest Asia.

This work investigates the modulation in dust activity over southwest (SW) Asia attributed to changes in the mean sea level pressure (MSLP) between the Caspian Sea (CS) and Hindu Kush (HK) during the summer months (JJAS) of the period 2000-2014. The MSLP anomalies obtained via NCEP/NCAR re-analysis are evaluated via a new climatology index, the Caspian Sea - Hindu Kush Index (CasHKI), which is defined as CasHKI = MSLPanom.CS - MSLPanom.HK, over specific domains taken over the CS and HK. The changes in CasHKI intensity are examined against dust activity and rainfall distributions over south Asia. The satellite remote sensing (Meteosat, OMI, MODIS) analyses show that high CasHKI values corresponding to enhanced pressure gradient between the CS and the HK, are associated with intensification of northerly winds, increased dust emissions and transportation over SW Asia and north Arabian Sea. In contrast, variations in CasHKI intensity do not seem to have a significant effect on the Indian summer monsoon. Only a slight decrease of precipitation over the southern Indian peninsula and the neighboring oceanic areas and an increase of precipitation along the Ganges Basin and Himalayan range are found to be related to high CasHKI values. Model (MIROC-SPRINTARS) simulations of dust concentration and dust AOD (Aerosol Optical Depth) over SW Asia are consistent with the satellite observations, highlighting for the first time the modulation of the SW Asian dust activity by CasHKI.






























The atmospheric circulation characteristics favoring snowfall in an area with complex relief in northwestern Greece.

The connection between atmospheric circulation over Europe and snowfall in Ioannina, NW Greece, is examined. The study deals with 160 snow events, referring to the 56-year period 1956–2011. For each of the events, the patterns of 500 hPa and 1000 hPa Geopotential Height, 500 hPa and 850 hPa Air Temperature, 1000-500 hPa, 1000-700 hPa and 700-500 hPa thickness are constructed for the European area, whereas static stability K-Index, Relative Humidity at 500 hPa, 700 hPa and 850 hPa levels and finally Relative Vorticity at 500 hPa and 1000 hPa are also used for the SE Mediterranean region, for the previous day (D-1), the starting day (D) and the day following the cessation of snowfall (END). Factor Analysis and Cluster Analysis are applied to the above data sets and the evolutions of the above circulation parameters are classified into 8 clusters. Snowfall in NW Greece is generally associated with a low pressure system over the Ionian Sea combined with an anticyclone over W or NW Europe, causing cold northwesterly flow in the lower troposphere over NW Greece. In the middle troposphere, a deep 500 hPa trough usually extends from W Russia to Italy and the Ionian Sea. The 8 atmospheric circulation structures differ mainly in the exact location and/or the intensity of the depression and/or the anticyclone, in the orientation of the 500 hPa trough axis and in the trajectories of the above synoptic systems between D-1 and END days.






























An aggregate of four anthrax cases during the dry summer 2011 in Epirus, Greece.

Human anthrax is currently a sporadic disease in Europe, without significant regional clustering. The objective of this work is to report an unexpected aggregate of anthrax cases and correlate local climatic factors with yearly anthrax admissions. The methodology followed includes clinical description of a geographical-temporal anthrax aggregate, correlation of disease admissions with local weather data in the period 2001-2014 and literature reports of anthrax clusters from Europe in the last 20 years. We identified five cases, all cutaneous: an unexpected four cases aggregate in mid-summer 2011 (including a probable human to human transmission) and a sporadic case in August 2005, all in relatively dry periods (p<0.05). Remarkably, 3/6 reports of human anthrax aggregates from Europe were observed in Balkan Peninsula countries in the year 2011. In the light of the predicted climatic change, unexpected anthrax aggregates during dry periods in southern Europe underscore the risk of future anthrax re-emergence in this continent.






























Development of a Greek solar map based on solar model estimations.

The realization of Renewable Energy Sources (RES) for power generation as the only environmentally-friendly solution, moved solar systems to the forefront of the energy market in the last decade. The capacity of solar power doubles almost every two years in many European countries, including Greece. This rise has brought the need for reliable predictions of meteorological data that can easily be utilized for proper RES-site allocation. The absence of solar measurements has, therefore, raised the demand for deploying a suitable model in order to create solar maps. The generation of a solar map for Greece could provide solid foundations on the prediction of the energy production of a solar power plant installed in the country, by providing an estimation of the solar energy acquired at each longitude and latitude of the map. In the present work, the Meteorological Radiation Model (MRM), a broadband solar radiation model, is engaged. This model utilizes common meteorological data, such as air temperature, relative humidity, barometric pressure and sunshine duration, in order to calculate solar radiation for areas where such data are not available. Hourly values of the above meteorological parameters are used from 39 meteorological stations, evenly dispersed around Greece; hourly values of solar radiation are, therefore, calculated through MRM. Then, by using an integrated spatial interpolation method, a Greek solar energy map is generated, providing annual solar energy values all over Greece.






























Building comprehensive meteorological data files with the use of artificial neural networks.

Synoptic meteorological observations all over the world are usually carried out on the basis of 3-hour intervals, as specified by the World Meteorology Organization. However, in many cases, e.g. for the detailed calculation of buildings' energy balance and the energy production of solar power systems, cognition of the hourly time series of some meteorological parameters such as air temperature, air relative humidity, atmospheric pressure, etc. is necessary. For this purpose, in the specific study, artificial neural networks are developed and applied in order to transform 3-hour meteorological data files of air temperature, relative humidity and atmospheric pressure of different regions to full 24-hour data files. The hourly values predicted by the developed artificial neural networks are accordingly compared with the actual hourly values of parameters examined with the results obtained showing a very satisfying agreement at a statistical significance level of p<0.01.






























Comparison of long-term broad band model results with experimental measurements of solar radiation.

A rapid development of solar energy systems has been noticed in the last decade worldwide, with the photovoltaic systems’ installed global power capacity exceeding 100GWe. Moreover, the European Union has realized that one of the key sectors for the implementation of energy saving techniques is the building sector for which Member States have already adopted measures for reducing energy consumption. A key parameter for energy calculations with regards to photovoltaics and building structures, along with the external temperature, is the possession of long-term solar radiation data. Nevertheless, due to the frequent lack of real field measurements in the area under investigation, the use of validated solar radiation models comprises a key step in the process allowing analysts and scientists to obtain the required background information. In this respect, the present work investigates the reliability of a theoretical model’s (MRM) solar radiation estimations through experimental 35,064 hourly solar radiation data points obtained in the area of Ioannina, NW Greece. The MRM routine was ran throughout a four (4)-year period, separating also each year into two main phases in order to demonstrate seasonal patterns between cloudy (cold season) and less cloudy (hot season) sky conditions. According to the results obtained, in general, a fairly good agreement was observed between experimental data and model’s results, however, deviations derived mostly in cases of partly cloudy weather mainly due to the use of recorded-daily instead of recorded-hourly values of sunshine duration.






























Energy consumption based on heating/cooling degree days within the urban environment of Athens, Greece.

The degree-day method is considered to be a fundamental and a rather simple method to estimate heating and cooling energy demand. This study aims in a detailed and accurate assessment of cooling and heating degree days in different locations within the Greater Athens area (GAA), Greece. To achieve this goal, hourly values of air temperature from eight different locations within the GAA, covering the period 2001-2005, were used. Thus, the monthly and the annual number of cooling and heating degree days for each one of the examined locations could be estimated separately. Furthermore, an effort is made to evaluate the energy consumption for a specific building, based on the degree-day method, to indicate the impact of the canopy layer urban heat island on neighboring regions within the GAA. Results reveal that there is great spatial variability of energy demand and energy consumption along with significant differences in expenses for heating and cooling among neighboring regions within the GAA. Finally, regarding the energy demands of buildings, it is important to take into account intra-urban variability of canopy layer climates against an ensemble mean throughout the city, because the latter can result in inaccurate estimations and conclusions.






























Atmospheric circulation evolution related to desert dust episodes over the Mediterranean.

In the present study, the cases of atmospheric circulation evolution favouring the occurrence of Desert Aerosol Episodes (DAEs) over the broader Mediterranean region are investigated. For this purpose, an objective and dynamic algorithm, using daily satellite measurements of various aerosol optical properties, is set up for the identification of DAEs, at 1x1 degree geographical cell level, over the period 1 March 2000 – 28 February 2007. According to the applied methodology, 98 Dust Aerosol Episode Days (DAEDs) and 62 cases of consecutive DAEDs (Desert Aerosol Episode Cases - DAECs) are defined. For each DAEC, the lower tropospheric circulation evolution phases one and two days before, during the initiation and after the cease of the DAEC are considered. S-mode Factor Analysis and K-Means Cluster Analysis are applied on NCEP/NCAR Mean Sea Level Pressure and 700hPa geopotential height fields, classifying the 62 cases of atmospheric circulation evolution into 6 homogenous and discrete clusters. According to the results, the mean intra-annual variation of the DAECs reveals a primary maximum in July (2 DAECs or 22.6%), while their mean annual number is equal to 8.9 DAECs. On a seasonal basis, the highest percentage of the DAECs is found in spring (54.9%) when dust transport activity is intense. Regarding the DAECs’ duration, results show that 64.5% of them last one day, while the maximum duration is 5 days. During the year, the mean monthly number of DAEs varies from 35.0 (September) to 61.5 (October). The western parts of the Mediterranean are affected by DAEs when a southwesterly flow at 700hPa and cyclonic conditions prevail in the central Mediterranean and NW Africa, respectively. On the contrary, the central and eastern parts of the study region are affected by dust storms when a low pressure system in the central Mediterranean or central Europe and an anticyclone in the eastern Mediterranean prevail. According to the cluster intercomparison, the intensity (aerosol optical depth at 550nm) of strong DAEs varies from 0.74 to 0.90, while the corresponding values for the extreme DAEs are 1.25 and 2.21, respectively. Finally, the contribution of the strong DAEs is higher than that of the extreme ones in five out of the six clusters, while extreme DAEs dominate in one cluster only.






























Relationship between the Indian summer monsoon and the large-scale circulation variability over the Mediterranean.

In this study the impact of the Indian summer monsoon on the large scale variability of the atmospheric circulation over the Mediterranean is investigated on an inter-annual time scale. Composite and correlation analysis results are presented, outlining different circulation patterns in the upper and lower troposphere for strong and weak monsoon years respectively. For this purpose, ERA-40 Reanalysis monthly mean data at various isobaric levels together with the standardized All India Rainfall Index for boreal summer (June-July-August-September) of a 44-year period were employed. During strong monsoon years many atmospheric circulation systems appear strengthened over Eurasia, resembling a well organized Rossby wave train over the area. In the upper troposphere, a meridional shift of the jet streams over the examined area was also identified during extreme monsoon years. On the other hand, in the lower troposphere enhanced northerlies (Etesians) appear to dominate over Eastern Mediterranean along with intensified subsidence during strong monsoon years.






























Influence of orography on precipitation amount and distribution in NW Greece; a case study.

In this work the impact of topography on the distribution and the amount of precipitation in Epirus, NW Greece is investigated, during the cold period of the year. A precipitation event with typical wintertime characteristics is considered. In the selected case, extended and heavy precipitation over Epirus is caused by a low pressure system centered northwest of Epirus and moving southeastwards, generating southerly winds over the Ionian Sea. The case is simulated by applying the numerical meteorological model MM5 in a high resolution grid (2x2km). Various sensitivity tests are performed, searching for the model initialization time and the convective parameterization scheme (CPS) leading to the best simulation of the observed event. Then, the model is applied again incorporating modified data of topography, in order to study the differentiations in the fields of the most important meteorological parameters, which are directly related with precipitation. The topography is modified with two ways: either by reducing the height of the mountains (type RT) or by elevating the terrain height (type ET). The results show that in the RT type, the precipitation field appears more homogeneous, with weaker maxima on the windward sides and above the mountain ridges. As a consequence, less rain falls in these areas and humid air passes easier to the lee sides reducing rain shadows. In the ET type, rain increases in the windward coastal zone of Epirus while it is reduced in the inland areas where less humid air is available. Generally, the modification of topography changes the flows near the surface, causing a displacement of the convergence zones and thus a new distribution of vertical velocities, which, combined with the new water vapour field, has a direct impact on precipitation.






























Meteorological conditions associated with severe dust storms in the Sistan region, Iran.

The present work focuses on examining the synoptic and dynamic meteorological conditions associated with dust-storm outbreaks in the Sistan region, southeastern Iran during the summer season (June-September) of the years 2001 to 2012. The dust-storm days (total number of 356) are related to visibility records below 1 km at Zabol meteorological station, located well within the dust source region. RegCM4 model simulations reveal that the local intense Levar wind, blowing from northern directions during the summer season, plays the major role in dust outbreaks. Furthermore, the high surface heating and the valley-like characteristics of the region strongly control the meteorological dynamics, the Levar wind and the formation of a low-level jet over the region that are strongly linked with dust exposures. NCEP/NCAR reanalysis reveals that the dust storms are associated with low sea-level pressure conditions over the whole south Asia during summer, while at 700 hPa a trough of low geopotential heights over India along with a ridge over Arabia and central Iran is the common scenario.






























Modes of diurnal variation of illuminance in Bratislava, Slovakia, during summer.

Ôhe diurnal variation of illuminance during summer is studied in Bratislava, Slovakia. The database consists of mean 5-minute values of global horizontal illuminance (CIE IDMP station - 48.17N, 17.08E, alt. 195m) around summer solstice (2 May - 8 August, up to 90% of solstice day length) for the 14-year period 1994-2007. Two sets of data are determined (sunrise-noon, noon-sunset) consisting of the semi-diurnal courses of illuminance anomalies from the mean semi-diurnal course of each calendar day of the year. By using an appropriate statistical methodology, at first Factor Analysis to reduce the dimensionality of the data set and then Cluster Analysis for grouping days with similar semi-diurnal courses, the most significant modes of semi-diurnal variations of illuminance anomalies, and then of illuminance, are revealed. Six typical semi-diurnal courses for sunrise-noon and seven for noon-sunset are found. Some of these courses present the almost smooth ascending/descending diurnal variation of illuminance, while others show significant disturbances during various daytime periods, due to development of clouds. Finally, for each Cluster the most representative day is selected and the corresponding atmospheric circulation patterns near the surface and at 500hPa level as well as the distribution of precipitation over Europe are presented and discussed.






























Atmospheric circulation characteristics favouring precipitation in Cyclades Islands, southern Aegean.

The main atmospheric circulation characteristics favouring precipitation in Cyclades Islands, southern Aegean, are examined. The data used consist of daily values of: 1) precipitation at the meteorological stations of Naxos, Santorini and Milos and 2) temperature, humidity and circulation parameters at 2.5x2.5 degrees grid over Europe (NCEP/NCAR), for the period 1974-2011. Precipitation values are used for the identification of the sequences of consecutive precipitation days (precipitation events). The grid point data are used for the construction of the daily patterns of the meteorological parameters for the day before the initiation (D-1), the first day (D) and the last day (END) of the events. Factor Analysis and K-means Cluster Analysis are applied to the above parameters for D-1, D and END days and the precipitation events are classified in 7 clusters. Each cluster corresponds to a characteristic evolution (D-1, D, END) type of the above patterns. The tracks of 1) the upper air troughs and the depressions, 2) the vorticity, humidity and instability maxima and 3) the warm/cold fronts or advection areas constitute the main characteristics of the 7 types.






























Meteorological aspects associated with dust storms in the Sistan region, southeastern Iran.

Dust storms are considered natural hazards that seriously affect atmospheric conditions, ecosystems and human health. A key requirement for investigating the dust life cycle is the analysis of the meteorological (synoptic and dynamic) processes that control dust emission, uplift and transport. The present work focuses on examining the synoptic and dynamic meteorological conditions associated with dust-storms in the Sistan region, southeastern Iran during the summer season (June-September) of the years 2001 to 2012. The dust-storm days (total number of 356) are related to visibility records below 1 km at Zabol meteorological station, located near to the dust source. RegCM4 model simulations indicate that the intense northern Levar wind, the high surface heating and the valley-like characteristics of the region strongly affect the meteorological dynamics and the formation of a low-level jet that are strongly linked with dust exposures. The intra-annual evolution of the dust storms does not seem to be significantly associated with El-Nino Southern Oscillation, despite the fact that most of the dust-storms are related to positive values of Oceanic Nino Index. NCEP/NCAR reanalysis suggests that the dust storms are associated with low sea-level pressure conditions over the whole south Asia, while at 700 hPa level a trough of low geopotential heights over India along with a ridge over Arabia and central Iran is the common scenario. A significant finding is that the dust storms over Sistan are found to be associated with a pronounced increase of the anticyclone over the Caspian Sea, enhancing the west-to-east pressure gradient and, therefore, the blowing of Levar. Infrared Difference Dust Index (IDDI) values highlight the intensity of the Sistan dust storms, while the SPRINTARS model simulates the dust loading and concentration reasonably well, since the dust storms are usually associated with peaks in model simulations.





























Synoptic weather conditions and aerosol episodes over Indo-Gangetic Plains, India.

The present study focuses on identifying the main atmospheric circulation characteristics associated with aerosol episodes (AEs) over Kanpur, India during the period 2001-2010. In this respect, mean sea level pressure (MSLP) and geopotential height of 700 hPa (Z700) data obtained from the NCEP/NCAR Reanalysis Project were used along with daily Terra-MODIS AOD550 data. The analysis identifies 277 AEs [AOD500 > mean AOD500 + 1STDEV] over Kanpur corresponding to 13.2% of the available AERONET dataset, which are seasonally distributed as 12.5%, 9.1%, 14.7% and 18.6% for winter (Dec-Feb), pre-monsoon (Mar-May), monsoon (Jun-Sep) and post-monsoon (Oct-Nov), respectively. The post-monsoon and winter AEs are mostly related to anthropogenic emissions, in contrast to pre-monsoon and monsoon episodes when a significant component of dust is found. The multivariate statistical methods Factor and Cluster Analysis are applied on the dataset of the AEs days’ Z700 patterns over south Asia, to group them into discrete clusters. Six clusters are identified and for each of them the composite means for MSLP and Z700 as well as their anomalies from the mean 1981-2010 climatology are studied. Furthermore, the spatial distribution of Terra-MODIS AOD550 over Indian sub-continent is examined to identify aerosol hot-spot areas for each cluster, while the SPRINTARS model simulations reveal incapability in reproducing the large anthropogenic AOD, suggesting need of further improvement in model emission inventories. This work is the first performed over India aiming to analyze and group the atmospheric circulation patterns associated with AEs over Indo-Gangetic Plains and to explore the influence of meteorology on the accumulation of aerosols.






























On the link between Indian summer monsoon and the Etesian pattern over the Aegean Sea.

The Indian summer monsoon (ISM) is a key factor influencing the eastern Mediterranean climate. During early summer period, the thermal low over Pakistan and northwestern India extends towards the Mediterranean through inner Iraq and Turkey. On the other hand, the Azores high expands eastwards and meets the Balkan high pressure system, forming one common system. Greece lies at the transitional zone between these two pressure systems, where persistent northeasterly wind currents form especially over the Aegean, known as the Etesians. The objective of this study is to investigate the interrelationbetween the ISM and the Etesian windsover the Aegean Sea during summer for an extended period of 44 years (1958-2001)with the aid of composite and correlation analyses and ERA 40 datasets.More specifically, the study will focus on the ISM activity during and prior to or after ‘Etesian episodes’ in order to determine the extent to which one system may lead another. Furthermore, the impact of ISM on the variability of Etesian wind speed and frequency will be examined. The present analysis reveals that during enhanced monsoon years two upper level ridges prevail over the greater area of interest, one over western Europe and northern Africa, which is part of the Azores high and a second one over Pakistan region, Persian Gulfand Middle Asia, whichextends to the west and connecting to the first one forms a slight trough over Eastern Mediterranean.At lower levels, an intensification of the thermal Asian low as well as of thehigh pressure system prevailing over western and central Europe is apparent. Concurring with this pattern in upper and lower troposphere, anomalously strong northerlies appear over the Aegean during periods with enhanced monsoon activity, as horizontal surface wind anomalies depict. In agreement with previous studies, mid-level subsidence during excess monsoon rainfall periods is found to be more intense over the Eastern Mediterranean, Iraq, Saudi Arabiaand east of the Caspian Sea.






























Investigation for a possible influence of Ioannina and Metsovo Lakes (Epirus, NW Greece), on precipitation, during the warm period of the year.

The aim of this work is to investigate if and to what extend the most important lakes of Epirus, northwestern Greece, namely, Ioannina and Metsovo Lakes, affect the precipitation regime (height and distribution) in the area during the warm period of the year. For this reason, an afternoon thermal precipitation event is considered and simulated by applying the numerical meteorological model MM5. Sequential sensitivity tests showed that the precipitation event is satisfactorily predicted by the model by applying Kain-Fritsch-2 convective parameterization scheme even in the finest domain of 2 Km. The comparison of simulations with unmodified and modified land use data showed that the humidity over and around the lakes, in the afternoon, is mainly controlled by air masses coming from the Ionian Sea. In other words, the lakes do not affect the events via their contribution in humidity and this is one of the reasons that thermal precipitation in Epirus does not occur exclusively in the areas of Ioannina and Metsovo but also in many other inland places. The role of the lakes is found to be indirect, as they influence the air flows above them and thus affect the updrafts which cause precipitation.






























Climatic characteristics of summer human thermal discomfort in Athens and its connection to atmospheric circulation.

The climate characteristics of summer human thermal discomfort in Athens and its connection to atmospheric circulation is studied for the period 1954-2012. The human thermal discomfort is examined in terms of the Predicted Mean Vote (PMV) discomfort index for calm and light wind (3 ms-1) conditions. Its inter-annual variability is characterized by a significant increase from the middle 1980’s to the end of the study period. The onset and the cessation of the discomfort period are found to take place around the beginning of July and the end of August respectively, but from middle 1980’s the dates of onset and cessation have slightly moved earlier and later, respectively, leading to a longer summer discomfort period. The connection between human thermal discomfort and atmospheric circulation is studied by examining the distribution of discomfort cases across six objectively defined circulation types over Europe, based on Athens weather characteristics. High values of the PMV discomfort index are mainly associated with two typical high-summer pressure patterns with the intensity of discomfort depending on the pressure gradient over the Aegean Sea. On the contrary, low PMV discomfort index values prevail mainly on days typified by the other four circulation types, which are more frequent during May, June and September.






























Large-scale variability over Mediterranean associated with the Indian summer Monsoon.

The objective of this study is the investigation of the large scale vari-ability of the atmospheric circulation over the Mediterranean region in relation to the Indian summer monsoon. For this purpose composite anomalies of selected fields at various isobaric levels are analyzed for strong versus weak monsoon years. Gridded, monthly mean data, such as geopotential height, horizontal wind components, vertical velocity and relative vorticity at 300 hPa were used, as ob-tained from the ERA-40 Reanalysis Data Base, with 2.5x2.5 degrees resolution for the boreal summer (June-September) and for a 44-year period (1958-2001). The stan-dardized Dynamic Indian Monsoon Index by Wang and Fan (1999) was employed to determine the strong and weak monsoon years in the 44-year period. It was found that there are significant differences between strong and weak composites for all fields, especially in the upper troposphere. The results suggest that these differences may be related to the existence of Rossby wave trains as well as to the intensity and the meridional shift of the upper-level jet streams.






























A study of convergence and divergence of the wind field over Europe and the Mediterranean.

In this work, the convergence and divergence of the wind field over Europe, the Mediterranean and North Africa is studied for the 60-year period 1950-2009. Monthly values (s-1) are utilized at 273 grid points, spaced by 2.5x2.5 degrees, at 10 atmospheric pressure levels from 1000hPa up to 100hPa. It is found that on a seasonal basis considerable differences appear among the various European regions both near the surface of the Earth and in the upper atmosphere. During winter, near the surface, high values of convergence appear in the cyclogenesis areas and along the tracks of depressions. Specifically: in the Mediterranean, with maximum values in the Gulf of Genoa, the Black Sea and from the North Sea up to the Baltic Sea. On the contrary, over the rest of the areas divergence prevails. This situation is reversed above approximately 400hPa with positive values (divergence) over the Mediterranean and the Seas of northern Europe and negative (convergence) over central Europe. During summer, over the Seas, where air is cooler than over land, positive values are observed (divergence). This difference is more intense over lower latitudes. Similarly to winter, in the upper atmosphere, the convergence-divergence field becomes smoother changing signs at about 400hPa.






























On the climate characteristics of convection contribution to precipitation in southeastern Europe.

The spatial and temporal variability of the convective percentage of precipitation in the Italian and Balkan peninsulas and the surrounding areas are examined for autumn, winter and spring. The data used consists of monthly grid point values of Convective Precipitation Rate (CPR) and Total Precipitation Rate (TPR) in the southeastern Europe for the 60-year period 1950-2009. The seasonal values are calculated from the monthly ones and the Convective Precipitation Percentage (CPP) is calculated, dividing CPR by TPR. The seasonal average patterns of CPP are characterized by a maximum over the sea and a minimum over the northern continental areas. The CPP over the sea is highest in autumn and lowest in spring, while CPP over the land is highest in spring and lowest in winter. The inter-annual variability of CPP is examined by applying S-mode Factor Analysis to the three seasonal data matrices. Only 12, 11 and 10 factors are retained for autumn, winter and spring. For all seasons, statistical significant of CPP trends are found: a decrease in central Europe and northern Italy and an increase in the area west of Sicily. These trends are possibly connected to the variability of the large-scale synoptic systems affecting the region.






























The impact of topography on the precipitation regime over Epirus, NW Greece, during the cold period of the year.

The aim of this study is to investigate the impact of orography on the distribution and the amount of precipitation in Epirus, NW Greece, during the cold period of the year. For this reason, a precipitation event with typical wintertime charac-teristics is considered; i.e. a low pressure system centered northwest of Epirus and moving southeastwards, generating southerly winds and causing extended and heavy precipitation over the area. The case is firstly simulated by applying the numerical meteorological model MM5 in a high resolution grid (2x2km), in which the actual topographical data are incorporated. Then, the model is applied again incorporating modified data of topography, in order to study the differentiations in the fields of the most important meteorological parameters, which are directly related with precipitation. According to the results, the modification of topography changes the flows near the surface, causing a displacement of the convergence zones and thus a new distribution of vertical velocities, which, combined with the new water vapor field, has a direct impact on precipitation.






























Wind energy in N.W. Greece.

In this paper the wind energy in northwestern Greece is studied at 80 meters above ground level. The Penn State/National Center for Atmospheric Research Mesoscale Model version 5 (MM5) is used to obtain the necessary meteorological data to estimate the wind energy potential. This model is used operationally by the Laboratory of Meteorology of Ioannina University, Greece, for daily weather forecast. The model runs in 3 domains (Europe - Greece - Epirus) with the one-way nesting technique. The third domain contains 12,544 (112x112) grid points with a spatial resolution of 2x2 km. By using a statistical analysis for 1 year wind data, the mean annual power density map and the resultant wind direction map at 80 meters above ground level is calculated for N.W. Greece. The mean annual power density is calculated from the average 2-hour wind data of MM5 from the 1st of June 2007 until the 31st of May 2008. The highest values of wind energy are found in the mountainous areas. The maximum estimated value is 815 W/m2. Finally, the wind regime of the area with the highest value of wind energy for the region of Epirus is analyzed.






























Influence of anomalous dry conditions on aerosols over India: transport, distribution and properties.

A synergy of satellite and ground-based radiometric observations, along with chemical transport modeling, was used for the assessment of the influence of drought monsoon conditions of 2002 and prolonged dry pre-monsoon period of 2003 on aerosol properties over south Asia, with emphasis over northern India. Reanalysis data are also examined for studying the dry anomalous period from the climatological mean, that show prevalence of westerlies under anticyclonic circulation and subsidence favoring the accumulation of aerosols. TRMM observations over south Asia indicate significant rainfall deficit over northwestern India in July 2002 as well as May-June 2003. Subsequently, the anomalous and prolonged dry conditions favored heavy aerosol buildup as indicated by strong positive anomalies (20-80%) of MODIS aerosol optical depth (AOD) as well as significant increase in TOMS aerosol index (AI) during July 2002 and May-June 2003 compared to the long-term monthly means of 2000-2009 and 1979-2005, respectively. The largest increase in aerosol loading is observed over northern India, encompassing the Indo-Gangetic Plains (IGP) that is in the downwind region of dust outflow from the Thar Desert and long-range transport from Arabia and Middle East. Thus, the deficit of precipitation caused an unusually heavy aerosol loading period that persisted well into July, i.e., during the active period of the summer monsoon season over northern India. Ground-based sunphotometer observations at Delhi and Kanpur also show enhanced presence of desert-dust aerosols during July 2002 and May-June 2003, characterized by large AOD and significantly low Angstrom Exponent. In addition, modifications in columnar aerosol size distribution towards larger coarse-mode fraction and higher single scattering albedo at longer wavelengths were observed, thus supporting the observation of enhanced dust influx. SPRINTARS model simulations also show the enhanced dust loading over northern India during July 2002 and May-June 2003, which is in general agreement with the satellite and ground-based observations. Detailed analysis presented here suggests a cause-and-effect association between the deficit of monsoon rainfall, and increased dust activity as well as prolonged aerosol lifetime that influences the dynamics and persistence of the spatio-temporal aerosol loading, and associated optical properties over northern India.






























Forecasting the discomfort levels within the greater Athens area, Greece using artificial neural networks and multiple criteria analysis.

In this work Artificial Neural Networks (ANNs) were developed and applied in order to forecast the discomfort levels due to the combination of high temperature and air humidity, during the hot season of the year, in eight different regions within the Greater Athens Area (GAA), Greece. For the selection of ANNs-forecasting models best type and architecture the Multiple Criteria Analysis (MCA) technique was applied. Three different types of ANNs were developed and tested with the MCA method. Concretely, the Multilayer Perceptron (MLP), the Generalized Feed Forward Networks (GFFN) and the Time-Lag Recurrent Networks (TLRN) were developed and tested. Results showed that the best ANNs type performance was achieved by using the GFNN model for the prediction of discomfort levels due to high temperature and air humidity within GAA. For the evaluation of the constructed ANNs appropriate statistical indices were used. The analysis proved that the forecasting ability of the developed ANNs models is very satisfactory, at a significant statistical level of p<0.01






























Large scale atmospheric circulation favoring deep and intermediate water formation in the Mediterranean Sea.

Atmospheric circulation patterns that are conducive to extreme ocean heat loss are investigated at four sites of special interest in the Mediterranean Sea. The Gulf of Lions, the South Adriatic Sea, the Cretan Sea, and the Levantine Sea are all areas, where anomalously high winter heat loss may lead to deep or intermediate water formation. At each of the above sites, the atmospheric circulation during such events is derived by averaging the sea level pressure (SLP) fields during the lower decile of the winter time series of the net heat exchange. A relatively simple SLP pattern dominated by an anticyclone over north-west Europe with a weaker cyclone to the south-east is found to be associated with strong heat loss in the selected sites with minor variations in pattern structure depending on the site. The SLP composite pattern reflects the combined effect of different atmospheric modes of variability and we consider the impacts on heat loss of a number of these modes (NAO, EA, EAWR, SCAN), together with the NCP and MI indices. The extremes in heat loss are strongly connected with the intensity and the positions of the poles of these patterns which modulate, through the necessary SLP gradient and associated northerlies, the transfer of cold and dry air masses over the areas of dense water formation. Analysis of air-sea temperature difference, specific humidity and evaporation anomalies, indicates that the extremes of the net heat fluxes are primarily due to the latent and sensible heat flux components.






























Intra-annual variation of atmospheric static stability in the Mediterranean region: a 60-year climatology.

The seasonal characteristics of atmospheric static stability in the Mediterranean region are examined, for the 60-year period 1948-2007 and for the four 15-year sub-periods 1948-1962, 1963-1977, 1978-1992 and 1993-2007. S-Mode and T-Mode Factor Analysis are applied to the mean 5-day values of K static stability index over the Mediterranean region. Three dominant modes are revealed for both, the intra-annual variation and the spatial distribution of K-index. It is found that these modes are connected to the seasonal characteristics of the main atmospheric circulation systems affecting the region and the thermal properties of the Earth's surface (land or sea). The differences among the results of the four sub-periods partially reflect the inter-decadal variations of the strength of the above factors.






























Verification of precipitation forecasts of MM5 model over Epirus, NW Greece, for various convective parameterization schemes.

The mesoscale meteorological model MM5 is applied to 22 selected days with intense precipitation in the region of Epirus, NW Greece. At first, it is investigated whether and to what extend an increased horizontal resolution (from 8 to 2km) improves the quantitative precipitation forecasts. The model skill is examined for the 12-hour accumulated precipitation recorded at 14 meteorological stations located in Epirus and by using categorical and descriptive statistics. Then, the precipitation forecast skill for the 2km grid is studied: a) without and b) with the activation of a convective parameterization scheme. From the above study, the necessity of the use of a scheme at the 2km grid is assessed. Furthermore, three different convective parameterization schemes are compared: a) Betts-Miller, b) Grell and c) Kain-Fritsch-2 in order to reveal the scheme resulting in the best precipitation forecast skill in Epirus. Kain-Fritsch-2 and Grell give better results with the latter being the best for the high precipitation events.






























Sea-level pressure-air temperature teleconnections during northern hemisphere winter.

The relationship between sea-level pressure (SLP) and 1,000 hPa air temperature (AT) in winter is investigated over the northern hemisphere, and a statistical forecasting of one of the two parameters from the other is attempted on a monthly basis. Mean monthly SLP and 1,000 hPa level AT values at 563 grid points over the northern hemisphere are utilized for January, February and March, for the period 1949-2002. At first, factor analysis is applied to the data sets as a dimensionality reduction tool. Then, canonical correlation analysis is applied to the resultant factor scores time series for the five SLP-AT pairs: SLP(J)-AT(J), SLP(J)-AT(F), SLP(J)-AT(M), AT(J)-SLP(F) and AT(J)-SLP(M), and the synchronous and time-lag connections between the two parameters are investigated. The areas characterized by a satisfactory monthly or/and bimonthly forecasting ability are detected. The most satisfactory results refer to the areas affected by the Southern Oscillation. It is found that the SLP teleconnection between the areas of the eastern and the western Pacific in January is related to the regime of AT in the central Pacific Ocean, in both February and March. Also, SLP over the Aleutian and Icelandic lows in January is related to AT over their southwestern and southeastern neighbouring areas in February and March. Finally, it appears that there is also ability for monthly/bimonthly statistical prediction for some areas affected by the well-known oscillations of North Atlantic Oscillation and Pacific/North American Oscillation. A validation of the statistical prediction methodology is carried out, using realtime series of AT and SLP parameters for some characteristic cases. The results show that the statistical prediction presents a remarkable success. The success rate varies from 67% to 83% for the analysis period 1949-2002 and from 71% to 86% for the recent period 2003-2009.






























Synoptic conditions favouring the occurrence of aerosol episodes over the broader Mediterranean basin.

In the present study, the synoptic conditions that favour the occurrence of aerosol episodes over the broader Mediterranean basin are investigated. For this purpose daily satellite measurements of aerosol optical depth (AOD) at 550nm from MODIS-Terra (Collection 5) for the period from 1 March 2000 to 28 February 2007 were used together with mean sea level pressure (MSLP) and geopotential height at 700 hPa obtained from the NCEP/NCAR Reanalysis Project. An objective and dynamic algorithm was developed in order to determine strong and extreme aerosol episodes in the study area. According to the applied methodology and the relevant criteria 322 days with aerosol episodes have been identified. The atmospheric circulation of each day was objectively classified by the implementation of Factor Analysis and Cluster Analysis, resulting in 8 representative synoptic conditions (clusters). The aerosol episodes days (AEDs) are most frequently observed during the dry period of the year, and especially in July-August (108 days or 33.5%) and April-May (106 days or 33%) and give rise to desert dust, anthropogenic pollution and biomass burning episodes. The most frequent synoptic conditions are those of Clusters 4 (30.1%) and 5 (21%), inducing episodes that occur primarily in the western and secondarily in the central Mediterranean. In the former case, which is observed in summer, Europe is under the influence of the extended subtropical anticyclone of the Atlantic (Azores), while the eastern Mediterranean is under the influence of the extended thermal low of south-west Asia. In the latter case, in spring and summer, anticyclonic conditions prevail over central Europe and over the east Atlantic Ocean, and low pressures at the northwestern parts of the broader Mediterranean basin and across the Sahara. During the aerosol episode days of the various clusters, the mean regional AOD can reach values up to 0.8±0.2 and 2.3±1.3 for strong and extreme episodes, respectively.






























Factors regulating the air-sea heat fluxes regime over the Aegean Sea.

We examine the impact of low frequency atmospheric forcings on the air-sea heat fluxes over the Aegean Sea. The correlation between the air-sea heat flux components and three established (NAO, EAWR and NCP) and two testing climatic indices of potential effect over the Eastern Mediterranean Sea region underlines significant discrepancies between the radiative (shortwave and longwave radiation) and the turbulent (sensible and latent heat) components. The NAO index affects the air-sea heat fluxes over the Aegean Sea region much less than the two innovative indices, the "Mediterranean Index", and the "Eastern Europe Index", which play more effective roles. Moreover, the influence of the sea level atmospheric pressure (SLP) variability over an extended area (Europe and North Africa) on surface fluxes regime is investigated. The SLP anomalies are corroborated as a prominent regulating factor of the air-sea heat fluxes over the Aegean Sea region, especially during the cold season of the year. The analysis of the extreme values in the heat exchange anomalies for the period 1958-2001 highlights the role of SLP field on determining the air-sea heat fluxes regime mainly during winter, when, occasionally, large amounts of heat loss from the sea surface trigger the mechanism of intermediate and deep water formation. It is suggested that wind regime and turbulent components are the modulators of the net air-sea heat flux anomalies throughout the year.






























On the atmospheric circulation characteristics associated with fog in Ioannina, north-western Greece.

The evolution of the atmospheric circulation over Europe and the Mediterranean during fog events in Ioannina, north-western Greece, is examined for the period 1957-2002. A 24-hour period starting at 12:00UTC is defined as a fog day, when at least one of the present and/or past weather codes of the corresponding SYNOPs refers to fog. A sequence of such fog days is defined as a fog event. In total, 1019 fog events are found during the period under study. A methodology scheme using S-Mode Factor Analysis and K-Means Cluster Analysis is applied to the patterns of 850hPa air temperature and 1000hPa and 500hPa geopotential heights for the day before the initiation, the first day of occurrence and the day after the dissipation of fog, resulting in the classification of the 1019 fog events into 10 clusters. For each one of the 10 clusters, the mean 00:00UTC patterns of the above parameters are constructed for the three days. These patterns correspond to the evolution of the atmospheric circulation associated with fog formation and dissipation in the Ioannina region. Most of the patterns are characterized by anticyclonic activity or a weak southerly flow over Greece. The differences among the 10 clusters refer mainly to the position and the intensity of the surface and upper air synoptic systems involved.






























Correlation between air-sea heat fluxes over the Aegean Sea and the total precipitable water over Europe and North Africa.

The relation between the air-sea heat fluxes in the Aegean Sea and the total precipitable water (TPW) over Europe and North Africa is investigated. A one-point linear correlation between the components of surface heat exchange and TPW is employed. During winter and for the shortwave radiation, a dipole of opposite correlation (see-saw teleconnection), is observed between the northeast Europe and the East Mediterranean Sea. This pattern is inverted for the longwave radiation and especially during the summer is limited to a strong positive south pole. Both spatial correlation patterns underline the influence of cloudiness during the winter season and the specific humidity during summer on the radiative fluxes behaviour. Regarding the turbulent air-sea heat flux components - latent and sensible heat - the correlation pattern is enhanced. The winter pattern exhibits positive correlation over Europe with the highest values (r > 0.75) over the Balkan Peninsula. The identified correlation patterns, although they remain dominant, weaken during the summer. The herein findings suggest that the stronger relation between the TPW and the turbulent fluxes is regulated by the wind regime.






























The RISKMED project: philosophy, methods and products.

This paper presents RISKMED, a project targeted to create an Early Warning System (EWS) in case of severe or extreme weather events in the central and eastern Mediterranean and specifically in southern Italy, northwestern Greece, Malta and Cyprus. As severe or extreme weather events are considered, cases when the values of some meteorological parameters (temperature, wind, precipitation) exceed certain thresholds, and/or a severe weather phenomenon (thunderstorm, snowfall) occurs. For an accurate weather forecast, selected meteorological models have been operated daily, based on a nesting strategy using two or three domains, providing detailed forecasts over the above mentioned areas. The forecast results are further exploited for the evaluation and prediction of human discomfort and fire weather indices. Finally, sea wave models have also been operating daily over the central and eastern Mediterranean Sea. In case a severe or extreme weather event is forecasted within the next 48 or 72 h for selected target areas (sub-regions defined by their morphological and population characteristics), the local authorities and the public are informed via a user-friendly graphic system, the so-called RISK MAP. On the web page of the Project (http://www.riskmed.net), additional information is provided about the real-time values of some meteorological parameters, the latest satellite picture and the time and space distribution of lightning during the last 24 h. The RISKMED project was financed by the EU and the Ministries of National Economy of Greece, Italy, Malta and Cyprus, in the frame of INTERREG IIIB/ARCHIMED programme.






























The interaction of northern wind flow with the complex topography of Crete Island - Part 2: Numerical study.

During the summer months, when northerly winds are blowing over the Aegean Sea the island of Crete modifies significantly the regional airflow as well as the pressure and temperature fields due to its complex topography. One of the major topographical elements of Crete Island is the major gap which is located between the two highest mountains Lefka Ori and Idi. On 24-25 August 2007 strong northerly winds, with gusts up to 25m/s, occurred at the exit of the major gap. In order to investigate the dynamics as well as the role of this elevated and sloping gap on the airflow modification, the event was simulated down to 1 km horizontal resolution using the non-hydrostatic model MM5. The model simulations show that the localized intensification of the flow downstream of the major gap is related to the channeling of the flow through the gap. The strongest winds are observed at the gap exit region, implying that the main cause of the strong winds is the pressure different between the gap entrance and exit, when the relatively cooler maritime air approached the island and dammed up the high mountains. Finally, sensitivity experiments with modified topography further supported the important role of the topography of the elevated gap, which reveals that the strong winds have aspects of both gap and downslope contributions over the gap exit






























Weather forecast in north-western Greece: RISKMED warnings and verification of MM5 model.

The meteorological model MM5 is applied operationally for the area of north-western Greece for one-year period (1 June 2007 to 31 May 2008). The model output is used for daily weather forecasting over the area. An early warning system is developed, by dividing the study area in 16 sub-regions and defining specific thresholds for issuing alerts for adverse weather phenomena. The verification of the model is carried out by comparing the model results with observations from three automatic meteorological stations. For air temperature and wind speed, correlation coefficients and biases are calculated, revealing that there is a significant overestimation of the early morning air temperature. For precipitation amount, yes/no contingency tables are constructed for 4 specific thresholds and some categorical statistics are applied, showing that the prediction of precipitation in the area under study is generally satisfactory. Finally, the thunderstorm warnings issued by the system are verified against the observed lightning activity.






























The interaction of northern wind flow with the complex topography of Crete Island - Part 1: Observational study.

The island of Crete with its mountain ranges is an excellent example of a major isolated topographic feature, which significantly modifies the regional airflow as well as the pressure and temperature fields. During summer, when northerly winds are blowing over the Aegean Sea (a large number of which are characterized as Etesians), the highly complex topography of Crete plays an important role in the modification of this northern wind flow. The main objective of this study is to determine the role of the topography of Crete Island during this wind flow on the strong downslope winds at the southern parts of the island as well as on the development of a gap flow between the two highest mountains of the island (Lefka Ori and Idi). For that purpose, observational data from four meteorological stations located along the aforementioned gap are used along with QuikSCAT satellite data. The observational analysis shows that the interaction of the northern wind flow with the mountains of Crete Island produces an upstream deceleration, a leftward deflection of the air as this approaches the mountains and an intensification of the winds at the southern coasts accompanied with a temperature increase. Furthermore, the maximum of the gap flow is observed at the exit region of the gap.































The main characteristics of atmospheric circulation associated with fog in Greece.

The characteristics of the atmospheric circulation over Europe and the Mediterranean associated with the formation and the dissipation of fog in Greece are examined. The data used consists of: i) 3-hourly meteorological observations recorded at 16 meteorological stations in Greece and ii) daily (00:00 UTC) 2.5x2.5 grid point values of mean sea-level pressure, 500hPa geopotential height, 850hPa and 500hPa air temperatures and 1000-500hPa thickness over Europe for the period 1957-2002. 1055 fog events are extracted from the 3-hourly meteorological observations. A specific methodology scheme including S-mode Factor Analysis and k-means Cluster Analysis is applied to the grid point data sets for the first day of a fog event (D day), the day prior to D day (D-1 day) and the day that follows the last day of a fog event (END day) and the 1055 evolutions of the atmospheric circulation associated with fog events in Greece are classified into 10 clusters. The mean patterns of MSL Pressure, 850hPa and 5 00hPa air temperatures, 1000-500hPa thickness and 500hPa geopotential height show that in most of the clusters, the presence of anticyclonic conditions over the Balkans, a warm front passage, or a weak, humid southerly flow induced by the presence of a shallow depression over the western Mediterranean favor fog formation in Greece, while the dissipation of fog occurs when drier air masses are transferred over the Balkans. The main differences among the 10 clusters refer to the exact position, the intensity and the specific evolution of the surface and the upper air systems, the season of their predominance and the area of the Greek territory that mainly refer to.































Winter air temperature covariability between the lower troposphere and higher tropospheric layers; the case of the North Atlantic Oscillation.

In the present work, the spatial and temporal air temperature covariability between the lower Troposphere and higher tropospheric layers is investigated for the winter (D-J-F) and for the 57-year period 1949-2006. The database consists of mean seasonal thickness values of the layers 1000-850hPa, 850-700hPa, 700-500hPa and 500-300hPa, over the whole northern hemisphere. The aim of this study is to examine the cases of air temperature covariance between the layers: i) 1000-850hPa and 850-700hPa, ii) 1000-850hPa and 700-500hPa and iii) 1000-850hPa and 500-300hPa. The similarities and/or the differences found among the various comparisons, reveal the degree of vertical homogeneity of the troposphere as far as it concerns air temperature variability. Furthermore, the vertical extend and intensity of some teleconnection patterns, as for example the one attributed to the North Atlantic Oscillation, are also examined. At first, S-mode Factor Analysis is applied separately for each layer in order to reduce the number of the initial variables (grid points), and define sub-areas within each layer comprising grid points with common long-term air temperature variability. Then, Canonical Correlation Analysis is applied on the Factor Analysis results for each of the above three pairs of layers revealing areas in both layers presenting common temperature variations. In order to achieve an interpretation of the resultant canonical variates, each of them is correlated with all the original time series of the corresponding layer and then, for each canonical pair, the correlation coefficients are plotted on two maps. By comparing the two maps of each canonical pair, it is possible to elucidate the similarities as well as the differences in temperature variability between the two layers. The application of the two statistical methods revealed covariability cases that refer mainly to areas that are influenced by already known atmospheric teleconnections such as the North Atlantic Oscillation, the Pacific North American Pattern, the West Pacific Pattern etc. The present work focuses on the area affected by the North Atlantic Oscillation, where a case of temperature covariability appears in all pairs of layers. It concerns a temperature see-saw teleconnection between northern Europe and, with an opposite sign, both the Labrador Sea and West Asia, implying that temperature changes over these areas are vertically spread. This see-saw teleconnection pattern is directly related/attributed to the well known large-scale pressure teleconnection called North Atlantic Oscillation (NAO). This air temperature teleconnection is detected without major differences at the lower two pairs of layers: 1000-850hPa / 850-700hPa and 1000-850hPa / 700-500hPA, but it is much weaker between the layers 1000-850hPa / 500-300hPa where the role of the equatorial warming appears more important. The time series of the canonical variates (in fact, temperature time series) indicate a statistically significant positive trend over northern Europe for all layers and negative trends for the other areas. The variability over northern Europe is in accordance with the one of the NAO index and the general temperature increase recorded on the ground during the last decades.































Numerical study of a downslope windstorm in Northwestern Greece.

This paper describes the leeside wind storm of 25-26 March 1998, the most intense wind storm of the last decade in Northwestern Greece. This wind storm produced wind gusts of ~30 m s-1 that resulted in tree uprooting, roof damaging, electric power network disruption and flooding in the lake-side areas of Ioannina city in Northwestern Greece. With the aim to identify the role of Mountain Mitsikeli near the city of Ioannina on the windstorm and to investigate the physical mechanisms responsible for such orographically induced weather events, numerical simulations with MM5 model have been performed. The model results showed that a resolution of 2-km resolution is necessary in order to reproduce the localized character of the wind storm. The analysis revealed that a synergistic combination of the cross-barrier northeasterly flow, the stable layer above the mountain top and the presence of a critical level, led to the intensification of the lee side winds during the studied wind event. Sensitivity experiments with modified topography, further supported the important role of mountain Mitsikeli that stands as an isolated obstacle, on the modification of the wind field during the observed windstorm.































Regional Covariability and Teleconnection Patterns in Surface Solar Radiation on a Planetary Scale.

The spatial and temporal distribution of the downward solar radiation (DSR) at the Earth's surface, which is a key parameter for the Earth-atmosphere climate system, is studied on global scale by applying the multivariate statistical method of Factor Analysis (FA) on mean monthly DSR values for the period 1984-2000 for winter and summer. DSR fluxes have been computed with a physical deterministic radiation transfer model, which uses satellite and reanalysis climatological input data. FA objectively groups grid points with common temporal variability of solar radiation, identifying areas with characteristic solar radiation variability and revealing teleconnection patterns. The globally distributed DSR exhibits strong variability and can be represented by about 30 factors (sub-areas) explaining approximately 85% of the total variance. The main factors are located in the tropical Pacific (El Nino - Southern Oscillation), the northern Pacific (Aleutian low), desert areas (Africa, Middle East, Australia), and oceanic areas (storm track zone around 60oS). Furthermore, some of these areas are teleconnected indicating common DSR variability in remote places of the Earth. The primary physical parameter for the covariability of surface solar radiation is found to be cloud cover. The time series of the factor scores (solar radiation) exhibit features associated with natural climatic phenomena (e.g. ÍÁĎ, ENSO) revealing thus that DSR might be considered as an indirect indicator for these phenomena. On the other hand DSR determines these phenomena, through its effects on major climate parameters such as surface temperature or evaporation, playing thus a key role for their formation and explanation. Finally, our analysis revealed some statistically significant trends of solar radiation, for example a reduction in the northern Pacific and an increase in tropical and subtropical regions, which are important for global dimming/brightening and associated surface warming of the Earth.
































Daylight climatology in Athens Greece: types of diurnal variation of illuminance levels.

In the present work, the diurnal variation of illuminance levels in Athens, Greece, is studied during winter and summer. The database consists of 5-minute values of global horizontal illuminance for the period 1992-1996 (National Observatory of Athens - NOA). For each season, two sets of data are determined (sunrise-noon, noon-sunset) consisting of the semi-diurnal courses of illuminance anomalies from the mean semi-diurnal course of each day of the year. By using the multivariate statistical methods Factor Analysis and Cluster Analysis, the identification of the most characteristic modes of diurnal variation of global illuminance is attempted. It is found that, 6 types of semi-diurnal courses (sunrise-noon, noon-sunset) of global illuminance are the most characteristic for winter, while for summer, 6 types of courses are found for sunrise-noon and 5 for noon-sunset. Some of these courses present the almost smooth ascending / descending intra-daily variation of illuminance, while others show significant disturbances during various daytime periods, due to development of clouds.
































Comparison of objective air-mass types and the Peczely weather types and their ability to classify levels of air pollutants in Szeged, Hungary.

This paper compares the efficiency of a system of objective air-mass types and the Peczely's weather types in classifying pollution levels over the Carpathian Basin for the winter and summer months. Based on the ECMWF data set, daily sea-level pressure fields analysed at 00 UTC were related to the levels of air pollutants for both the objective air-mass types and the Peczely-types in Szeged. The data base comprises daily values of 12 meteorological and eight pollutant parameters for the period 1997-2001. Mean sea-level pressure fields of the Peczely-types show higher independence from each other than those of the objective clusters in both seasons. In the winter months, anticyclonic types are mostly favourable, while cyclonic ones are mostly negligible in classification of pollutant levels both for the objective and the Peczely-types. In the summer months, neither the objective nor the Peczely classifications are effective in categorisation of pollutant concentrations.
































Atmospheric circulation patterns associated with extreme precipitation amounts in Greece.

The main synoptic conditions associated with extreme precipitation amounts in Greece are examined by using a multivariate statistical methodology comprising S-mode Factor Analysis and k-means Cluster Analysis. The following data were used : i) daily precipitation amounts (measured at 06:00 UTC) for the meteorological stations of Hellenikon (Athens), Thessaloniki (northern Greece) and Ioannina (western Greece) and ii) daily (18:00 UTC) 2.5x2.5 grid point values of 500hPa geopotential height, mean sealevel pressure and 1000-500 hPa thickness at 273 grid points over Europe (10W to 40 E and 30N to 60 N), for the period 1970-2002. The dates corresponding to the upper 5% of the frequency distribution of precipitation are selected for each one of the three stations. In total, 369 dates are used, some of them being common among the three stations. The corresponding 369x273 data matrices of 500 hPa geopotential height, mean sea-level pressure and 1000-500 hPa thickness are constructed. The rows refer to the 369 extreme precipitation cases and the columns refer to the 273 grid points. The three matrices are unified into one 369x819 matrix. In order to reduce the dimensionality of the data set, S-mode Factor Analysis is applied to the unified matrix, revealing 7 factors accounting for 85% of the total variance. Finally, k-means Cluster Analysis is applied to the factor scores matrix, classifying the 369 cases into 9 clusters. For each one of the 9 clusters, the mean 18:00 UTC patterns of the above parameters are constructed and presented. These patterns correspond to the main distinct atmospheric circulation structures favoring extreme precipitation amounts in Greece. Most of the patterns are characterized by enhanced cyclonic activity over or near the Greek area. The differences among the 9 circulation structures refer mainly to the position and the intensity of the surface and the upper air synoptic systems involved. Some of the 9 synoptic conditions favor extreme precipitation amounts mainly at one or two of the three stations while some others equally affect the three stations.
































Relationship between the groups of Peczely's large-scale weather types and air pollution levels in Szeged, Hungary.

This paper investigates the relationship between the 6 groups of the 13 Peczely's large-scale weather situations over the Carpathian Basin and the levels of the main air pollutants for the winter (DJF) and summer (JJA) months, in Szeged, Hungary. Based on the ECMWF data set, mean daily sea-level pressure fields (00 UTC) are prepared from all the days belonging to each of the 6 groups of Peczely's weather types for the period 1997-2001. Then, by using daily values of twelve meteorological and eight pollutant parameters for the same period and an appropriate statistical approach, it is possible to relate the 6 sea-level pressure patterns with the levels of air pollutants in Szeged. It is found that group V (an anticyclone is located over Hungary) is beneficial for high pollutant concentrations, while group VI (cyclonic flow over Hungary) is related to moderate or low pollutant concentrations in winter. On the other hand, the 6 groups of the Peczely's macro-synoptic types play no important role in the classification of pollutant levels in summer. Hence, the groups of the Peczely's large-scale weather situations cannot be considered as a fully suitable system for categorization of pollutant concentrations. Although, they have an emphasized role in winter, they are inefficient in summer.
































Contribution to the seasonal air temperature forecast in the Northern Hemisphere; a statistical approach .

A seasonal air temperature forecasting is attempted by using statistical methods. The data basis consists of seasonal surface air temperature values at 514 grid points of the northern hemisphere, for the period 1948-2006. At first, grid points with covariant seasonal air temperatures for the period 1948-1996 are objectively grouped, by using Factor Analysis. Then, Canonical Correlation Analysis is applied on the time series of factor scores for each of the 4 pairs of sequential seasons as well as for the 4 pairs of 'crosswise' seasons. The results show that the number of the statistically significant pairs of canonical variates (Wi, Vi) ranges between 4 and 9 and the correlation coefficients between the canonical variates are higher than 0.92. Then, for every analysis, the Wi time series is correlated to the air temperature ones of the predictor season and the Vi time series to the air temperature ones of the predictant season for all the grid points. By plotting the correlation coefficients on maps, the isopleths indicate the areas where seasonal air temperature can be forecasted. The best results (r>0.70) are found for three low latitude areas, where persistence prevails: a) autumn-winter: western Indian Ocean - central and eastern Indian Ocean, b) autumn-winter: central Pacific - eastern Pacific and c) spring-autumn: eastern Pacific - eastern Pacific. The results in the middle and high latitudes are less significant and practically they cannot be used for a seasonal air temperature forecast. Finally, for the areas characterized by high correlation coefficients between the canonical variates and the temperature time series, a validation process is carried out by comparing the temperature anomalies time series of the corresponding seasons for the period 1997-2006. The results confirm that prediction may be considered practically satisfactory for some low latitude areas of the Indian, the Atlantic and the Pacific Oceans.
































A comparative study on various statistical techniques predicting ozone concentrations: implications to environmental management.

The objective of the present work is to compare various techniques for modeling the dependence of the tropospheric ozone concentrations on several meteorological and pollutant parameters. The study focuses on two different sites in the metropolitan area of Athens, Greece; one in the city centre and another one in the suburbs. It is found that although simple Linear Regression Analysis fails to construct accurate equations due to the existence of multi-collinearity among the independent variables, still various combinations of a Multivariate Method (PCA) and Stepwise Regression Analysis manage to produce equations free of the multicollinearity issue. The derived formulas are validated and prove to have R**2 values in the order of 0.8 approximately. However, the equations are found to be unsuccessful in case of severe episodes. For this reason, a new procedure is followed for estimating the ozone values in case of episodes exclusively. The new R**2 value is estimated to be 0.9, approximately
































On the intra-annual variation of sea surface temperature on global scale.

The intra-annual variation of the sea surface temperature (SST) is studied on a global scale. The study is based upon mean monthly values at 1430 grid points spaced by 5o in latitude and longitude for a 30-year period. The highest mean monthly SST values are recorded in the tropical Oceans where, in some areas, temperature exceeds 30oC, while the lowest are recorded near the Poles, where SST, next to the ice, reaches -1oC. The annual temperature range varies from 2oC in the tropics and near the Polar areas to 15oC or even higher in the middle latitudes of the W Atlantic and the W Pacific. In general, the intra-annual variation of SST is characterized by a simple sinusoidal curve with maximum values in late summer and minimum values in late winter. However, in the tropics, where the conventional seasons do not exist, the intra-annual variation is characterized by two maxima and two minima. The intra-annual variation of SST is analyzed in a sum of harmonic terms by applying Fourier analysis on the 12 mean monthly SST values in every grid point on the Earth. The 1st harmonic term is very strong and describes well the intra-annual variation of SST, explaining more than 80% of the total variance, almost in all areas outside the tropics. Its maximum contribution appears in middle latitudes where, in some cases, it reaches 95%, while the minimum appears in the tropics (lower than 70%). Its amplitude, C1, varies from 1oC in the tropics and the very high latitudes, up to 8oC in the middle latitudes of the W Atlantic and the W Pacific where strong sea currents prevail. Its maximum, T1, in general, occurs in late summer or early autumn; in the northern hemisphere in August-September while in the southern hemisphere in February-March. However, in the tropics, where the variation is not simple, the maximum occurs in almost every month with most dominant the period April-June. The 2nd harmonic is weaker and it hardly explains more than 15% of the total variance. It appears stronger in the tropics and in the Arabian Sea, where its percentage exceeds 30%, but even there, in most cases, it is still weaker than the 1st one. Its amplitude, C2, is lower than 1oC everywhere except in the middle latitudes of the W Atlantic and the W Pacific where, however, C1, is also strong and thus its contribution to the formation of the intra-annual variation of SST is negligible. The only exception to this is the Arabian Sea where C1 is 2oC and thus the two harmonic waves are comparable. The 1st of its maxima, outside the tropics, occurs in late summer - early autumn and the 2nd in late winter - early spring, while in the tropics, its two maxima appear in May and in November. Although the harmonic analysis does not always yield to physically interpretable results, in the case of the intra-annual variation of SST, it appears that the incoming solar radiation, the position of the intertropical convergence zone and the prevailing sea currents are responsible for the formation of its two main harmonic components.
































On the intra-annual variability of atmospheric circulation in the Mediterranean region.

Some aspects on the intra-annual variability and the seasonal peculiarities of the atmospheric circulation in the Mediterranean region are examined. T-mode Factor Analysis is applied on the long-term (1948-2003) 5-day mean values of relative geostrophic vorticity for the 73 5-day periods of the year. The analysis is applied for sea-surface and 500hPa levels. For sea level, two circulation types are found to be dominant: the type of the warm period and the type of the cold period, characterizing the so-called Mediterranean climate; they are strongly depended on the thermal and humidity characteristics of the earth's surface and the position and strength of the subtropical (Azores) anticyclone. For 500hPa level, two extra but weaker circulation types are added, prevailing mainly during the transitional seasons and modifying the smooth intra-annual variation of the atmospheric circulation. In this case, the thermal influence of the earth's surface appears to be weaker, while other circulation features, such as blocking activity and/or cut-off lows, seem to play an important role too.
































The atmospheric conditions over Europe and the Mediterranean, favoring snow events in Athens, Greece.

The 3-dimensional structure and the evolution of atmospheric circulation favoring snowfall in Athens are examined. The study refers to 61 snow events, which occurred during the period 1958-2001. For each one of the events, the patterns of MSL pressure, 850hPa and 500hPa air temperatures, 500hPa geopotential height and 1000-500hPa thickness are constructed for the European region, for the day before the event (D-1), the first day of the event (D) and the day after the end of the event (END). A statistical methodology involving Factor Analysis and Cluster Analysis is applied to the above data sets and the 61 cases are finally classified into five clusters. These clusters are generally characterized by a north-easterly flow in the lower troposphere over the Athens area. This flow is associated with the presence of a low pressure system around Cyprus and an anticyclone over Europe. The position, the intensity and the trajectories of the surface and the upper air systems during D-1, D and END days are generally different among the five clusters.
































Relationship between the Peczely's large-scale weather types and air pollution levels in Szeged, southern Hungary.

This paper determines the 13 Peczely's large-scale weather situations over the Carpathian Basin with the levels of the main air pollutants for the winter and summer months. Based on the ECMWF data set, daily sea-level pressure fields analysed at 00 UTC were prepared for each Peczely-type in order to relate their sea-level pressure patterns to the levels of air pollutants in Szeged. The database comprises daily values of 12 meteorological and 8 pollutant parameters for the period 1997-2001. Concentration of air pollutants occured exclusively during anticyclonic conditions, or anticyclone-ridge situations. Their dispersion can be experienced not only during cyclonic, but also anticyclone-ridge weather types. CO, SO2 and TSP were sensitive to Peczely's weather classification, while NO2/NO, O3 and O3max were completely insensitive. After all, anticyclonic weather types are determinant in winter, while their role in classifying concentrations of the air pollutants is less predominant during summer.
































Generation of daylight reference years for two European cities with different climate: Athens, Greece and Bratislava, Slovakia.

In this work, daylight reference years (DRYs), based on daylight and solar radiation measurements, are designed for two European cities, Athens, Greece and Bratislava, Slovakia, by using the Danish method, the Festa-Ratto technique and the Modified Sandia National Laboratories methodology. The data basis consists of 5-minute values of global and diffuse horizontal illuminance, global and diffuse horizontal irradiance, zenith luminance and solar altitude as well as of daily values of sunshine duration for 5 years. Moreover, Linke's turbidity factor, luminous turbidity factor and relative sunshine duration are calculated and utilized. Then, for each DRY, the predominant sky luminance distributions over Athens and Bratislava are identified, by using the methodology of Kittler et al., who corresponded the main sky conditions to 15 theoretical sky standards in diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. For both cities the three aforementioned methods do not create identical DRYs. Despite the differences, the sky types defined for each of the two places seem not to depend on the choice of DRY. The predominant sky standard, for all of them, is a cloudless, polluted sky with a broad solar corona for Athens and an overcast sky with slight brightening towards the sun as well as very clear sky with low atmospheric turbidity for Bratislava. However, the selection of the DRY, which represents best the daylight conditions, is necessary for studies in saving energy in buildings. The DRY, which is created by the Modified Sandia National Laboratories method, is chosen for most cases, while the one created by the Danish method is also useful on certain occasions.
































A new statistical methodology for classification of sky luminance distributions based on scan data.

The sky luminance distribution in Garston, south England, is studied using the multivariate statistical methods of Factor Analysis and Cluster Analysis. The database consists of sky luminance scan data for the year 1992. The whole methodology is applied for 5 solar altitude intervals (6o-18o, 18o-30o, ., 54o-66o). At first, Factor Analysis defines the main categories of sky conditions (overcast, partly cloudy, clear) and then Cluster analysis gives a more detailed grouping of the available scans. For the final classification of the sky luminance distributions, the Cluster Analysis results along with the functions of "relative gradation" and "relative scattering indicatrix", which have been introduced by Kittler et al., are taken into account. For an assessment of the methodology proposed, the results have been compared to the ones found by using the Standard Sky Luminance Distribution method (Kittler et al.). For solar altitudes up to 42o the differences in the frequency of occurrences, for the three main categories, is less than 10%. The statistical approach used in this paper is recommended as an additional means to "standard" methods for classification of sky luminance conditions, when scan measurements are available.
































Comparison of various methods for modelling the dependence of Ozone concentrations on meteorological and pollutant parameters in Athens.

The present study compares various methods for modeling the dependence of the surface ozone concentration on several meteorological and pollutant parameters in two different sites in the metropolitan area of Athens, Greece. Although linear regression models fail to produce accurate equations due to multicollinearity, still combinations of principal component and stepwise regression analyses manage to produce prediction models free of the multicollinearity issue.
































Relationship between the Peczely's large-scale weather types and airborne pollen grain concentrations for Szeged, Hungary.

This paper discusses a subjectively defined system of air mass types, the thirteen Peczely's large-scale weather situations over the Carpathian Basin in relation to the detected airborne pollen grain concentrations. Based on the ECMWF (European Centre for Medium-Range Weather Forecasts) sea-level pressure data set, daily sea-level pressure fields analyzed at 00:00 UTC were prepared for each Peczely's weather type in order to relate the sea-level pressure patterns with the average pollen levels in Szeged. The data basis for this study comprises daily values of twelve meteorological parameters and daily average pollen concentrations for twenty four species in a five-year period (1997-2001). It was found that Peczely's anticyclonic ridge types 2 and 11 as well as cyclonic types 4 and 7 are favourable for pollen production and dispersal unlike the cyclonic types 3 and 13. Hence, the Peczely's large-scale weather situations cannot alone be considered as an overall system in predicting pollen concentrations.
































Extreme precipitation events in NW Greece.

In this work, the extreme precipitation events in NW Greece are studied. The data used are daily precipitation totals recorded at the meteorological station of Ioannina University for the period 1970-2002. 156 days with precipitation totals above 35mm (5% upper limit) are only considered. It is seen that, a minimum frequency of extreme precipitation events appears in the period 1986-1991, which is characterized by a high positive NAO index. For each of the 156 extreme precipitation days, at first, the mean sea level pressure pattern over Europe is constructed by using 273 grid point values. Using Factor Analysis, the dimensionality of the 156x273 data matrix is reduced to 156x5 (84% of the total variance) and then, Cluster Analysis is applied on the results of Factor Analysis. Thus, the 156 cases are grouped objectively to 11 clusters, revealing the main pressure patterns, which favour extreme precipitation in NW Greece. Seven of the patterns are encountered in winter and autumn, while three of them cover a period from autumn to spring and one appears mainly in summer. In all of them the cause of the extreme precipitation event is a low pressure system centred west of Greece or a low pressure trough extended eastwards or southwards up to Greece. In some cases the depression is so strong and extended that it covers the whole Europe and the Mediterranean. In the single summer pattern, rainfall is caused by an extension of the SW Asia thermal low up to the central Mediterranean.
































An objective classification system of air mass types for Szeged, Hungary with special attention to plants' pollen levels.

This paper discusses the characteristic air mass types over the Carpathian Basin in relation to the plants' pollen levels for the annual pollination period. Based on the ECMWF data set, daily sea-level pressure fields analysed at 00 UTC were prepared for each air mass type (cluster) in order to relate sea-level pressure patterns to the pollen levels in Szeged. The data basis comprises daily values of twelve meteorological parameters and daily pollen concentrations of twenty four species for their pollination periods between 1997 and 2001. The characteristic air mass types were objectively defined via Factor Analysis and Cluster Analysis. According to the results, nine air mass types (clusters) were detected for the pollination period of the year corresponding to the pollen levels that appear with higher concentrations when irradiance is moderate, while wind speed is moderate or high. This is the case, when an anticyclone prevails in the region west of the Carpathian Basin and when Hungary is under the influence of zonal currents (wind speed is high). The sea level pressure systems belonging to low pollen concentrations are mostly similar to those connected to higher pollen concentrations and arise, when wind speed is low or moderate. Low pollen levels occur when an anticyclone prevails the region west of the Carpathian Basin, as well as when an anticyclone covers the region with Hungary in its centre. Hence, anticyclonic or anticyclonic ridge weather situations seem to be relevant in classifying pollen levels.
































A contribution to the predetermination of daylight reference conditions.

The importance of effective utilization of natural resources is evident and has increased after last economy prognosis. As daylight is a free daily available commodity any better solution for its utilisation could help to save and decrease energy consumption in buildings. The time variability and permanent changes of daylight levels are characteristic and have to be considered not only in building design but also in its long-term exploitation. Therefore the determination of reference daylight conditions representing prevailing daylight situations during the whole year is needed. The concept of the predetermination of typical annual profiles of daylighting in exterior, i.e. Daylight Reference Year (DRY) based on the measured data and daily sunshine duration values is presented in this paper.
































Extreme temperature events in NW Greece.

In this work, the extreme temperature events in, NW Greece are studied for winter and summer. The data used are the maximum and minimum daily temperatures recorded at the meteorological station of Ioannina University for the 33-year period 1970-2002. Days with temperature above 35.8 C in summer (JJA) and below -7.6 C for winter (DJF) are only considered. These thresholds are the 95% upper and lower limits respectively (quintiles). It is seen that, in winter, the maximum frequency of minimum temperature values below the above threshold appears in the 5-year period 1989-93, a period characterized by a high positive North Atlantic Oscillation index. In summer, the maximum frequency of maximum temperatures in the upper quintile appears the recent years, in agreement with the general warming trend in eastern Mediterranean. For these extreme temperature days (approximately 150 days for each season), at first, the pressure patterns over Europe are constructed (from 40W to 50E and from 30N to 70N - NCEP-NCAR data). Then, using Factor Analysis, the 150 charts (days) are grouped objectively in order to reveal the main pressure patterns associated with extreme temperature values in NW Greece. In winter, 4 factors were selected, explaining 79% of the total variance. In all of them the main feature of the synoptic conditions is an anticyclone centered either north or northwest or southwest of Greece, and extended over Greece. Thus, clear nights prevail, favouring extremely low minimum temperatures in the morning. In one case, the position of the anticyclone favours a northeasterly flow over NW Greece, which transfers cold air masses from eastern continental Europe. In summer, 5 factors were selected, explaining 81% of the total variance. Here, also, the extremely high maximum temperatures are associated with an anticyclone. The anticyclone is either located over NW Africa (extension of the Azores subtropical anticyclone), transferring warm air masses from the Sahara desert, or it is located over Greece causing sunshine and very low wind or no winds at all, conditions obviously related to heat waves.

































A study on evaporation in Ioannina, NW Greece.

Evaporation in Ioannina, NW Greece is studied for the period 1988-2003. The data basis comes from the readings of the Wild evaporigraph charts of the meteorological station of Ioannina University. The total number of data consists of 70,080 bi-hourly values. The intra-annual variation of evaporation resembles to a simple sinusoidal curve with one maximum (July, 153mm) and one minimum (December, 16mm). The maximum inter-monthly evaporation change is found from August to September (-56mm) and the minimum from December to January (+6mm). The diurnal variation is also simple with a maximum at 14:00-16:00 hour (0.15mm in winter and 0.90mm in summer) and a minimum at 4:00-6:00 hour (0.03mm in winter and 0.05mm in summer). During winter, daily evaporation, in 90% of the cases, is less than 1.80mm. The maximum frequency appears for daily values less than 0.20mm, while the maximum values recorded are around 5mm. In summer, the curve is closer to the Gaussian distribution with maximum frequency of daily evaporation at around 4-6mm and maximum values above 10mm. A long-term study revealed that there is a statistically significant negative trend during winter and autumn. For spring, summer and the year as a whole, trends were not found statistically significant. Examination of the number of days with evaporation above or below specific thresholds showed that in winter and autumn, the number of days with low evaporation increases while that with high evaporation decreases, in agreement with the general winter trend. The lower winter evaporation values during the recent years are related to the increase of precipitation in Ioannina and NW Greece after 1990 due to the increase of cyclonic circulation in C Mediterranean. Finally, the synoptic patterns over Europe and the Mediterranean, associated with cases of extreme evaporation values in NW Greece, are investigated. Factor Analysis was applied on the pressure space-series of the days of the upper decile for winter and lower decile for summer. The approximately 145 days (pressure patterns) of each category were grouped objectively to 4 factors in winter and 5 factors in summer, explaining around 85% of the total variance. In winter, it appears that the wind plays the major role since in the first 3 factors the main feature of the pressure pattern in the area of NW Greece is a dry and katabatic easterly or north-easterly flow. In summer, in the 4 stronger factors, a cyclonic flow appears over the Ionian sea.
































An objective classification system of air mass types for Szeged, Hungary with special interest to air pollution levels.

This paper determines the characteristic air mass types over the Carpathian Basin for the winter (December, January, and February) and summer (June, July and August) months with the levels of the main air pollutants. Based on the ECMWF data set, daily sea-level pressure fields analysed at 00 UTC were prepared for each air mass type (cluster) in order to relate sea-level pressure patterns with the levels of air pollutants in Szeged. The data basis comprises daily values of twelve meteorological and eight pollutant parameters for the period 1997-2001. Objective definition of the characteristic air mass types occurred by using the methods of Factor Analysis and Cluster Analysis. According to the results, during the winter months five air mass types (clusters) were detected corresponding to levels of the primary pollutants that appear with higher concentrations when irradiance is high and wind speed is low. This is the case when an anticyclone is found over the Carpathian Basin and when an anticyclone prevails over the region south of Hungary, influencing the weather of the country. Low levels of pollutants occur when zonal currents exert influence over Hungary. During the summer months, anticyclones and anticyclone ridge situations are found over the Carpathian Basin. As a result of high irradiance and very low NO levels, secondary pollutants are highly enriched.

































Comparison between winter and summer sky-luminance distribution in Central Europe and in the Eastern Mediterranean.

This work studies the illuminance levels at a city in Central Europe (Bratislava) and another in the Eastern Mediterranean (Athens) and defines the prevailing sky-luminance distributions for winter and summer. The data used are five-minute values of global and diffuse horizontal irradiance and illuminance along with zenith luminance from the IDMP stations at the two cities for a period of five years. The study is based on the new set of the sky types recently adopted by CIE. These sky standards correspond to 15 theoretical diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. The theoretical curves converge above 35° of solar elevation and in many cases they intersect each other. For this reason each observation is classified into one of the 15 sky standards, only when the value of the ratio of zenith luminance to diffuse horizontal illuminance lies in a zone of ±2.5% around the theoretical curve describing the specific category. If any observation is classified into more than one sky standards, the corresponding ratio of global horizontal illuminance to extraterrestrial horizontal illuminance is compared to the average ratio of the sky types implicated, for the same solar altitude, and the observation is classified in the sky type with the nearest value. It is found that the most frequent sky types for Bratislava are the overcast sky with steep luminance gradation and slight brightening towards the sun (winter) and the white-blue sky with a distinct solar corona (summer), while for Athens the cloudless polluted sky prevails in both seasons. It is also found that during a summer day, a noticeable shift appears in the sky-luminance distribution at both sites.

































Relation between sensible and latent heat fluxes in the Mediterranean and precipitation in the Greek area during winter.

Sensible and latent heat fluxes from sea surface to the atmosphere are two parameters with an important contribution in the supply of energy to the Mediterranean air masses during the cold period of the year. In this work the relation of these parameters with precipitation in the Greek area is examined for winter (December-January-February) and for the period 1959-1997. The investigation of statistical significant relations between the above parameters is carried out with the use of Factor Analysis (FA) and Canonical Correlation Analysis (CCA). FA was applied separately on the time series of each of the parameters in order to reduce the dimensionality of the original data set. Then, CCA was applied on the FA results for the pairs: precipitation-sensible heat flux and precipitation-latent heat flux in order to detect the statistically significant cases of covariability. The application of CCA on the pair precipitation-sensible heat flux led to one only statistically significant pair (W,V) which explains 50% of the common variance of the two fields with a canonical correlation r=0.70. The canonical variate W is strongly correlated with precipitation in west Greece and the east Aegean, while the canonical variate V corresponds to sensible heat flux over the west Mediterranean. Similar results were found for the pair precipitation-latent heat flux (one canonical pair, 54%, r=0.71). It is then revealed a statistically significant relation between heat fluxes (sensible and latent) in the west Mediterranean and precipitation at the windward areas in west Greece and the east Aegean. High values of sensible and latent heat fluxes in the west Mediterranean imply an enrichment in energy of the air masses traveling over the area because of the Westerlies and consequently more cases of cyclogenesis. Also, the depressions formed cause cold and dry NW flow at their western parts, which contributes to a further enhancement of these fluxes. The same depressions traveling to the east, cause precipitation at the west winward areas of Greece, due to the S-SW flow along with the presence of potential instability in the atmosphere. According to the variations of the values of the corresponding canonical variates, a slight decrease is observed in all the parameters (especially in precipitation) from early 70’s until early 90’s.

































Sky type classification for the region of Athens, Greece, during a typical daylight and solar radiation year.

In this work, three typical meteorological years (TMYs), based on daylight and solar radiation measurements, are designed for the area of Athens, Greece, using: a) the Danish method, b) the Festa-Ratto method and c) the Sandia National Laboratories method. The data basis of the study consists of 5-minute values of global and diffuse horizontal illuminance, global and diffuse horizontal irradiance, zenith luminance and solar altitude as well as daily values of sunshine duration for the period 1992-1996. Moreover, Linke’s turbidity factor, luminous turbidity factor and relative sunshine duration are calculated and utilized. Then, for each TMY, the predominant sky luminance distribution over Athens is identified, by using the methodology developed by Kittler, who has presented 15 theoretical standard sky types in diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. The TMYs so generated are not identical. The ones derived by the Festa-Ratto and the Sandia National Laboratories methods resemble more, as they have most months in common. Despite the differences, the sky types defined for the region of Athens seem not to depend on the choice of TMY. Also the predominant standard sky, for all the cases examined, is type 13, a cloudless, polluted sky with a broad solar corona.

































Frequency probabilities of daylight illuminance courses due to sunshine duration.

Lighting controls in daylit interiors have to correspond to exterior illuminance changes in various seasons associated with local climate conditions. Typical half-daylight illuminance courses indicated by long-term 5-minute regular measurements in Athens and Bratislava are related to monthly mean relative sunshine duration. During a prevailing clear half day the relative sunshine duration is very high; the higher its monthly value is more clear days have are expected. Cloudy sky conditions are associated with shorter sunshine duration while highest probabilities are related to monthly relative sunshine duration in the range 0.2-0.6. Overcast days have usually no sunshine data and, therefore, the higher the monthly sunshine duration is, the less the frequency for overcast half-days becomes. The broken and sparse cloudiness causes dynamic situations with frequent sunshine spells; thus maximum dynamic sky conditions are related to monthly sunshine duration in the range 0.5-0.75. After Athens and Bratislava IDMP measurements these frequency probabilities were analysed in relation to measured sunshine duration and this interrelation was determined by simple formulas. Frequency probabilities of these four typical daylight situations are important for modelling monthly differences derived from monthly average sunshine duration especially in locations where no other climatic parameters are available.

































Sky type classification in south England during the winter period.

In this work, we study the conditions of illuminance in south England (Garston area 51.71°Í, 0.38°W) during the winter period. This study is based on the 15 categories of sky luminance distribution defined by Kittler, Perez and Darula. These 15 sky types are represented in diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. The data used are diffuse horizontal irradiance and illuminance, global horizontal irradiance and illuminance, zenith luminance and solar altitude, for the period 1/10/1991-31/3/1992. The classification of the observations into the 15 sky types was performed for the following situations: a) For solar altitude greater than 5° and less than or equal 35°, where the theoretical curves are not intersected. In this case, the ratio of zenith luminance to diffuse horizontal illuminance of each observation was compared with the corresponding ratios of the 15 theoretical sky types, for the same solar altitude and then the observation was classified in the sky type with the nearest value. b) For all solar altitude greater than 5°. In this case, due to the fact that the theoretical curves converge and above the 35° solar altitude they are intersected, each observation is classified in one of the 15 sky types, only when the value of the ratio of zenith luminance to diffuse horizontal illuminance lies in a zone of ±2.5% around the theoretical curve, which describes a specific category. In case that, an observation is classified in more than one sky types, the ratio of global horizontal illuminance to extraterrestrial horizontal illuminance is compared with the average ratios of the various sky types (estimated from all observations), for the same solar altitude and the observation is classified in the sky type with the nearest value. The above study has shown that the predominant sky type in south England and probable in regions with similar geographic and climatic conditions is the sky type 2 (I.2), which is an overcast sky with steep gradation and slight brightening toward the sun.

































Reconstruction of missing measured illuminance values in regular daylight data recordings

During routine daylight measurements disruptions due to equipment failure may occur resulting in data losses. These data losses can be attributed to power failure or recording of incorrect values due to lightning, snow or sensor interference by birds. After the appropriate quality control some data can be rejected too. Such irregularities cause gaps in data sets, which must be mended when fluent data sequences have to simulate illuminance courses or time availability. Therefore, in some evaluation studies the problem of missing data reconstruction has to be faced. There are several methods to deal with this task. The simplest is a linear interpolation between marginal values in narrow gaps assuming no interference of sunshine. However, in wider gaps, when fluent curves follow solar altitude influences, the quadratic or cubic interpolation is more suitable to express the nonlinearity of illuminance variations. Another more appropriate approach based on the identification of diffuse and global illuminance descriptors is proposed here. If only illuminance gaps are present but irradiance measurements are correct and fluent, then sunshine periods even under dynamic conditions can be followed. Thus using luminous efficacy ratios the illuminance values can be approximated satisfactorily. When no measurements are available during the gap, then the marginal measured diffuse and global levels are applied as descriptors for the fluent courses assuming a homogeneous case within the gap. This method can be recommended because it offers a new approach in filling gaps in global and diffuse illuminance data sets.

































Sky type classification in Central England during winter

In the present work there are studied the exterior daylight conditions in the area of Sheffield in central England (53.38°Í, 1.50°W) during the winter period. The study makes use of the fifteen standard sky types defined by Kittler et al. These sky types are presented in diagrams of the ratio of the zenith luminance (Lz) to the diffuse horizontal illuminance (Dv) against solar altitude (ă). The illuminance parameters used are time series of half-hourly values of Dv, Lz, global (Ge) and diffuse (De) horizontal irradiance and ă for the period November 1st, 1993 to February 10th, 1994. This study shows that the most frequent winter sky types in central England are: (i) I.2 (overcast with the steep gradation and slight brightening toward sun), with frequency of occurrence of 19.5%, (ii) I.1 (overcast with the steep gradation and with azimuthal uniformity) (10.5%) and (iii) II.1 (overcast moderately graded with azimuthal uniformity) (9.7%). The most predominant cloudless sky is that of V.5 (cloudless polluted with a broader solar corona) with frequency of occurrence of 7.6%.

































Sky luminance distribution in central Europe and the Mediterranean during summer period.

In this work the conditions of illuminance in Central Europe and the Mediterranean are studied and the prevailing sky luminance distributions are defined for the summer period. The data used are five-minute values of global and diffuse horizontal irradiance and illuminance along with zenith luminance from Bratislava and Athens for a period of five years. The study is based on the 15 theoretical sky types defined by Kittler et al., which are represented in diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. Due to the fact that the theoretical curves converge and above 35° they are intersected, each observation is classified in one of the 15 sky types, only when the value of the ratio of zenith luminance to diffuse horizontal illuminance lies in a zone of ±2.5% around the theoretical curve, which describes the specific category. If any observation is classified in more than one sky types, the corresponding ratio of global horizontal illuminance to extraterrestrial horizontal illuminance is compared to the average ratios of the various sky types, for the same solar altitude and the observation is classified in the sky type with the nearest value. It is found that the most frequent sky types in Bratislava are: the white-blue sky with a distinct solar corona and the very clear sky with low illuminance turbidity, while in Athens the cloudless polluted sky with a broad solar corona prevails.

































Dynamics of radiation and daylight climate linked with standard skies.

A system of twin radiometric and photometric descriptors defining the radiation and daylight climate at any location is based on the local extraterrestrial flow of solar radiation. Due to atmospheric transmission, redirection and diffusion these quantities on ground level have to be studied as direct solar and diffuse sky components forming together the so called global values. The latter are usually measured either within the world-wide meteorological network or at general IDMP stations. Thus by the help of these basic descriptors can be documented daily variations using ratios of global, direct and diffuse to extraterrestrial levels corresponding to time or sun altitude changes. Secondary descriptors as Linke or luminous turbidity factors, relative sunshine duration, clearness index etc. can be deduced either by further measurements or calculation. The importance of radiance and luminance distribution on various sky types is linked to the basic descriptor system by the ratio of zenith radiance/luminance values to diffuse irradiance/illuminance levels respectively. Both these ratios on the other hand are linked to the diffusion indicatrix and gradation functions characterising sky patterns. Applying data of the set of five-year recordings gathered in Athens and Bratislava seasonally typical changes of their radiation and daylight climate will be presented considering the possibilities of their simulation by general standard skies.

































On the relation between sea surface and lower troposphere temperature over the northern hemisphere.

The covariability of sea surface and lower troposphere temperatures (SST-AT) is investigated for winter (DJFM) and summer (JJAS), over the whole northern hemisphere, by utilizing monthly anomalies in grid boxes for a 22-year period. Air temperature variations in the lower troposphere are examined in terms of 1000-500hPa thickness. At first, factor analysis (FA) is applied on both sets of data in order to reduce their dimensionality and to group objectively grid points with a similar long term temperature variability. Also, FA revealed numerous teleconnections in both fields, most important being those governed by PNA in the Pacific Ocean and NAO in the Atlantic Ocean. Then, by using the FA results as a guide, correlation coefficients between the mean temperature time series of the subareas defined by FA in the two fields are estimated. The highest values, up to r=0.70, were found for subareas located one over/under the other. Finally, by introducing time lags, an attempt of forecasting either SST form AT or AT from SST is made. Although in some cases the coefficients appear increasing, test showed that the increase is not statistically significant. In any case, it appears that, on monthly time scale, it is the AT that controls SST and not the vice versa.

































The effect of meteorological and pollution parameters on the frequency of hospital admissions for cardiovascular and respiratory problems in Athens.

This study aims to investigate the influence of meteorological and pollution parameters on the frequency of hospital admissions for cardiovascular and respiratory diseases. The study includes 1,554 patients who were hospitalized in a major hospital in Athens during an 8-year period. The meteorological and pollution database consists of daily values of 12 original parameters and 24 constructed ones. The statistical methods used are simple linear correlation and linear stepwise regression analysis and they were applied on 10-day period data. The results showed that temperature, humidity and smoke concentrations are significantly correlated with the events of these diseases.

































The 850hPa relative vorticity centres of action for winter precipitation in the Greek area.

In this work, the relationship between atmospheric circulation over Europe and precipitation in Greece is studied during high winter (January-February). Specifically, the 850hPa relative vorticity centres, affecting or related to precipitation in the various areas of Greece, are defined and the corresponding temporal variability of both parameters is examined. Factor Analysis is at first applied on both fields, in order to reduce the dimensionality of the original data sets. Canonical Correlation Analysis is then applied on the Factor Analysis results, in order to interrelate the two parameters. It is found that: (i) precipitation over west and north Greece, east Aegean islands and west Turkish coasts is mainly controlled by vorticity over Italy and the Gulf of Genoa, (ii) south Aegean Sea and Crete precipitation is controlled by relative vorticity west of Crete and (iii) east mainland areas precipitation is significantly affected by a vorticity seesaw between Tunisia and northern Aegean. The temporal variations of precipitation and relative vorticity parameters over the above areas show a precipitation reduction over most Greek stations, combined with enhanced anticyclonic activity over almost the whole Mediterranean during the late 1980s and the early 1990s, a period characterized by high North Atlantic Oscillation index values. Finally, Cluster Analysis is applied on the factor scores time series of precipitation, in order to classify the winter precipitation anomaly patterns into objectively defined clusters. Six main precipitation patterns were revealed and the corresponding relative vorticity mean anomaly regimes over southern Europe indicate the atmospheric circulation characteristics provoking these specific patterns.

































An objective definition of air mass types affecting Athens, Greece; the corresponding atmospheric pressure patterns and air pollution levels.

This work aims at defining characteristic air mass types that dominate in the region of Athens, Greece during the cold (November-March) and the warm (May-September) period of the year and also at evaluating the corresponding concentration levels of the main air pollutants. For each air mass type, the mean atmospheric pressure distribution (composite maps) over Europe and the Mediterranean is estimated in order to reveal the association of atmospheric circulation with air pollution levels in Athens. The data basis for this work consists of daily values of thirteen meteorological and six pollutant parameters covering the period 1993-97. The definition of the characteristic air mass types is attempted objectively by using the methods of Factor Analysis and Cluster Analysis. The results show that during the cold period of the year there are six prevailing air mass types (at least 3 % of the total number of days) and six infrequent ones. The examination of the corresponding air pollution concentration levels shows that the primary air pollutants appear with increased concentrations when light or southerly winds prevail. This is usually the case when a high pressure system is located over central Mediterranean or a low pressure system lays over south Italy, respectively. Low levels of the primary pollutants are recorded under northeasterly winds, mainly caused by a high pressure system over Ukraine. During the warm period of the year, the southwestern Asia thermal low and the subtropical anticyclone of the Atlantic Ocean affect Greece. Though these synoptic systems cause almost stagnant conditions, four main air mass types are dominant and ten others, associated with extreme weather, are infrequent. Despite of the large amounts of total solar radiation characterizing this period, ozone concentrations remain at low levels in central Athens because of its destruction by nitric oxide.

































A study on the intra-annual variation and the spatial distribution of precipitation amount and duration over Greece on a 10-day basis.

The intra-annual variation of precipitation amount and duration and their spatial distribution during the year are studied on a 10-day basis for the Greek region, using S-mode and T-mode Factor analysis. (i) For the intra-annual variation of precipitation amount, two modes were revealed: the first shows one broad maximum during the conventional winter in stations affected by the sea and the second presents two maxima, the first during late autumn – early winter and the second during late spring, corresponding to the northern mainland stations. (ii) For the spatial distribution of precipitation, three main patterns were revealed: the first one is the ‘winter’ pattern with the maximum over the west windward area, the second is the ‘summer’ pattern with a maximum over the north inland region and the third is the ‘autumn’ pattern with the maximum over north western Greece. (iii) For precipitation duration, two types of intra-annual variation were revealed. The first one is similar to the first of the analysis for precipitation amount, while the second presents two maxima, the first during the beginning of December and the second during the middle of February, corresponding to the areas of north western and north eastern Greece. (iv) For the spatial distribution of precipitation duration three main patterns were revealed: the first is the ‘summer’ pattern, which is similar to the second of the analysis for precipitation amount, the second is the ‘winter’ pattern, with the spatial maximum located over the eastern mainland and western Crete and finally, the third one is the ‘autumn’ pattern, with the maximum in north western Greece. During the third 10-day period of October and the second one of February, precipitation seems to present singularities, possibly due to fluctuations in atmospheric circulation. The above intra-annual variations and spatial distribution patterns are connected to: the seasonal variations of the depression trajectories, the atmospheric instability, the influence of sea surface temperature as a cyclogenessis factor and the windward or leeward character of the various areas (orographic effect).

































Sky luminance distribution in central Europe and the Mediterranean area during the winter period.

Five-minute data of global and diffuse irradiance and illuminance along with zenith luminance from Bratislava and Athens for a period of five years are used in order to define the prevailing sky luminance distributions at the two sites in the winter period. The classification into 15 sky types is mainly based on the value of the ratio of zenith luminance to diffuse illuminance with solar altitude as parameter (SSLD method). This is applied for the first time in a research work. It is found that the most frequent sky type in Bratislava is the overcast (with steep luminance gradation and slight brightening towards the sun); while in Athens it is the cloudless polluted sky (with a broad solar corona). That is, the winter daylight climate is quite different in Central Europe in comparison to the Mediterranean. An analysis for various solar altitudes reveals that the frequency distribution of the 15 sky types appears almost the same for every solar altitude.

































Weather conditions and sudden sensorineural hearing loss.

Climatic or meteorological condition changes have been implicated in the pathogenesis of Idiopathic Sudden Sensorineural Hearing Loss (ISSHL). We investigated the seasonal distribution of ISSHL and evaluated the influence of meteorological parameters (such as temperature, humidity, and atmospheric pressure), their variation and covariation on the incidence of the disease. A total of 82 cases of ISSHL, admitted to our University Hospital over a five-year period, were enrolled in the study. Seasonal distribution of the disease was investigated by dividing the year in four seasons. Meteorological data included daily values of 13 distinct parameters recorded at the meteorological station of the University of Ioannina during this period. A relationship between each meteorological variable and the incidence of ISSHL was investigated by applying (÷ 2) test on data from 13 contingency tables as well as by using logistic regression and t-test approaches. In addition, the influence of different weather types on the incidence of ISSHL was investigated using Cluster Analysis in order to create eight clusters (weather types) characteristic for the prefecture of Ioannina. The results of the study could not indicate any seasonal distribution of the disease. The incidence of ISSHL could not be significantly correlated either to any distinct meteorological parameter or to any specific weather type. Meteorological conditions, such as those dominating in the Northwestern Greece, and/or their changes have no proven effect on the incidence of ISSHL.

































Air pollution levels in Athens during winter weather types.

This work aims to define characteristic weather types over the region of Athens during winter period and then to evaluate the corresponding concentration levels of the main air pollutants. For each weather type, the mean atmospheric pressure distribution over Europe is estimated in order to reveal the association between atmospheric circulation and pollution levels in Athens. The data basis for this work consists of daily values of 13 meteorological parameters and 6 pollutants for the cold season of the year (November-March) and for the period 1993-97. The definition of the characteristic weather types is attempted objectively by using Factor Analysis and Cluster Analysis. Firstly, Factor Analysis is applied on the meteorological data in order to reduce their dimensionality by grouping objectively those parameters, which covary in time. Then, Cluster Analysis is applied on the factor scores time series in order to define clusters of days with similar weather. The results show that during the cold period of the year the prevailing weather types (at least 3% of the total number of days) are only 6. The examination of pollution concentration levels during these weather types showed that the primary pollutants appear with increased concentrations when light or southerly winds prevail. This is usually the case when a high pressure system is located over central Mediterranean or a low pressure system lays over south Italy, respectively. Low levels of the primary pollutants are recorded under northeasterly winds, mainly caused by a high pressure system over Ukraine. Very high concentrations of secondary photochemical pollutants in central Athens are rarely recorded since they are inhibited by primary pollutants.

































Effect of meteorological parameters on acute laryngitis in adults.

In this study we have investigated the monthly distribution of acute laryngitis and the influence of meteorological factors on the frequency of the disease on daily basis, in Ioannina prefecture, NW Greece. The study includes 825 cases referred to our department from 1995 until 1999. The meteorological parameters investigated were: temperature (minimum, mean, maximum, diurnal temperature range, etc), humidity (relative, water vapour pressure etc) and atmospheric pressure as well as their day-to-day changes. We examined the relationship between each meteorological parameter separately and the frequency of the disease, using Contingency Tables. Moreover, the 1825 days of the 5 years were divided into groups with common weather characteristics, using Factor Analysis and Cluster Analysis. For each group of days we calculated the number and the frequency of cases and then we searched for statistically significant influence of weather on the incidence of the disease. In the monthly distribution we observed maximum frequency of events in March and minimum in August. The detailed analysis revealed an increment in the frequency of cases when there was low temperature, low diurnal temperature range, low water vapour pressure, low atmospheric pressure, and when, in comparison to the previous day, we observed: a) great change of maximum temperature, and b) decrement in atmospheric pressure. Furthermore, we found a statistically significant influence of the weather type on the frequency of the disease. The highest frequency was observed during winter. Among the different winter weather conditions, peak frequency of cases was observed in those characterized by high humidity recordings.

































Seasonal water quality of shallow and eutrophic Lake Pamvotis, Greece: implications for restoration.

Lake Pamvotis is a moderately-sized (22 km**2) shallow (average depth=4 m) lake with a polymictic stratification regime located in northwest Greece. The lake has undergone cultural eutrophication over the past 40 years and is currently eutrophic (annual averages of FRP=0.07 mg P L**-1, TP=0.11 mg P L**-1, NH4+=0.25 mg N L**-1, NO3-=0.56 mg N L**-1). FRP and NH4+ levels are correlated to external loading from streams during the winter and spring, and to internal loading during multi-day periods of summer stratification. Algal blooms occurred in summer (Jul.-Aug. green algae, Aug.-Sep. blue-green algae), autumn (Oct. blue-green algae and diatoms), and winter (Feb. diatoms), but not in the spring (Mar.-Jun.). The phytoplankton underwent brief periods of N- and P-limitation, though persistent low transparency (secchi depth of 60-80 cm) also suggests periods of light limitation. Rotifers counts were highest from mid-summer to early autumn whereas copepods were high in the spring and cladocerans were low in the summer. Removal of industrial and sewage point sources a decade ago resulted in a rapid decrease in FRP. A phosphorus mass balance that examined external versus internal loading indicates further reductions to external loading from the predominately agricultural catchment will decreases phosphorus levels further. The commercial fishery and fish hatchery also provides opportunities to control algal biomass through biomanipulation measures.

































Weather conditions and Bell’s palsy: Five-year study and review of literature.

Background: Climatic or meteorological condition changes have been implicated in the pathogenesis of Bell’s palsy (BP).
Objective: To evaluate the influence of meteorological parameters, such as temperature, humidity, and atmospheric pressure, and their variation and covariation on the incidence of BP and to present a review of the literature on the effect of meteorological conditions on facial nerve function.
Materials and Methods: A total of 171 cases of BP admitted to our Department over a five-year period were studied. The meteorological database included daily values of 13 distinct parameters recorded at the meteorological station of the University of Ioannina during this period. A relationship between each meteorological variable and the incidence of BP was investigated by applying (÷2) test on data from 13 contingency tables. In addition, the influence of different weather types on the incidence of BP was also investigated. For this purpose Cluster Analysis was used to create eight clusters (weather types) for the Ioannina prefecture and (÷2) test was applied on the contingency tables consisting of the days of BP cases for each cluster.
Results: No significant correlation was found either between BP and each distinct meteorological parameter or between BP and any specific weather.
Conclusions: Meteorological condition changes have little effect on the incidence of BP. Multicenter studies taking into account atmospheric pollution, and climatic differences between countries, are necessary to scrutinize the environmental effects on facial nerve function.

































Spatial and temporal 850hPa air temperature and SST covariances in the Mediterranean region and their connection to atmospheric circulation.

The spatial and temporal covariability between the lower troposphere and sea surface temperatures are studied in the Mediterranean basin for the period 1958-98. Monthly air temperature anomalies for the 850hPa pressure level (T-850hPa) at grid points 2.5°x2.5° and sea surface temperature anomalies (SST) in grid boxes 5°x5° are utilized. As a first step, Factor Analysis (FA) is applied on both sets of data in order to reduce their dimensionality. Then, Canonical Correlation Analysis (CCA) is applied and leads to one statistically significant pair of canonical variates for winter and to two pairs for summer. In winter, a teleconnection (see-saw) between western Europe and eastern Mediterranean at 850hPa level is revealed, corresponding to a weaker one between the areas of central-west and eastern Mediterranean for SST. The correlation between T-850hPa and SST appears higher over the eastern part. In summer, the first pair of canonical variates reveals a covariability between T-850hPa and SST in the western Mediterranean, while the second one shows a covariability in the eastern Mediterranean, without an existence of any strong spatial teleconnection. The analysis is repeated, using time lags of one month, or longer, in order to detect any possible non-synchronous relation. Statistically significant results are found only when T-850hPa leads SST for one month. In particular, the results are statistically significant for winter only and the findings are similar to the ones of the first analysis. Therefore, the existence of an one-month time scale persistence in SST is detected for winter months.

































An objective assessment of the relation between meteorological parameters and the main air pollutants in Athens on a daily basis.

This work aims to reveal the relations between meteorological parameters and the concentrations of the main air pollutants in Athens. The data basis consists of daily values of 8 pollution and 14 meteorological parameters for the cold and the warm season of the year (November-March and May-September respectively) and for the period 1993-97. Factor Analysis (P-mode) was applied in order to group objectively the parameters showing common variation in time. In this way, the 22 initial variables were reduced to three Factors for the cold period and four for the warm. Consequently, the final three or four Factors can be considered as the main contributors to the formation of the weather and air pollution conditions in Athens basin. Wind speed is the crucial parameter for the accumulation of air pollutants in Athens basin throughout the year. The role of wind direction on air quality appears stronger during the warm period. The secondary pollutant of O3 is controlled by the primary ones and this is more apparent during summer.

































The influence of meteorological parameters on the frequency of epistaxis.

This retrospective study aims to search out the influence of temperature, atmospheric pressure and humidity parameters on the frequency of epistaxis events. The study includes 701 patients, who have suffered from epistaxis and they have been treated in the ENT department of the University of Ioannina Hospital, during the years 1995 and 1996. The statistical methods used are simple linear correlation and linear stepwise regression analysis. The results of simple correlation analysis showed that the daily number of epistaxis depends mainly on mean, maximum and minimum temperature and water vapour pressure. The corresponding correlation coefficients are statistically significant, indicating an influence of weather on epistaxis up to 9% of its total variance. By applying stepwise regression analysis, we managed to increase the linear correlation coefficient and the corresponding amount of variance of epistaxis explained by meteorological parameters. This percentage was found to approach 20% for the cold period and 10% for the warm period of the year.

































An evaluation of the nature and timing of summer human thermal discomfort in Athens, Greece.

An assessment of the summer human thermal climate over the period 1966 to 1995 for Athens, Greece was undertaken using the Predicted Mean Vote (PMV). At the seasonal (diurnal) time scale human thermal discomfort reaches a maximum over a 20 day (4-6 hour) period centred on the end of July (1400 hours LST). Not only are general levels of thermal discomfort variable at the intra- and inter-annual time scales but the timing of discomfort season onset and cessation is highly variable from year to year. Man-Kendall tests for trends of discomfort season diagnostics revealed that there has been a tendency towards an increase in the length of the discomfort season over the study period. The implications of Athens’ harsh human thermal climate for staging athletic events are discussed and the concept of cool and warm synthetic summers is introduced as a basis for the planning of human activities.

































Daylight climate specification based on Athens and Bratislava data. Comparison of daylight conditions.

Available horizontal daylight illuminance and zenith luminance data gathered in 5-minute intervals during five years at IDMP general stations in Athens, Greece and Bratislava, Slovakia were analysed. Using the set of fifteen sky types of the proposed CIE General Sky Standard the seasonal and yearly occurrence frequency of sky types have shown relevant differences in daylight climates of Central Europe and the Mediterranean regions. The methods of statistical analysis can be applied in any location where long term measurement data are recorded and archived.

































Winter temperature covariances in the middle and the lower Troposphere over Europe and the north Atlantic ocean.

In this work, the variability and covariability of winter temperature in the middle and the lower troposphere are studied over Europe and the North Atlantic Ocean. Temperature variations of the middle and the lower troposphere are examined in terms of a) 500-700hPa and 700-1000hPa thickness and b) air temperature on the isobaric surfaces of 500hPa and 700hPa. At first, Factor Analysis, defined areas with characteristic temperature variability in each layer (and on each isobaric surface) and then, Canonical Correlation Analysis revealed areas in both layers (and on both isobaric surfaces) presenting common temperature variations. A temperature seesaw between North Europe and West Asia was revealed for both layers and isobaric surfaces implying that temperature changes in these areas are vertically spread. Another well defined area, appearing in both analyses, is the area of Labrador Sea and S Greenland. This region is also teleconnected to other regions, though not very clearly in every height. These temperature patterns are mainly attributed to the Eurasian (EU) and North Atlantic Oscillation (NAO) circulation patterns, which are responsible for large air mass exchanges in the area, being vertically extended in the middle and the lower troposphere.

































Sea surface temperature-850hPa relative vorticity relations in the Mediterranean region during winter.

In this work, a relation between 850hPa relative vorticity and sea surface temperature in the Mediterranean region was sought for winter during the last 40 years. For this reason, at first, Factor Analysis was applied on both sets of time series and then Canonical Correlation Analysis on the resultant factor scores. The results showed that there is only one statistically significant canonical pair which relates relative vorticity over central Mediterranean and Middle East with sea surface temperature over the Aegean and the Alboran sea. High relative vorticity over Middle East is connected with low relative vorticity over central Mediterranean and they imply high sea surface temperature over the Alboran sea and low sea surface temperature over the Aegean sea and vice versa. These seesaws have presented an one-sided tendency during the last 40 years corresponding to stronger or/and more frequent blocking anticyclones over central Mediterranean, being responsible for the reduction of precipitation observed over S Italy and the Balkans during the same period.

































A statistical study of precipitation in NW Greece.

In this work, using 20 year mean monthly precipitation totals from 54 stations, the general statistics for time and space distribution of precipitation in NW Greece, a very particular area, is at first described. Precipitation varies from about 1000mm at coasts to 2500mm on mountains, with a large variability. Then, the intra-annual variation is studied, using Fourier Analysis. This variation is described well by two harmonics, explaining about 90% of the total variance. The first harmonic, over 80% of the variance, shows a maximum from late December (coastal areas) to early January (continental areas) and is caused by the NW Mediterranean depression activity. The second harmonic exhibits maxima in May and November, also delayed in the continental areas, and is mainly caused by upper air troughs and instability. The spatial distribution of precipitation is then studied by using Factor Analysis. The 12 months of a year are classified in two parts corresponding to two statistically significant factors, explaining 88.5% of the total variance. Finally, the combination of positive and negative standardized scores of the two factors showed that NW Greece is characterized by four precipitation regimes, depending on the combination of depression activity, effect of sea and effect of topography.

































Temporal variations of the harmonic analysis parameters of the annual pressure march over the Atlantic, the Mediterranean and Asia.

In this study the temporal variations of the harmonic analysis parameters of the annual pressure march are studied over the Atlantic, the Mediterranean and Asia. Mean monthly pressure values have been used for the period 1873-1994 at grid points between 35N-50N and 40W-110E. Factor analysis revealed three representative areas inside which the above parameters covary in time. It was found that over the Atlantic the time of maximum of the first harmonic appears somewhat earlier the recent years. It was also found that the last years the amplitude of the first harmonic increases in the Mediterranean and decreases over Siberia. In most cases a turning point appears at about 1970. These changes are due to changes in mean monthly pressure values at specific months of the year. Finally it is especially mentioned that there exists a teleconnection (sea-saw) between the Atlantic and the Mediterranean in spring. During the last 50 years when pressure in spring is above normal over the Atlantic, it is below normal over the Mediterranean and vice versa.

































Analysis of the periodic variations of atmospheric pressure over Greece.

The annual variation of atmospheric pressure over Greece presents a double fluctuation with a primary maximum in October-November and a primary minimum in July. The diurnal variation presents a double fluctuation, too, with a maximum around 10:00, a primary minimum around 16:00 a secondary maximum around 22:00 and a secondary minimum around 05:00. The three harmonic terms reproduce the original curve well, as they explain 87% of the total variance. Most important, and almost identical to the final curve, is the 2nd harmonic term.




















Estimation of the eddy thermal diffusivity coefficient in water.

In this paper the eddy thermal coefficient in water is estimated. The experiment took place at Lake Washington, Seattle, USA. Water temperature in surface layer, wave frequency as well as all the other meteorological parameters were recorded simultaneously. After data processing, the value of the coefficient was found 20x10**(-5 ) m**2/s. Taking into account that the coefficient is depended on weather conditions the results are considered satisfactory.























Annual variation of pressure over the Mediterranean area.

This study deals with the annual variation of pressure over the Mediterranean area using monthly values from 24 stations, for a period of 22 years. The Azores and the Siberian anticyclones as well as the low pressure system of SW-Asia are responsible for the observed pressure pattern over the area and for the seasonal changes at each station. Fourier analysis shows that the reproduction of the annual variation of pressure is better at stations where the amplitudes of the successive harmonic terms converge faster. The strongest harmonic is the first one which presents its maximum from October (central Mediterranean) to January (east and west Mediterranean). Finally for central Mediterranean stations, which record many minima and maxima of pressure during the year, the amplitude of the second harmonic is smaller than the amplitude of the third.























The annual variation of pressure over the Northern Hemisphere.

The annual variation of the mean monthly surface pressure is studied over the Northern Hemisphere using the harmonic analysis. The study is based upon grid point data, 1900-1974. Eventually, the annual variation of mean pressure systems is studied, too. Two types of annual pressure variation are found, viz. one with a single harmonic and the other with two harmonics. The first is found around the middle of the large continents, with a pressure maximum in winter, and over the large oceans with a maximum in summer. In this type the first harmonic contributes about 90% of the total variance and the residual harmonics are negligible. The two-harmonics type is found in transitional zones between the continents and the oceans. The mean pressure systems are of thermal mostly nature, showing their maximum strength either in June-July (Asiatic and Californian Lows) or in December-January (Icelandic and Aleutian Lows and Siberian anticyclone).























Temperature fluctuations in the Mediterranean area during the last 120 years.

Based upon sea-surface temperature (SST) data, recently corrected for bucket-intake observational change, the SST fluctuations since 1873 in the Mediterranean are studied and compared with air temperature (AT) fluctuations at some land stations of this area and for the Northern Hemisphere (ATNH). Instead of the actual temperature, we used the frequency difference (per cent) of the number of the warm minus cold months, (Fw-Fc) annually and seasonally, smoothed using 10-year moving averages. This frequency difference was found to be a linear function of the actual temperature. For the whole Mediterranean, the trend of SST is simple enough, with a minimum in about 1910 and a double maximum in about 1940 and 1965. A secondary minimum follows in 1975-1980, very strong in the eastern Mediterranean, with a rise after that to the present. This trend when compared with ATNH presents some differences: in the ATNH a minimum appears earlier, in 1890, but this is shown only slightly in the Mediterranean SST. The second SST maximum, in 1965, is not shown in ATNH, and the general rise of recent years starts in the ATNH about 10 years earlier. When considering the AT of the Mediterranean stations, it can be seen that the rise, after the minimum of 1910, stops 15 years earlier than the SST and ATNH. Then, an anomalous and slow decrease follows until about 1975-1980, followed by a rise to the present, which is delayed in the eastern Mediterranean.























Sea surface temperature in the Mediterranean. Statistical properties.

Mediterranean Sea Surface Temperature is studied using various statistical properties. The whole area is divided in 5 long x 5 lat squares and the data used are mean monthly values for each year in these squares, since 1876. Harmonic analysis shows that one sinusoidal wave can represent well the annual variation of SST. The time series shows that there is a minimum in about 1910, a split maximum between 1947 and 1964, a secondary minimum in about late 70's, and a continuous increase since then. Spatial correlation, using the stepwise regression analysis shows that the SST of each square strongly depends on the SST of the West and East neighbouring ones. Finally, the ARIMA prediction models were applied and gave rather good results and better than the temporal multiple correlation coefficients, given by the stepwise regression analysis.























Atmospheric circulation anomalies in dry and wet winters in Greece.

The synoptic condition during the very dry winter of 1989 in Greece and most Europe is at first described. Similarly, the synoptic condition is investigated for the upper and lower decile of January rainfall, using Athens rainfall for its long record. The analysis has indicated that the winter of 1989 has been a very extreme case. In all cases, the persistence of a blocking anticyclone appeared to be the dominant synoptic feature. When this anticyclone covers Europe and the Mediterranean area, then dry conditions prevail over most Europe. On the other hand, when the anticyclone is located over the northeast Atlantic ocean west of the previous position, then wet conditions prevail over Greece and large part of Europe.























Covariability and climatic changes of the lower-Troposphere temperatures over the Northern Hemisphere.

The covariability and the long-term fluctuation of the lower-troposphere temperatures is studied, applying factor analysis on grid point (1000-500)hPa thickness data for the Northern Hemisphere. About 30 factors were retained and rotated for each season and year, but only about half of them were found to be significant. The grouping of the grid points according to the factor loadings has been mainly influenced by the land and sea distribution. Three teleconnections were found only, one in winter and two in summer. The winter one is a see-saw phenomenon between West Asia and North-West Scandinavia. In summer, the British Isles were found to covary with East Canada and the same is the case for almost all the subtropical deserts. The study of the factor scores revealed that the recent warming of the Earth's surface in the lower troposphere appears over the continents only.























Spatial and temporal sea-surface temperature covariances in the Mediterranean.

Using sea-surface temperature data for the post-war period, the Mediterranean Sea is divided objectively (principal components factor analysis) into subareas with similar covariances. This division is not the same in every season of the year. In summer, two subareas are found, the west and the east. In spring and autumn three, and in winter four significant subareas denote physical oceanographic and meteorological processes. The scores of the analyses show clearly the minimum of the late 1970s and then the warming of the recent years somewhat delayed in the eastern Mediterranean.























Pressure covariability over the Atlantic, Europe and N. Africa.
Application: Centers of action for temperature, winter precipitation and summer winds in Athens, Greece.


This paper deals with surface pressure covariability over the Atlantic/European sector of the Northern Hemisphere, using monthly grid point data for the 100 year period 1890-1989. Factor Analysis is applied to 90 grid point time series for January, February, July, and August. The initial 90 pressure variables can be reduced to 7-8 factors in winter and 10 in summer. A winter teleconnection was identified, known as the seesaw phenomenon, between the Icelandic low and the Azores subtropical anticyclone. In order to define the centers of action for temperature, winter precipitation and summer northerly wind frequency (etesian days) in Athens and in the Aegean sea, the variability of the factor scores and of these weather elements is compared. It is shown that the center of action for temperature in Athens is found to be in north and northwest Europe (centered over southern Scandinavia). For winter precipitation, the center of action is located in the west and southwest Mediterranean and northwest Africa. Finally, for the etesian winds frequency variability, this center of action is found over the northern Adriatic and northern former Yugoslavia, while there is no evidence of influence by the southwest Asia thermal low.























Temporal variations of atmospheric tides over Athens, Greece.

The mean daily march of pressure (hourly values) for each month of the 96 year period 1894-1989 in Athens, Greece is studied using Fourier analysis. The annual variation of the harmonic parameters (amplitude, time of maximum, x and y-components) is examined. The globally excited 2nd and 3rd harmonics show in general the same behaviour as in other places of the Earth. Similar features show the corresponding parameters of the daily solar radiation march. The first harmonic displays some local characteristics. The long period fluctuations study revealed statistically significant trends in most parameters of the 2nd harmonic, most strikingly the 0.5h shift in the time of its maximum during the 20th century. Power spectrum analysis showed the existence of a biennial oscillation in B1, the x-component of the first harmonic, but this oscillation is mainly confined to the second half of the entire period. A periodicity of about 6 years which could be associated to the Southern Oscillation was also found in B1 while in A1, the y-component, a periodicity close to the 11-year solar cycle and another one close to the nutation of the Earth's axis are present. The lunar tides are too small to be detected by the same methods. However the semilunar tide was revealed to be 10-20 times smaller than the semisolar.























Daily precipitation variability in semi-arid agricultural regions in Macedonia, Greece.

The spatio-temporal variability of daily precipitation series was investigated in a semiarid region of central Macedonia in northern Greece. Ten years of daily rainfall records for seven stations in the region constituted the data base. The spatial characteristics were examined by drawing composite correlation diagrams for the cool (October-March) season and the warm (April-September) season, and the results confirmed the regional homogeneity of the data sets. Furthermore, the temporal analysis indicated that the non-rainy days constituted the major portion of days throughout the year at all the stations. Similarly, light rainfall represented the majority of rainy days. Moreover the annual rainfall variation showed high values in March, April and November with low values occurring in the summer and autumn. A sharp increase of rainfall between the 185th and 195th day of the year must be taken into account when the harvest is scheduled. Harmonic and Power Spectrum analyses applied to the annual variation of rain depths using 5-day intervals revealed significant periodicities of 26, 122, 365 and 55 days. Finally the analysis of the annual variation of rain occurrences revealed periodicities of 365 and 122 days.























Factor Analysis of some climatological elements in Athens, 1931-1992: Covariability and Climatic Change.

The climatological parameters of a station are, in general, intercorrelated to various degrees (e.g. cloudiness with sunshine duration or possibly rainfall with temperature). In this work, 15 climatological parameters of the National Observatory of Athens are distributed in groups where the contained parameters are highly covariant. This classification is achieved using P-mode Factor Analysis, based on the monthly values for the period 1931-1992. Using several known criteria, we found up to four significant factors. The first two are the most important ones including the "temperature" and the "precipitation" parameters, respectively, with loading values above 0.85. The other two are weaker and variable, depending on the month. Finally, climatic trends of the grouped parameters (factors) are examined and the results are compared with those of studies using classical methods.























Northern Hemisphere gross circulation types. Climatic change and temperature distribution.

In this work the atmospheric circulation patterns over the N Hemisphere are objectively revealed and discussed, applying Factor Analysis, T-mode, on monthly grid point data of surface pressure. The analysis has been effected for January and February on the one hand and for July and August on the other. The results were similar in adjacent months in almost every respect. Using Monte-Carlo technique, up to 6 circulation types (factors) were found statistically significant (at 0.05 level) during the high winter months, in the 100 year period 1890-1989, but 2 of them comprise very few years only. The first pattern refers mainly to Europe and Asia, the second to the Atlantic, the Pacific and the adjacent coasts, the third to the whole N Hemisphere and the fourth (5 cases only) mainly to the N Pacific. For the high summer months, only two factors were found, affecting mainly the polar regions. In the first, a polar low appears and in the second a polar high. For the remaining regions worth mentioning pressure anomalies appear over NW Europe only. The time series of patterns indicates a climatic change in atmospheric summer circulation, leading to corresponding changes also in other meteorological parameters (e.g., precipitation). Finally, using 1000-500hPa thickness data for the post-war years, the temperature distribution of the revealed patterns is shown to be in accordance with the prevailing wind field.























Variability and covariability of basic air pollutants in Athens.

The mean annual variability and covariability of the basic air pollutants (Smoke, O3, SO2, NO2) monitored in 9 stations of the Greater Athens area, is studied for the period 1991-94. On the time series of each pollutant Factor Analysis has been applied in order to define the main modes of variation and to group the stations in subgroups with a particular annual variation. The results showed that NO2 is the only one exhibiting different annual variation in the coastal than in the inland stations. This is due to the Etesian winds which in summer affect the penetration of the sea breeze over the land.























Phenomenological features of precipitation series in agricultural regions.

In this paper phenomenological features of daily rainfall series are investigated in homogeneous agricultural regions. A region in central Macedonia of northern Greece is used for the analysis, where the availability of water is mandatory for the agricultural activities of the region. Seven stations are employed with a 10-year record of daily rainfall depths. At first, composite correlation diagrams are used, which justify the regional and spatial homogeneity of the data sets. Temporal characteristics of the rainfall data have shown that the majority of days throughout the year are without rain for all the stations using either mean annual or monthly rain occurrences. Similarly, the small rainfall amounts constitute the majority of the rainy cases. Moreover, harmonic analysis of the annual variation of rain occurrences is attempted using 5-day values of rainfall depths. The five strongest harmonics are presented along with phases, amplitudes, temporal cycles and total variance explained .























Spatial covariability of the climatic parameters in the Greek area.

In this work, 12 monthly climatic parameters of 49 Meteorological stations in Greece are classified according to their spatial covariability, in order to define groups of climatic elements having the same characteristic spatial distribution. The grouping of the parameters is achieved objectively by using the method Factor Analysis. The analysis is applied for the similar winter months, January and February and for the summer months July and August. In winter the parameters are classified in 4 groups as following: (a) maximum, mean and minimum temperature and, negatively, pressure and number of frost days, (b) wind and, negatively, diurnal temperature range, (c) precipitation and (d) cloudiness and, relative humidity. In summer the groups are three: (a) precipitation, cloudiness and, negatively, minimum temperature, (b) maximum and mean temperature, relative humidity and (c) wind. From the above groups the most interesting is (a) of summer, as the comprised parameters, in winter belong to different groups. Finally, the study of the spatial distribution of the above groups showed that some depend on latitude, others on the continentality and others on the leeward or windward character of the various areas.























Climatic fluctuation of temperature and air circulation in the Mediterranean.

The global warming is tested in the Mediterranean area during the last 45 years using SST and 1000/500mb thickness data, in order to avoid urban effects. The area is divided in two parts vis. the West and the East Mediterranean. Since warming is not apparent in the East Mediterranean but only during the last years, the time series of surface pressure and relative geostrophic vorticity were examined for possible explanations. It is found that there exists a continuous and considerable increase of the mean vorticity in the extreme East Mediterranean, meaning increased frequency and/or strength of N. winds there, causing a delay of the warming. The opposite phenomenon appeared in the extreme West Mediterranean where increased S. winds may contribute to the warming there.























Circulation types over the Atlantic, Europe and N. Africa and their temperature distribution.

The main pressure patterns that dominate over the Atlantic, Europe and N Africa in winter and in summer are investigated using grid point surface pressure data for the 100 year period 1890-1989. The method used is Factor Analysis, T-mode, which revealed 5-6 patterns for the winter months and 2 for the summer ones. In winter, the first factor exhibits lower than normal pressure values in high latitudes; the second factor shows positive anomalies over Scandinavia and negative over central Europe. The other factors are very weak and only one or two years depend on them. In summer, the first factor shows positive anomalies in the polar region and negative over the British Isles while the second one exhibits the opposite characteristics. Finally, the the 1000-500hPa thickness anomalies for the years corresponding to each factor are found to be in good agreement with the expected conditions by the described circulation patterns.























On spatial and temporal variations of drought in selected regions of central and southeastern Europe.

Spatial and diachronic characteristics of drought periods are examined. Assessment of droughts is performed by employing the Palmer Drought Severity Index (PDSI). The Palmer's model is applied to monthly data in the period 1951-1990 (1957-83 for Greece) at 27, 19, 16, and 26 stations in Poland, Czech Republic, Hungary and Greece, respectively. Factor analysis is employed to define subareas in each country, inside of which PDSIs covary in time. The same is also performed for Z-index. The station with the highest factor loading in each grouping is used for estimation of the basic statistical parameters and for ARMA modeling techniques.























The variability of the relative geostrophic vorticity in Europe, the Atlantic and N. Africa, during the year.

The intra-annual variation of relative geostrophic vorticity over the Atlantic, Europe and N. Africa is studied using daily and pentad surface pressure and 500hPa height grid point data, for the 20 year period 1970-1989. Factor analysis is used in order to reduce the dimensionality of the 117 grid points. Six factors were retained explaining about 65% of the total variance. The first one, which is the dominant, covers the Mediterranean, N. Africa and the Middle East. These areas display one maximum and one minimum during the year. However, the time of this maximum is not the same in all the above areas, as numerous seesaw phenomena were revealed.























Factor Analysis as a tool for coherence and variability studies in climatology.

In this paper the outlines of Factor Analysis are presented, along with some applications in Climatological Studies. The main achievement of this analysis is at first the objective reduction of the dimensionality of a data set, which then can be handled more easily, in order to reveal its hidden constituent features. The variables denote either space (stations or grid points) or time (years, months or days) or meteorological parameters and are examined for possible covariability. The combinations of each two of these variables result in the various modes of this analysis. So we have the following examples:
S-mode (S for space): The covariability (and the climatic change) of the lower-troposphere temperatures are studied, using monthly grid point 1000-500hPa thickness data for the N hemisphere. The factor scores trends showed that since 1945 only the recent warming is apparent and over the continents.
T-mode (T for time): The atmospheric circulation types over the N hemisphere are investigated, using monthly values of surface pressure for the period 1890-1989. In winter, up to 6 circulation types appeared (4 important) while in summer 2 only. The validity and the significance of the results were confirmed by examining the 1000-500hPa thickness anomalies corresponding to each factor.
P-mode (P for parameters): The covariability of 15 climatological parameters at the National Observatory of Athens is studied using their monthly values since 1931. Four factors are found but two of them are the most significant, viz. the "temperature" and the "precipitation" ones. The other two factors are weaker and variable, depending on the season.