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This paper presents a comparative sensitivity analysis of six of the widely used human thermal comfort indices. The analysis consists of the evaluation of the effect of indices 'input parameters' variation and change rate on the output of human energy balance and simple thermohygrometric indices. For the implementation of the sensitivity analysis, the generalized additive model's methodology is applied on a long period and high temporal resolution dataset from Athens, Greece. The results indicate that the proposed methodology of generalized additive models is adequate for such an analysis. Moreover, this research revealed the differences in index behaviour. The thermohygrometric indices (i.e. Thermohygrometric Index and HUMIDEX) exhibit a clearly deferent sensitivity pattern in comparison to the human energy balance indices (i.e. physiologically equivalent temperature (PET), perceived temperature (PT), modified physiologically equivalent temperature (mPET) and Universal Thermal Comfort Index (UTCI)), and they are incapable to handle the complexity of the atmospheric stimuli on human thermal perception. On the other hand, human energy balance indices can follow the input parameters fluctuations but with different grades of sensitivity. PET and mPET present a moderate and gradual sensitivity both in terms of the input variation and input change rate. PT is the less sensitive index among the human energy balance investigated, but it is able to follow efficiently the input parameters variation during the measurements period. Moreover, UTCI is the most sensitive among all the selected indices for low values (and low change rate) of the input parameters but for high input parameter values (except the wind speed), UTCI exhibits a low sensitivity in comparison to the other human energy balance indices. In terms of sensitivity, the most influential input parameter is global radiation, and the less influential is vapour pressure. [ABSTRACT FROM AUTHOR]

In this paper, the surface and lower tropospheric temperature trends in Greece and their relationship to the atmospheric circulation for the period 1955-2013 were examined, updating the study of Feidas et al. (Theor Appl Climatol 79:185-208, 2004) for data observed during the 12-year period 2002-2013. The trend analysis is based on a combination of three statistical tests. The trends are now examined for all the seasonal time series, new atmospheric circulation indices were added in the analysis, and maps with the spatial distribution of correlation between air temperature and atmospheric circulation were constructed and analysed. The series updated to 2013 for 18 stations reveal a clearer positive trend than that found for the period 1955-2001 on both the annual and the seasonal timescales. The warming signal detected only in summer in the study of Feidas et al. (Theor Appl Climatol 79:185-208, 2004) has now intensified and spread in other seasons. This warming appears to be mainly caused by the very high temperatures in the last decade (after 2004) of the record. At the national scale, there is now a match between surface temperature trends in Greece and Northern Hemisphere (NH) but only for summer, spring and annual time series, which are the only time series presenting a statistically significant warming trend in Greece. Satellite-induced lower tropospheric temperatures now show a statistically significant tropospheric temperature warming trend for the period 1979-2013, for both areas (Greece and NH). Lower tropospheric and surface air temperatures for the same period (1979-2013) show a very good agreement, with differences only in winter and summer for Greece. The influence of atmospheric circulation on the temperature variability in Greece was also examined using two more circulation indices: the Eastern Mediterranean Pattern Index (EMPI) and the North-Sea Caspian Pattern Index (NCPI). EMPI and especially NCPI explain better now the temperature variance in Greece, principally in winter. This connection, however, is not only developed during winter, as expected, but is also present for annual and other seasonal temperatures. [ABSTRACT FROM AUTHOR]

The paper presents a semi-supervised weather classification method based on 850-hPa isobaric level maps. A preprocessing step is employed, where isolines of geopotential height are extracted from weather map images via an image processing procedure. Α feature extraction stage follows where two techniques are applied. The first technique implements phase space reconstruction, and yields multidimensional delay distributions. The second technique is based on chain code representation of signals, from which histogram features are derived. Similarity measures are used to compare multidimensional data and the k-means algorithm is applied in the final stage. The method is applied over the area of Greece, and the resulting catalogues are compared to a subjective classification for this area. Numerical experiments with datasets derived from the European Meteorological Bulletin archives exhibit an up to 91 % accurate agreement with the subjective weather patterns. [ABSTRACT FROM AUTHOR]

This paper aimed to investigate the potential of using spectral and textural features in the Meteosat Second Generation-Spinning Enhanced Visible and Infrared Imager (MSG-SEVIRI) data for developing techniques capable of classifying convective and stratiform rain areas in the satellite image. Two different classification methods were introduced that use the brightness temperature (BT) T and brightness temperature differences T- T T- T T- T T- T T- T T- T, and T- T as spectral parameters, along with textural parameters derived from the thermal infrared MSG-SEVIRI channel to discriminate between convective and stratiform rainy clouds. The first is an algorithm based on the probability of convective rainfall (PCR) for each pixel of the MSG satellite data and the second is an artificial neural network multilayer perceptron (MLP) model that relies on the correlation of satellite data with convective and stratiform rain. Both schemes were trained using as reference convective/stratiform classification data from 88 stations in Greece for 20 rainy days with high convective activity and evaluated against an independent dataset of gauge data for six rainy days. Two PCR and two MLP algorithms were constructed based on the previous results: the PCR1 and MLP1 algorithms that use only spectral measures and the PCR2 and MLP2 algorithms based on both spectral and textural measures. It was found that the introduction of textural parameters as additional information to a technique (PCR2 and MLP2 ) works in improving the delineation between convective and stratiform rainy pixels compared to the techniques based only on spectral information (PCR1 and MLP1). The comparison of the two schemes revealed a clear outperformance of the MLP techniques over the PCR techniques. Best skill is provided by MLP2 (POD = 74.5 %, FAR = 44.3 %, POFD = 22.5 %, CSI = 0.47, ETS = 0.31, bias = 1.34) followed by MLP1 and the two PCR techniques. All algorithms overestimate the convective rain occurrences observed by the rain gauge network. These findings showed that the combined use of spectral and textural features in the MSG-SEVIRI imagery can be beneficial for the classification of convective and stratiform precipitating clouds. [ABSTRACT FROM AUTHOR]

During the last decades, wildfires have received growing attention because of their major impact on the natural and anthropogenic infrastructures worldwide. The present paper focuses on investigating the relationship between the meteorological/climatological conditions and wildfires in Greece on a variety of temporal and spatial scales. This is performed through the identification of the mean surface temperature, the sea-level pressure, and the absolute humidity anomalies during wildfires at 26 meteorological stations covering both maritime and land environments in the Greek domain and the subsequent construction of the derived GIS surface contour maps. A case study analysis through composite anomaly maps of temperature, pressure, precipitable water, 500-hPa geopotential height, specific humidity, and vector wind is also performed. It is concluded that positive surface temperature, absolute/specific humidity, 500-hPa geopotential height, and vector wind anomalies are observed during wildfires while sea-level pressures are anomalously negative. Furthermore, western regions display lower magnitude anomalies compared to the more central and central-eastern regions during wildfires. The above meteorological/climatological findings in conjunction with medium range to seasonal climate forecasts could be used by wildfire risk managers to provide increased wildfire prediction accuracy and thus benefit many aspects of the natural and built environment. [ABSTRACT FROM AUTHOR]

This paper investigates whether there is any association between the daily mortality for the wider region of Athens, Greece and the thermal conditions, for the 10-year period 1992-2001. The daily mortality datasets were acquired from the Hellenic Statistical Service and the daily meteorological datasets, concerning daily maximum and minimum air temperature, from the Hellinikon/Athens meteorological station, established at the headquarters of the Greek Meteorological Service. Besides, the daily values of the thermal indices Physiologically Equivalent Temperature (PET) and Universal Thermal Climate Index (UTCI) were evaluated in order to interpret the grade of physiological stress. The first step was the application of Pearson's χ test to the compiled contingency tables, resulting in that the probability of independence is zero ( p = 0.000); namely, mortality is in close relation to the air temperature and PET/UTCI. Furthermore, the findings extracted by the generalized linear models showed that, statistically significant relationships ( p < 0.01) between air temperature, PET, UTCI and mortality exist on the same day. More concretely, on one hand during the cold period (October-March), a 10°C decrease in daily maximum air temperature, minimum air temperature, temperature range, PET and UTCI is related with an increase 13%, 15%, 2%, 7% and 6% of the probability having a death, respectively. On the other hand, during the warm period (April-September), a 10°C increase in daily maximum air temperature, minimum air temperature, temperature range, PET and UTCI is related with an increase 3%, 1%, 10%, 3% and 5% of the probability having a death, respectively. Taking into consideration the time lag effect of the examined parameters on mortality, it was found that significant effects of 3-day lag during the cold period appears against 1-day lag during the warm period. In spite of the general aspect that cold conditions seem to be favourable factors for daily mortality, the air temperature and PET/UTCI exceedances over specific thresholds depending on the distribution reveal that, very hot conditions are risk factors for the daily mortality. [ABSTRACT FROM AUTHOR]

This paper studies the maximum intensity of the urban heat island (UHI) that develops in Volos urban area, a medium-sized coastal city in central Greece. The maximum temperature difference between the city center and a suburb is 3.4°C and 3.1°C during winter and summer, respectively, while during both seasons the average maximum UHI intensity is 2.0°C. The UHI usually starts developing after sunset during both seasons. It could be attributed to the different nocturnal radiative cooling rate and to the different anthropogenic heat emission rate that are observed at the city center and at the suburb, as well as to meteorological conditions. The analysis reveals that during both seasons the daily maximum hourly (DMH) UHI intensity is positively correlated with solar radiation and with previous day's maximum hourly UHI intensity and negatively correlated with wind speed. It is also negatively correlated with relative humidity during winter but positively correlated with it during summer. This difference could be attributed to the different mechanisms that mainly drive humidity levels (i.e., evaporation in winter and sea breeze (SB) in summer). Moreover, it is found that SB development triggers a delay in UHI formation in summer. The impact of atmospheric pollution on maximum UHI intensity is also examined. An increase in PM10 concentration is associated with an increase in maximum UHI intensity during winter and with a decrease during summer. The impact of PM10 on UHI is caused by the attenuation of the incoming and the outgoing radiation. Additionally, this study shows that the weekly cycle of the city activities induces a weekly variation in maximum UHI intensity levels. The weekly range of DMH UHI intensity is not very large, being more pronounced during winter (0.4°C). Moreover, a first attempt is made to predict the DMH UHI intensity by applying regression models, whose success is rather promising. [ABSTRACT FROM AUTHOR]

This paper deals with the most recent trends in meteorological and hydrological variables, which include air temperature and precipitation ( P), potential and actual (ET) evapotranspiration, surface runoff (RO), water recharge into the soil ( R) and water loss from the soil ( L). Most hydrological variables were calculated via Palmer's algorithm. For this purpose, two rank-based statistical tests (the Mann-Kendall (MK) and a change-point analysis (CPA) approach) and the basic linear regression-based model were applied on the weekly precipitation and temperature from 17 stations all over Greece, during 1961-2006. Only in winter, all variables except for R, which showed no clear signal, presented downward trends. The declining trends of P and L in spring and summer were counterbalanced by reductions in RO (and R in the case of summer) as opposed to increases in ET. In autumn, the declining tendencies of P and L were offset by RO reductions and R increases. Annually, the trends in water cycle components were analogous to that of spring, summer and autumn. The number of stations with statistically significant (at 95%) trends greatly varied with season and meteorological/hydrological variable. [ABSTRACT FROM AUTHOR]

This paper presents a method for grouping weather types that occur over an area, which combines meteorological parameters, reflecting air mass characteristics at the surface, with synoptic conditions prevailing over an area. Five quantitative meteorological parameters are used in the procedure: temperature, precipitation, relative humidity, wind velocity and sunshine duration. In addition, two qualitative variables related to the prevailing circulation type and whether it is cyclonic or anticyclonic are also included. The study period is 43 years (1958–2000) and is restricted to the cold and wet sub-period of the year, December–March. Weather types are defined using a relatively new method of cluster analysis, two-step cluster analysis, which allows the simultaneous use of both quantitative and qualitative variables. The aim of the present study is to distinguish primary weather patterns so that the investigation into the relationship between weather patterns and circulation types will be more effective. For Athens, six weather types are created, whereas for Thessaloniki five are produced. For both stations, only two weather types are related to anticyclonic situations. The majority of the identified weather types correspond to a distinctive and well-defined synoptic situation. Each weather type differs from the others, not only in terms of the circulation conditions referring to it, but also with reference to meteorological variables such as temperature and precipitation. The results of the evaluation of the aforementioned procedure are considered to be highly satisfactory. [ABSTRACT FROM AUTHOR]

The aim of the paper lies in the identification of possible significant linear trends at monthly, seasonal and annual timescales in the Mediterranean during the second half of the 20th century. Monthly and daily records of 63 stations have been used to elaborate several precipitation indices: sum of daily precipitation (SDP) for rainfall >0.1 mm, >10 mm and >95th percentile, of number of rainy days (RD) >0.1 mm and >10 mm and of mean daily precipitation (MDP) >0.1 mm and >10 mm. For each index the stations have been gathered together by Rotated Principal Component Analyses to determine 8 sub-areas which can be considered as identical for all the timescales at the spatial scale of the research. Trends have been estimated from the scores of each eigenvector retained in all RPCAs. They are mainly non existant or non significant decreasing, even if a few monthly trends appear to be significantly diminishing, primarily during winter months, March in the Atlantic region, October in the Mediterranean Spain, December in the Lions and Genoa Gulfs, January, winter and the year in Greece, winter and the year in Italy and winter in the Near East and increasing in April in the two gulfs. Correlation coefficients between SDP>0.1 mm and other indices have been computed: the significant trends seem mainly related to RD>10 mm, which represents a high percentage of the total rainfall amount. Greece is remarkable: SDP>0.1 mm and >10 mm decrease significantly during January, winter, the rainy season and the entire year whereas SDP>95th percentile increases significantly, in accordance with the climatic change scenarios for the end of this century as does the decreasing of the total monthly and seasonal rainfall. [ABSTRACT FROM AUTHOR]

Climate change is inherently linked to long-term non-stationary changes in the characteristics and frequency of weather patterns. The present study attempts to identify the statistical changes of weather patterns in Athens Greece, from the comparative assessment of 96-h backward trajectories between historic (1980–2009) and future (2020–2049) climatology derived from the IPCC RCP4.5 and RCP8.5 scenarios. Arrival heights at 750 m, 1500 m, and 3000 m above sea level are considered to account for the impact of the planetary boundary layer and the lower free troposphere. The analysis of the historic period yields 7 dominant patterns for all heights determined independently, with similar spatial characteristics but varying frequency of occurrence. The classification of backward trajectories under future climate using the same historic clusters reveals percentage changes from locally short-distance travelling patterns to longer-distance ones with a predominant northbound direction. As a second experiment, backward trajectories are re-clustered independently reaching again the same type of clusters but with observable changes in the cluster origins and trajectory lengths. [ABSTRACT FROM AUTHOR]

Photosynthetically active radiation (PAR) is an important parameter in ecological research. However, it is not routinely measured and often is estimated as a constant ratio of the incoming solar shortwave radiation (Rs). There are only few reported PAR/Rs values worldwide concerning the Mediterranean climate and even fewer from rural or forest areas, especially at higher altitudes. Hourly PAR and Rs flux densities were measured above a deciduous oak forest in Greece from 1999 to 2005, and their relationship was investigated under various conditions. Results show that the annual mean hourly PAR/Rs is 0.454, ranging from 0.443 in spring to 0.478 in autumn, with intermediate values in summer (0.454) and winter (0.459). The ratio increases with dew point temperature but decreases as solar elevation angle or Rs increases. Atmospheric clearness index, Kt, and actual water vapor pressure, ea, are the key factors determining the ratio; however, relative humidity (RH) also seems to have an indirect effect by affecting Kt and ea values. PAR/Rs changes from 0.468 to 0.455 as sky conditions change from clear to overcast and appears to increase with ea. However, Kt affects the ratio when RH is lower than 60%, while ea has a more obvious effect at more saturated atmospheric conditions. [ABSTRACT FROM AUTHOR]

Recent studies have showed that there is a significant decrease in rainfall over Greece during the last half of the pervious century, following an overall decrease of the precipitation at the eastern Mediterranean. However, during the last decade an increase in rainfall was observed in most regions of the country, contrary to the general circulation climate models forecasts. An updated high-resolution dataset of monthly sums and annual daily maxima records derived from 136 stations during the period 1940-2012 allowed us to present some new evidence for the observed change and its statistical significance. The statistical framework used to determine the significance of the slopes in annual rain was not limited to the time independency assumption (Mann-Kendall test), but we also investigated the effect of short- and long-term persistence through Monte Carlo simulation. Our findings show that (a) change occurs in different scales; most regions show a decline since 1950, an increase since 1980 and remain stable during the last 15 years; (b) the significance of the observed decline is highly dependent to the statistical assumptions used; there are indications that the Mann-Kendall test may be the least suitable method; and (c) change in time is strongly linked with the change in space; for scales below 40 years, relatively close regions may develop even opposite trends, while in larger scales change is more uniform. [ABSTRACT FROM AUTHOR]

Notwithstanding technological developments, agricultural production is still affected by uncontrollable factors, such weather and climate. Within this context, the present study aims at exploring the relative influence of growing season climate on the yields of major cereals (hard and soft wheat, maize, and barley) on a regional scale in Greece. To this end, crop-climate relationships and the impacts of climate trends over the period 1978-2005 were explored using linear regression and change point analysis (CPA). Climate data used include maximum (Tx) and minimum temperature (Tn), diurnal temperature range (Tr), precipitation (Prec), and solar radiation (Rad). Temperature effects were the most substantial. Yields reduced by 1.8-7.1 %/°C with increasing Tx and by 1.4-6.1 %/°C with decreasing Tr. The warming trends of Tn caused bilateral yield effects (from −3.7 to 8.4 %/°C). The fewer significantly increasing Rad and decreasing Prec anomalies were associated with larger yield decreases (within the range of 2.2 % MJ/m/day (for maize) to 4.9 % MJ/m/day (for hard wheat)) and smaller yield increases (from 0.04 to 1.4 %/mm per decade), respectively. Wheat and barley-the most vulnerable cereals-were most affected by the trends of extreme temperatures and least by Tr. On the contrary, solar radiation has proven to be the least affecting climate variable on all cereals. Despite the similarity in the direction of crop responses with both analyses, yield changes were much more substantial in the case of CPA analysis. In conclusion, regional climate change has affected Greek cereal productivity, in a few, but important for cereal production, regions. The results of this study are expected to be valuable in anticipating the effects of weather/climate on other warm regions worldwide, where the upper temperature limit for some cereals and further changes in climate may push them past suitability for their cultivation. [ABSTRACT FROM AUTHOR]

This study presents a methodology for modeling and mapping the seasonal and annual air temperature and precipitation climate normals over Greece using several topographical and geographical parameters. Data series of air temperature and precipitation from 84 weather stations distributed evenly over Greece are used along with a set of topographical and geographical parameters extracted with Geographic Information System methods from a digital elevation model (DEM). Normalized difference vegetation index (NDVI) obtained from MODIS Aqua satellite data is also used as a geographical parameter. First, the relation of the two climate elements to the topographical and geographical parameters was investigated based on the Pearson's correlation coefficient to identify the parameters that mostly affect the spatial variability of air temperature and precipitation over Greece. Then a backward stepwise multiple regression was applied to add topographical and geographical parameters as independent variables into a regression equation and develop linear estimation models for both climate parameters. These models are subjected to residual correction using different local interpolation methods, in an attempt to refine the estimated values. The validity of these models is checked through cross-validation error statistics against an independent test subset of station data. The topographical and geographical parameters used as independent variables in the multiple regression models are mostly those found to be strongly correlated with both climatic variables. Models perform best for annual and spring temperatures and effectively for winter and autumn temperatures. Summer temperature spatial variability is rather poorly simulated by the multiple regression model. On the contrary, best performance is obtained for summer and autumn precipitation while the multiple regression model is not able to simulate effectively the spatial distribution of spring precipitation. Results revealed also a relatively weaker model performance for precipitation than that for air temperature probably due to the highly variable nature of precipitation compared to the relatively low spatial variability of air temperature field. The correction of the developed regression models using residuals improved though not significantly the interpolation accuracy. [ABSTRACT FROM AUTHOR]

The present study aims at forecasting hard wheat ( Triticum turgidum L. var. durum) yield in northern Greece, a season prior to harvest. It is based on (a) crop simulated, with CERES-Wheat indicators at four planting dates and (b) reported crop yields at two regional levels (three NUTS2 [ Nomenclature of Units for Territorial Statistics] and 16 NUTS3 regions), for the years 1979-2006. Principal component analysis (PCA) was applied to explore major patterns of joint variability in 20 crop simulated agroclimatic indicators of the growing season before harvest. Stepwise regression and hindcast were employed for the selection of the modes identified by PCA as predictors in multivariate linear regression models used for forecasting yield a season ahead of harvest. Forecasting skill varied to a large extent by spatial scale and planting date. When the simulation results aggregated to the larger spatial level (NUTS2), the yield forecasting skill, in terms of R, was rated as high (ranging from 0.48 to 0.73) in three out of four planting dates for Central Macedonia and in one planting ( R = 0.57) for Thrace. Harvest index, nitrogen leaching and related soil water crop simulated output of the previous season, were the most important predictors. No forecasting skill was found in the third NUTS2 region. The performance of the regression models substantially deteriorated at the higher resolution spatial level (NUTS3). In four regions only (including the one where CERES-Wheat was calibrated) yield forecasting skill was moderate ( R > 0.25). The results demonstrate the potential of this approach for regional crop yield forecasting before the beginning of the cropping season. However, crop model calibration is required before its application. [ABSTRACT FROM AUTHOR]

Raingauge data from four meteorological stations in Chalkidiki peninsula (Greece) were used to identify the characteristics of the synoptic circulation patterns associated with cold-season heavy rainfall events and corresponding flooding over the area. Precipitation climatology over the complex topography of Chalkidiki is characterized by limited annual rainfall, but in the occurrence of heavy rainfall episodes daily accumulations are exceptionally high with increased precipitation rates, leading often to severe flooding. Fifty-five cases of high daily accumulations for the period of 1997 to 2010, mostly observed during December and October, were classified into eight clusters by applying S-Mode Factor Analysis and Cluster Analysis to the ERA-Interim grid point reanalysis data. The results revealed that, in most cases, intense rainfall and flooding is produced by synoptic scale disturbances that generate and sustain cyclonic activity over south Italy, the Ionian, and less frequently over the Aegean Sea. In particular, the atmospheric circulation patterns associated with heavy rainfall are characterized by the presence of a southeasterly-easterly low-level humid flow over Chalkidiki in conjunction with the potentially unstable lower troposphere influenced by mid-level cyclonic vorticity advection and enhanced low-level convergence over the complex terrain. [ABSTRACT FROM AUTHOR]

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 the best type and architecture of ANNs-forecasting models, 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, the generalized feed forward networks (GFFN), and the time-lag recurrent networks were developed and tested. Results showed that the best ANNs type performance was achieved by using the GFFN 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. [ABSTRACT FROM AUTHOR]

The present study examines the variability of the precipitation regime across the Ionian Islands complex, Greece (Central Mediterranean), for a period spanning more than one century. Significant negative long-term linear trends in the annual precipitation totals are observed, more pronounced in the southern parts of the studied area, while a climatic discontinuity possibly occurred during the 1970s, manifested first in the southern Ionian. Statistically significant nonlinear trends and subdecadal intermittent oscillations were detected using Monte Carlo singular spectral analysis. The correlation of the winter precipitation variability at Ionian complex with the North Atlantic Oscillation anomalies was also investigated and extended in the frequency domain. Finally, future projections were performed using an ensemble of Regional Climatic Models. Model simulations suggested a decrease of the order of ∼20% or more in the mean annual precipitation of the area by the end of the century. [ABSTRACT FROM AUTHOR]

The goal of this study is the analysis of heat waves and their impact on humans, using human biometeorological indices, which are based on the energy balance of the human body. The implications for humans are not only described through the intensity of the heat waves, but also through their duration over consecutive days. Both intensity and duration were analyzed for the Greater Athens Area during the period 1955 to 2001. The analysis was carried out using the daily physiologically equivalent temperature and the daily minimum air temperature. Based on these two parameters, the results showed an increase in the average duration of heat waves. Furthermore, the use of the Gaussian filter revealed the intra-annual variation of heat stress conditions and their relevance to humans. The results could be used for the management of the negative consequences of heat waves in cities suffering from environmental pollution and also for climate impact studies. [ABSTRACT FROM AUTHOR]

n extensive validation of two of the most popular and recently upgraded satellite rainfall products, 3B42 and 3B42RT, was performed over the Evros catchment in southeastern Europe using data recorded from January 2000 to April 2009. For conducting this validation study, the Climate Prediction Center's (CPC) ground data were used. The satellite data products were aggregated to daily time series, remapped to spatial resolution of 0.5°, validated against CPC, and intercompared using a variety of statistical indices and coefficients. After the validation process, all three data sets (CPC, 3B42, and 3B42RT) were separately fed in a statistical rainfall-runoff model, in order to predict the five major recorded flood events which occurred in the Evros catchment during the last decade. It has been found that post-calibration with ground data, which is present only in 3B42 product, is a necessity for operational flood forecasting and similar studies conducted in areas at mid-latitudes. Knowledge of rainfall events with small intensities is crucial for estimating the total rainfall height and drastically improves the skill of the satellite product. [ABSTRACT FROM AUTHOR]

The present study aimed to investigate the potential of possible rain area delineation schemes based on the enhanced infrared spectral resolution of the Meteosat Second Generation-Spinning Enhanced Visible and Infrared Imager. The proposed schemes use the brightness temperature (BT) Τ along with the brightness temperature differences (BTDs) Τ − Τ, Τ − Τ, and Τ − Τ as spectral cloud parameters. Two different methods were used to develop the rain area delineation models. The first is a common threshold technique in the multispectral space of the spectral cloud parameters, and the second is an algorithm based on the probability of rain (PoR) for each pixel of the satellite data. Both schemes were trained using as rain information gauge data from 41 stations in Greece for 107 rainy days, covering a period of 1 year. As a result, one single-infrared model (TB), three two-dimensional (BTD, BTD, and BTD), and two multidimensional models (BTD and PoR) were constructed and verified against an independent sample of rain gauge data for four daily precipitation events. It was found that the introduction of BTDs as additional information to a model works in improving the discrimination of rain from no-rain events compared with the single-infrared model BT. During the training phase, BTD exhibited the best performance among the threshold techniques, while the PoR model outperformed all the threshold techniques, producing scores slightly better than those of BTD model. When verifying against the independent dataset, all models exhibited the same performance with that of the dependent dataset according to the ETS score but less skill according to the HK score. The proposed techniques, however, still perform better than the single-infrared technique but with different ranking; BT performs best followed by PoR and BTD. Finally, two case studies are presented to gain a visual impression of the performance of the new developed rain area delineation schemes, showing the effectiveness of BTD in delineating the thicker part of cirrus clouds and of PoR in detecting rain from the low-level thick clouds. [ABSTRACT FROM AUTHOR]

Six widely available satellite precipitation products were extensively validated and intercompared on monthly-to-seasonal timescales and various spatial scales, for the period 1998–2006, using a dense station network over Greece. Satellite products were divided into three groups according to their spatial resolution. The first group had high spatial (0.5°) resolution and consists only of Tropical Rainfall Measuring Mission (TRMM) products: the TRMM Microwave Imager (TMI) precipitation product (3A12) and the TRMM multisatellite precipitation analysis products (3B42 and 3B43). The second group comprised products with medium spatial (1°) resolution. These products included the TRMM 3B42 and 3B43 estimates (remapped to 1° resolution) and the Global Precipitation Climatology Project one-degree daily (GPCP-1DD) analysis. The third group consisted of low spatial (2.5°) resolution products and included the 3B43 product (remapped to 2.5° resolution), the GPCP Satellite and Gauge (GPCP-SG) product, and the National Oceanographic and Atmospheric Administration Climate Prediction Center (NOAA-CPC) Merged Analysis (CMAP). Rain gauge data were first gridded and then compared with monthly and seasonal precipitation totals as well as with long-term averages of the six satellite products at different spatial resolutions (2.5°, 1°, and 0.5°). The results demonstrated the excellent performance of the 3B43 product over Greece in all three spatial scales. 3B42 from the first and second group and CMAP from the third exhibited a reasonable skill. [ABSTRACT FROM AUTHOR]

Two discontinuities were detected in the air-temperature time series at the meteorological station of the National Observatory of Athens. The first discontinuity reflects the instrumental change, which took place in June 1995 and the second discontinuity (and most pronounced) the application of a correction factor to the temperature values (in January 1997), after a calibration of the new thermometers. As a result, a cooling bias was observed after June 1995 and a warming bias after January 1997. The magnitude of bias exhibited a seasonal variability being more pronounced and reaching up to 0.67°C during the warm period of the year. The common period of operation of the ‘old’ and ‘new’ instrumentation was used for the estimation of monthly correction factors and the removal of the bias. The application of the new correction factors, restore the continuity in the air temperature record after June 1995. [ABSTRACT FROM AUTHOR]

A discriminant analysis is applied in order to determine the relationships between circulation types in the middle troposphere and prevailing weather types over two major Greek cities, Athens and Thessaloniki. In order to describe the synoptic conditions, an automatic classification scheme for the Greek region is used. For each circulation type identified (14 in total), several meteorological parameters at the 500 hPa level are calculated such as geopotential heights and their anomalies, temperature and relative vorticity. Weather group-types that reflect the conditions at the surface, were previously defined using a two-step cluster analysis. These types result from a combination of five meteorological parameters—maximum temperature, precipitation amount, relative humidity, wind velocity and sunshine duration. The study period is 43 years long (1958–2000) and is restricted to the cold and wet period of the year, from December until March. For Athens, six weather types are developed, whereas for Thessaloniki five are produced. By means of a stepwise discriminant analysis (DA) model, the most important variables from the 500 hPa level are found and are used to generate the necessary functions that can discriminate weather types over the two stations. The aim of the present study is first to discriminate weather types effectively and to identify the most important discriminating variables, and second, to connect these weather types to elements of the prevailing synoptic pattern, through mathematical functions provided by DA. The results of the evaluation of the aforementioned procedure are considered to be very satisfactory. [ABSTRACT FROM AUTHOR]

Tropical days (TD) are defined as the days with a maximum air temperature greater than 30.0 °C. It is clear that the study of TD includes also the absolute maximum temperatures, which are of great interest for the description of a region’s climate. These days are considered as very hot, and they particularly are of great importance not only for bioclimatology and applied sciences, but also for the individuals who are sensitive in the heat-stress. The regime of the TD in Greece is the focus of this study. The aim is to demonstrate their changes from decade to decade, for the time period 1960–2000. For this study, the Annual Number of Tropical Days (ANTD) recorded by each of the 26 meteorological stations of National Meteorological Service, which are uniformly distributed in the Hellenic peninsula, was calculated and analysed. In terms of quantifying the conditions in a humanbiometeorological manner, the thermal index Physiological Equivalent Temperature (PET) and the consecutive days for Athens have been included in this study. The trends of the TD for each station were analysed through the Mann–Kendall technique, while the spatial distribution per decade reveals the regions with change (increase or decrease) in the ANTD during the examined period. Two characteristic periods of change for the ANTD appear in the majority of the meteorological stations in Greece. The first period (1955–1976) is determined by a negative trend, which is statistically significant (c.l. 95%), for adequate stations. In the period between 1976 and 2000, the increase in the ANTD and the maximum temperature exceed the corresponding maximum that appeared in the beginning of the 1950s for several of the examined meteorological stations. The human-biometeorological analysis shows that the consecutive days of PET > 35 °C have had a positive trend in the last two decades of the last century. [ABSTRACT FROM AUTHOR]

(1) The main objectives of this study are the spatial and temporal analysis of annual 24-hour extreme precipitation amounts for different return periods, and (2) the interannual variability of the frequency of month-to-month maximum rainfall using harmonic analysis. Long time series data from 110 well-scattered rain gauges are analysed using extreme values theory and harmonic analysis techniques. The results indicate that the Jenkinson solution is more appropriate than the linear Gumbel solution for the extreme value law. The maximum expected values of 180–200 mm are observed along the southern coasts of the Greek peninsula and in the islands close to the Turkish coast, as well as in the north–west parts of the country. The harmonic analysis showed that the highest percentage of variance for rainfall extremes explained by the first harmonic (PVR(1)) appears over the southern and south-eastern parts of Greece in association with the greatest values of the amplitude of this harmonic, therefore explaining more than 90% of the variation of the frequency of maximum annual 24-hour rainfall amounts. In contrast, for regions north of 38° N and especially mountainous regions, more harmonics (3–5) are necessary to describe the parameter under investigation. [ABSTRACT FROM AUTHOR]

Statistical downscaling techniques have been developed for the generation of maximum and minimum temperatures in Greece. This research focuses on the four conventional seasons, and three downscaling approaches, Multiple Linear Regression using a circulation type approach (MLRct), Canonical Correlation Analysis (CCA) and Artificial Neural Networks (ANNs), are employed and compared to assess their performance skills. Models were developed individually for each variable (Tmax, Tmin), station and season. The accuracy of downscaled values has been quantified in terms of a number of performance criteria, such as differences of the mean and standard deviation ratios between observed and modelled data, the correlation coefficients of the two sets, and also the RMSEs of the downscaled values relative to the observed. All methods revealed that during the cool season Tmax tends to be better reproduced, whereas Tmin is overestimated, particularly over western Greece, which is characterised by higher orography. With respect to the warm season, the simulation of Tmax reveals greater divergence, whereas Tmin is better generated. The distinction between the three techniques is somewhat blurred. None of the methods were found to be superior to the others and each has been shown to be a good estimator in some cases. This study concludes that all proposed methods comprise useful tools for simulating daily temperatures, as the high correlation coefficients, between observed and downscaled values, have demonstrated. However, the importance of local factors, which affect the natural variability of temperature, has been emphasised indicating that the geography of a region constitutes an important and rather complex factor, which should be included in models to improve their performance. [ABSTRACT FROM AUTHOR]

In the present study the Standardised Precipitation Index (SPI) is used to detect drought events in spatial and temporal basis. Using monthly precipitation data from 23 stations well spread over Greece and for a period of 51 years, a classification of drought is performed, based on its intensity and duration. Results indicate that, mild and moderate droughts reduce from north to south and from west to east on the 3- and 6-months time scale, while for the class of severe drought, the frequencies in the southern part of Greece are higher than in the other parts of the country. Furthermore the frequency of occurrence of severe and extreme drought conditions is very low over the whole Greek territory on the 12-month running time scale. Finally SPI was compared to the “de Martonne aridity index (I)” and a satisfactory correlation between them was found. [ABSTRACT FROM AUTHOR]

The evolution of synoptic scale atmospheric circulation at least two days before the occurrence of thunderstorm or shower activity with high precipitation amounts is examined for the months of July and August for the area surrounding the city of Thessaloniki in Northern Greece. In a period of sixteen years (1985–2000), seventeen cases with precipitation amounts greater than 10 mm were found in these two months. The most important factor for the occurrence of the above activities was the approach of cold air, usually of mP origin, into Northern Greece. The advection of cold air was examined at the 700 hPa level. This cold air was advected toward the southern Balkans either from Northwestern Europe, or more directly from higher latitudes. In a few cases cold air that had initially been advected into the Central Mediterranean region finally moved eastward affecting Northern Greece. From these seventeen cases, the first four with the highest precipitation amounts (all exceeding 20 mm) were analysed in detail with the help of weather maps and radiosonde data available for the station at Micra. For these four cases, apart from the detailed analysis of the cold air advection, an attempt was made to investigate whether in the lowest part of the troposphere, the area of Northern Greece and the interior of the Balkan Peninsula to the north, was dominated by warm and humid air in the twenty four hour period that preceded each thunderstorm outbreak. For this purpose the evolution of the spatial distribution of the equivalent potential temperature (Θe) at the 850 hPa level was determined for the two or three preceding days. Finally, the presence of mesoscale disturbances, that could be considered responsible for triggering the ascending motions that are necessary for the initiation of the thunderstorm development, was investigated for each case. [ABSTRACT FROM AUTHOR]

In this study, the temporal and spatial distribution of bright sunshine hours and relative sunshine duration over Greece are presented. The datasets used for this study were obtained from the archives of the Hellenic National Meteorological Service (HNMS). Furthermore, mean annual and seasonal duration of bright sunshine has been estimated from empirical formulae, which depend on the following parameters: (i) percentage of land cover around each station (radius of 20 km), (ii) distance of each station from the nearest coast, (iii) height above sea level for each station location, (iv) latitude of each station, (v) longitude of each station. Differences between measured and estimated bright sunshine hours are accounted for. In general a good relationship exists between estimated and observed sunshine values. The annual march of sunshine is simple with a maximum in July and a minimum in January or December. The spatial distribution of the annual and seasonal bright sunshine duration shows minimum values in the interior mountain areas of the Greek region (Western Macedonia, Epirus, Central Greece), increasing gradually towards the coasts of the Ionian and Aegean seas as well as from north to south. The highest sunshine values occur at the southeastern islands of the Aegean Sea and above the southern coasts of Crete, followed by Attica (Athens area) and the surrounding coastal areas, the islands of the eastern Aegean Sea and the southwestern coastal and island parts of the Ionian Sea. [ABSTRACT FROM AUTHOR]

This study examines the characteristics of wet spells in Greece, using daily rainfall gauge data, over a 40-year period (1958–1997). The longest wet spells, computed for two different thresholds (0.1 mm and 1 mm), were observed in Western Greece and Crete, whereas the shortest ones were found in the central and south Aegean. A detailed analysis of the wet spells, dividing them into three classes, shows that on an annual basis their highest frequencies were observed in Western and Northwestern Greece. The seasonal results are also quite similar. The trends and the variability of the mean length of the wet spells were also analyzed. Negative trends were found in the case of the year-to-year analysis and during winter. Finally, this study presents an evaluation of two theoretical distribution models; the second order Markov Chains (MC2) and the Negative Binomial Distribution (NBD), by their adjustment to the empirical data. Both models can be used in the future for an estimation of the wet spells in the area under study. [ABSTRACT FROM AUTHOR]

In this study, trends of annual and seasonal surface air temperature time series were examined for 20 stations in Greece for the period 1955-2001, and satellite data for the period 1980-2001. Two statistical tests based on the least square method and one based on the Mann-Kendall test, which is also capable of detecting the starting year of possible climatic discontinuities or changes, were used for the analysis. Greece, in general, shows a cooling trend in winter for the period 1955-2001, whereas, summer shows an overall warming trend, however, neither is statistically significant. As a result, the overall trend of the annual values is nearly zero. Comparison with corresponding trends in the Northern Hemisphere (NH) shows that temperatures in Greece do not follow the intense warming trends. Satellite data indicate a remarkable warming trend in mean annual, winter and summer in Greece for the period 1980-2001, and a slight warming trend in annual, spring and autumn for the NH. Comparison with the respective trends detected in the surface air temperature for the same period (1980-2001) shows they match each other quite well in both Greece and the NH. The relationship between temperature variability in Greece and atmospheric circulation was also examined using correlation analysis with three circulation indices: the well-known North Atlantic Oscillation Index (NAOI), a Mediterranean Oscillation Index (MOI) and a new Mediterranean Circulation Index (MCI). The MOI and MCI indices show the most interesting correlation with winter temperatures in Greece. The behaviour of pressure and the height of the 500?hPa surface over the Mediterranean region supports these results. [ABSTRACT FROM AUTHOR]

Studies the existence of the heat island effect in urban places in Athens, Greece. Thermal structures between the urban and rural environments; Measurement of climatic parameters; District variations of the heat island intensity.