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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, examining the trends at a local scale, spatial and temporal patterns are revealed, 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 are detected, 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 anti-correlated (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.

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−1, but it has a strong spatial variability ranging from 20 mm year−1 (North Africa) to 1500 mm year−1 (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−1. 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.

International audience; 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 (June-July-August-September, 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 connection between atmospheric circulation over Europe and snowfall in Ioannina, Northwestern (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 and 1000 hPa geopotential height, 500 and 850 hPa air temperature, 1000–500, 1000–700 and 700–500 hPa thickness are constructed for the European area, whereas static stability K-index, relative humidity at 500, 700 and 850 hPa levels and finally relative vorticity at 500 and 1000 hPa are also used for the Southeastern (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 datasets and the evolutions of the above circulation parameters are classified into eight clusters. Snowfall in NW Greece is generally associated with a low pressure system over the Ionian Sea combined with an anticyclone over Western 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 Western Russia to Italy and the Ionian Sea. The eight 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.

In this work we present an accurate and cost-effective system for measuring S-band signal attenuation due to rainfall. The system is based on a pair of transmitter and receiver, operating at the lower limit of S-band, i.e. 2 GHz, separated at few meters distance and thus at any instant the rain's strength is uniform at this short path. The system is installed in two adjacent buildings inside University of Ioannina campus and at very close distance from the university's meteorological station. We present system's measurement modules as well as operating parameters. Finally, we present preliminary rain measurements for moderate, heavy, very heavy and extreme precipitation rates. Significant results, concerning correlation of the four precipitation rate cases with signal attenuation, are obtained.

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, 850 hPa and 500 hPa air temperatures, 500 hPa geopotential height and 1000–500 hPa thickness are constructed for the European region, for the day before (D-1), the first day (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.

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) 2.5×2.5 grid point values of 500 hPa geopotential height, mean sea-level pressure and 1000–500 hPa thickness at 273 grid points over Europe (10 W to 40 E and 30 N 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 369×273 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 369×819 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.

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. An 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 behavior. 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 publication of this article is sponsored by the Swiss Academy of Sciences.

The Indian summer monsoon (ISM) is a prominent feature of the summer circulation in the Northern Hemisphere (NH) and has been found to modulate the weather and climate conditions in many remote regions. This study investigates the most recurrent patterns of summertime midlatitude circulation, over the eastern Mediterranean (EM) and also globally, that are most associated with the ISM. Monthly data of 44 summers from the ERA40 dataset are used and two multidimensional statistical methods, the Principal Component Analysis (PCA) and Canonical Correlation Analysis (CCA), are implemented. The ISM is found to be related to subsidence anomalies in the middle and more extendedly in the upper troposphere over the central and eastern Mediterranean and with an Etesian-like pattern regarding the field of the lower troposphere winds. An equatorial Rossby wave pattern, extending westward from an ISM heat source up to EM and N. Africa, was identified to be associated with the variability of ISM. The observed relationship between the ISM and the EM circulation features can be attributed to this equatorial Rossby wave response to the monsoon forcing. CCA implementation revealed the interconnection of the aforementioned PCA results with an ISM action center over the northern Arabian Sea and the monsoon trough region. © 2018 by the authors.

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.

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 centred 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 (2 × 2km). 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.

Cases of atmospheric circulation evolution favouring the occurrence of desert aerosol episodes (DAEs) over the broader Mediterranean region were investigated using an objective and dynamic algorithm, with daily satellite data for the period 2000–2013. After identifying strong and extreme DAEs, at a 1°×1° geographical-cell level, 255 dust aerosol episode days (DAEDs) and 148 cases of consecutive DAEDs, namely desert aerosol episode cases (DAECs), are defined. For each DAEC, the evolution of the lower tropospheric circulation 1 and 2 days before, during the initiation and after the cessation of the DAEC, is considered. S-mode factor analysis and k-means cluster analysis are applied on mean sea-level pressure and 700 hPa geopotential height fields obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project, classifying the 148 cases of atmospheric-circulation evolution into six homogeneous and discrete clusters. The mean intra-annual variation of the DAECs reveals a primary maximum in May (18.9%), and their mean annual number is equal to 11.4 DAECs. On a seasonal basis, the highest percentage of the DAECs is found in spring (51.4%). Maximum duration of the DAECs is 7 days, with 58.8% lasting 1 day. Annually, the mean monthly number of the DAEs varies from 35.8 (September) to 58.0 (April). The western parts of the Mediterranean are affected by the DAEs when cyclonic conditions prevail in the western Mediterranean and northwestern Africa. In contrast, 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. As to the mean regional intensity (aerosol optical depth at 550 nm) the strong DAEs vary from 0.67 to 0.77, while the extremes vary from 1.14 to 2.06. Generally, strong DAEs are more frequent than extremes (in five out of six clusters).

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.

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 2001–2012. 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. National Center for Environmental Prediction/National Center for Atmospheric Research 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 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.

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 > $$ \overline{AOD} $$ 500 + 1STDEV (standard deviation)] 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.

Τhe 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.

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.

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 0.5° × 0.5° 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.