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Α new method, based on the definition of weather types (WTs), for the determination of seasons in the Mediterranean region is adopted. This method combines principal component analysis and k‐means Cluster Analysis on daily grid point meteorological data for the 70‐year period 1949–2018 obtained from the NCEP/NCAR database. At first, the method is applied to the classification of WTs and subsequently to the definition of seasons based on the mean intra‐annual variations of the WTs' frequency. The resulting seasons are compared to a previous method of the seasons' definition, based on the long‐term mean intra‐annual variations of climatological parameters. Furthermore, a synthesis of the two methods is proposed. The analysis is applied not only to the 70‐year period 1949–2018, but also to the five overlapping 30‐year sub‐periods 1949–1978, 1959–1988, 1969–1998, 1979–2008, 1989–2018 in order to investigate possible long‐term changes of the seasons' characteristics. Four seasons are identified for all periods and for both methodological approaches. These seasons broadly correspond to the four conventional ones, but they differ in their onset and cessation dates as well as their duration. For the composite method, it is found that winter lasts about 4 months, summer is a little longer than 3 months and spring and autumn last around 2.5 months. The most remarkable findings regarding the seasons' characteristics are: (a) the start/end dates and duration of winter are almost identical in all three approaches, while summers' characteristics differ; (b) shortening of winter and spring in recent periods, associated with the delayed start and expedited end dates, respectively; and (c) the prolongation of autumn and summer, due to the delay of its end date, and the earlier onset and delayed end dates, respectively. [ABSTRACT FROM AUTHOR]

An objective definition of seasons for the Mediterranean region is performed using the mean intra‐annual variations of 12 climatological parameters. The aim is to achieve a climatologically appropriate determination of the onset and cessation dates, as well as the duration, of each season, better than the conventional seasons (winter, spring, summer and autumn). Moreover, possible changes of the defined seasons during the 70‐year study period 1949–2018 are investigated over the climatologically sensitive Mediterranean region. The data used are daily NCEP/NCAR Reanalysis grid point values of precipitation rate, convective precipitation rate, 2 m temperature, total cloud cover, 2 m zonal and meridional wind, 500 and 1,000 hPa geopotential height, 500 and 850 hPa temperature, 850 hPa specific humidity and precipitable water over the Mediterranean region, for the period 1949–2018. Firstly, Principal Component Analysis is applied to the mean intra‐annual variations of the above parameters in order to reduce the dimensionality and then k‐means Cluster Analysis is applied to the resultant Principal Components in order to group dates with similar regimes of the above parameters. This methodology is applied for the 70‐year period 1949–2018 as well as for the five overlapping 30‐year subperiods 1949–1978, 1959–1988, 1969–1998, 1979–2008, and 1989–2018. According to the results, four seasons are defined for all periods. Although these seasons correspond to the four conventional seasons, there are differences regarding the onset and cessation dates and the duration. In general, it is found that winter lasts about 4 months, spring and summer are a little shorter than 3 months and autumn lasts about 2.5 months. The most remarkable long‐term changes are: (a) the warming during the last 30‐year period, which agrees with the ongoing climate change, (b) the shortening of winter and spring due to the delay of their onset date, and (c) the extension of autumn due to the delay of its cessation date. [ABSTRACT FROM AUTHOR]

Rain monitoring through signal attenuation has been a significant concern for the scientific community over the last decade. The lowest L-, S- and C-bands in the microwave spectrum are full of potential for environmental monitoring devices. It has not been extensively proven whether signal attenuation can be used for rain estimation throughout the microwave spectrum in the region of 1-6 GHz. The study derives a power-law rain estimation model by validating the one year data for the amount of rain and signal attenuation at the lowest limit of the S-band. This is achieved by a prototype accurate experimental setup in a laboratory environment at the campus of the University of Ioannina, northwest Greece. A comparison with global rain attenuation models as well as measurements near this frequency band are also presented and discussed. [ABSTRACT FROM AUTHOR]

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 National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP-NCAR) re-analysed mean sea-level pressure (MSLP), geopotential height at 700 hPa and surface meridional wind, along with meteorological data at Zabol, east Iran and Meteosat/Infrared Difference Dust Index (IDDI) retrievals. The analysis focuses on the summer period (June to September) of 2000-2014 and the winter period (November to March) of 1963-2014. The CasHKI values are mostly controlled by the MSLP anomalies over the Caspian Sea (CS) domain, varying from approximately -25 to +35 hPa in winter and from approximately-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 CS, 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 SWAsia, 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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

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-h period starting at 12:00 UTC 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 850 hPa air temperature and 1000 and 500 hPa 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 ten clusters. For each one of the ten clusters, the mean 00:00 UTC patterns of the above parameters are constructed for 3 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 ten clusters refer mainly to the position and the intensity of the surface and upper air synoptic systems involved. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The article presents a study on the spatial and temporal variability and co-variability and the teleconnection patterns of downward solar radiation (DSR) at the surface of the Earth. The study uses the multivariate statistical method of Factor Analysis (FA) on the DSR data that was computed with a physical deterministic transfer model. Furthermore, the result shows a strong variability of DSR due to the complex interactions of solar radiation with the atmosphere and surface of the Earth.

The article reports on the diurnal variations of illuminace levels in Athens, Greece from 1992-1996. It mentions that the spatial and temporal variations depend on latitude, orography, and synoptic conditions. It discusses that longer database is necessary to have a more accurate study on the diurnal variation of illuminance. It also notes that the Factor Analysis and Cluster Analysis are used in the global horizontal illuminance data as multivariate statistical method.

The article presents a study that measures the intra-annual variation of the atmospheric circulation and detecting potential seasonal oddments in the Mediterranean regions. It uses the geostrophic vorticity and analysis is enforced for lower troposphere and middle troposphere. The vertical homogeneity of the troposphere is tested as the vertical structure of depressions in the regions depicts an important spatial and seasonal variability. Moreover, the thermal influence of the surface of the Earth appears to be weaker, while the blocking activity and/or cut-off lows have also significant roles.

The variability of sensible and latent heat fluxes in the Mediterranean Sea and their effect on precipitation in the Greek area during the winter months are investigated for the 39 year period 1959–97 by using multivariate statistical methods. First, factor analysis is applied, mainly as a data reduction tool, and then canonical correlation analysis is applied on the factor scores time series of the two pairs, i.e. sensible heat flux–precipitation and latent heat flux–precipitation, to reveal their interrelations. The results show there is a statistically significant relationship between sensible and latent heat fluxes over the western Mediterranean Sea and precipitation in the western windward areas of Greece. The sensible and latent heat fluxes in the western Mediterranean Sea contribute significantly to depression development. The depressions formed advect cold and dry air masses over the western Mediterranean, reinforcing these sensible and latent heat fluxes. Furthermore, these depressions lead to a southerly or southwesterly airflow over Greece. This flow, along with the existence of potential instability, significantly contributes to precipitation formation in the windward areas of western Greece and also in the islands of the eastern Aegean Sea. Copyright © 2004 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

In this work, the relationship between atmospheric circulation over Europe and precipitation in Greece is studied during high winter (January–February). Specifically, the 850 hPa 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 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 western and northern Greece, the eastern Aegean islands and the western Turkish coasts is mainly controlled by vorticity over Italy and the Gulf of Genoa; (ii) southern Aegean Sea and Crete precipitation is controlled by relative vorticity west of Crete; and (iii) eastern mainland areas precipitation is significantly affected by a vorticity seesaw between Tunisia and the 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 causing these specific patterns. Copyright © 2003 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

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; 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 northwestern 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; 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 northwestern and northeastern 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; finally, the third one is the ?autumn? pattern, with the maximum in northwestern Greece. 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 cyclogenesis factor, and the windward or leeward character of the various areas.

Studies the spatial and temporal covariability between the lower troposphere and sea surface temperatures (SSTs) in the Mediterranean basin for the period 1958-98. Details of study using monthly air temperature anomalies for the 850 hPa pressure level and SST anomalies; Results suggesting the correlation between T-850hPa and SST appears higher over the eastern Mediterranean; Results indicating the existence of a one month time scale SST persistence is detected for winter months.

In this work, the variability and covariability of winter temperatures 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) 500–700 hPa and 700–l000 hPa thickness and (b) air temperature on the isobaric surfaces of 500 hPa and 700 hPa. At first, factor analysis (FA) defined areas with characteristic temperature variability in each layer (and on each isobaric surface) and then, canonical correlation analysis (CCA) revealed areas in both layers (and on both isobaric surfaces) presenting common temperature variations. A temperature see-saw between N Europe and W 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 the 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. Copyright © 2001 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

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 of factor analysis. The analysis is applied to the winter months of January and February and to the summer months of July and August. In winter, the parameters are classified into four groups as follows: (i) maximum, mean and minimum temperature and, negatively, pressure and number of frost days; (ii) wind and, negatively, diurnal temperature range; (iii) precipitation; and (iv) cloudiness and relative humidity. In summer, the parameters are classified into three groups, namely: (i) precipitation, cloudiness and, negatively, minimum temperature; (ii) maximum and mean temperature, relative humidity; and (iii) wind. From the above groups the most interesting is group (i) of summer, as its parameters belong to different groups in winter. 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. Copyright © 1999 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

In this paper, using 20 year mean monthly precipitation totals from 54 stations, the general statistics for the time and space distribution of precipitation in northwest Greece is at first described. Precipitation varies from about 1000 mm at the coast to 2500 mm on the 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 northwest 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 northwest Greece is characterized by four precipitation regimes, depending on the combination of depression activity, the effects of the sea and of topography. Copyright © 1999 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

This retrospective study aims to search out the influence of temperature, atmospheric pressure and humidity on the frequency of epistaxis. The study includes 701 patients who have suffered from epistaxis and 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 epistaxes 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 factors. This percentage was found to approach 20% for the cold period and 10% for the warm period of the year. [ABSTRACT FROM AUTHOR]