Atmospheric circulation patterns associated with extreme precipitation amounts in Greece

Abstract

Abstract. 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.

Trends in Daily Rainfall Intensity Over Israel 1950/1-2003/4

“…Precipitation reduction over most Greek stations, combined with enhanced anticyclonic activity over almost the whole Mediterranean was found during the late 1980s and the early 1990s, a period characterized by high North Atlantic oscillation index values [6]. Time series of extreme precipitation cases in Greece for the period 1970-2002, did not reveal significant positive or negative trend [7].…”

MEDEX: a general overview

“…Regarding the eastern Mediterranean, some studies also relate heavy rain to cyclonic presence (see Xoplaki et al, 2000;Kotroni et al, 2005;Ziv et al, 2006;Houssos et al, 2008;Houssos and Bartzokas, 2006;Tolika et al, 2007;Lagouvardos et al, 2007;among others). Especially over Greece, heavy precipitation amounts and/or heavy snowfall can be related to cold surges that are accompanied by the passage of a fast moving cold front from the north towards the south, and gale force winds, especially in the gaps between mountains in northern Greece and over the Aegean Sea.…”

Atmospheric circulation patterns associated with strong wind events in Catalonia

“…Moreover, given the complexity of atmospheric processes that produce strong wind events and the complex topography of Catalonia, which can intensify this phenomenon, we have adapted the methodology to consider both synoptic and mesoscale factors. Therefore, to consider weather variability, several atmospheric levels has been chosen: mean sea level pressure (SLP), temperature at 850 hPa (T850) and geopotential at 500 hPa (Z500) (Martínez et al, 2008;Houssos et al, 2008;Aran et al, 2010). With regard to the complex topography, we have included a matrix of the scalar wind gust values measured with the AWS which manages the SMC ).…”

“…However, multivariate analysis techniques are widely accepted by researchers to elaborate synoptic catalogues . There is also an important line of research that uses these techniques to look for some relationship between atmospheric patterns and extreme meteorological events, such as heavy rainfall (Houssos et al, 2008;Martín Vide et al, 2008;Martínez et al, 2008;Vicente Serrano et al, 2009), extreme summer temperatures and heat waves (García Herrera et al, 2005;Kyselý and Huth, 2008), heavy snow and variability in snow cover (Esteban et al, 2005;López Moreno and Vicente Serrano, 2007), and extreme episodes of avalanches ). These studies both use multivariate statistics like PCA, CA or both methods simultaneously (Esteban et al, 2006).…”