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

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