The objective of this work is to investigate the seasonal and daily lightning activity and the predominant, large-scale circulation regimes and their influence to the spatial lightning distribution in the eastern Alps. For this purpose seasonal and daily lightning climatologies for the summer 2010-2015 are created using cloud-to-ground lightning data from ALDIS, whereas for the spatial lightning distributions intra-cloud lightning strokes are used additionally. Circulation patterns for Europe are identified with an EOF-analysis combined with a Gaussian Mixture Model (GMM) using 00Z ECMWF analysis data with a 16km x16km resolution of the 700 hPa geopotential field. The accompanying spatial lightning distributions are computed on the same grid with Generalized Additive Models (GAMs) in order to increase the signal-to-noise ratio of the relatively small sample size and to obtain full distinctive lightning distributions. Composite maps of CAPE, lapse rate, wind shear, mean sea-level pressure and 2m dew point temperatures to give a comprehensive synoptic explanation of the spatial lightning patterns.
During strong south-westerly flow lightning probabilities reach 45% in the Italian pre-Alps and 30% in the south-eastern parts of Bavaria and the Austrian foreland. When southerly flow is present, thunderstorms frequently occur in the Italian foreland and northward to the south-eastern parts of Austria. Predominating westerly and north-westerly wind directions favor higher lightning activity south of the Alpine crest, whereas foehn inhibits the formation of thunderstorms in the central Alps. When weak south-westerly flow is present, higher lightning 18 UTC activity is found at in the Central Alps and the Bavarian foreland, likely due to a northward propagation of the thunderstorms originated in the northern Alps. The main trigger mechanisms for thunderstorms are approaching cold fronts and thermal heating during the day.
