The climatology of (severe) thunderstorm days is investigated on a pan-European scale for the period of 1979–2017. For this purpose, sounding measurements, surface observations, lightning data from ZEUS (a European-wide lightning detection system) and European Cooperation for Lightning Detection (EUCLID), ERA-Interim, and severe weather reports are compared and their respective strengths and weaknesses are discussed. The research focuses on the annual cycles in thunderstorm activity and their spatial variability. According to all datasets thunderstorms are the most frequent in the central Mediterranean, the Alps, the Balkan Peninsula, and the Carpathians. Proxies for severe thunderstorm environments show similar patterns, but severe weather reports instead have their highest frequency over central Europe. Annual peak thunderstorm activity is in July and August over northern, eastern, and central Europe, contrasting with peaks in May and June over western and southeastern Europe. The Mediterranean, driven by the warm waters, has predominant activity in the fall (western part) and winter (eastern part) while the nearby Iberian Peninsula and eastern Turkey have peaks in April and May. Trend analysis of the mean annual number of days with thunderstorms since 1979 indicates an increase over the Alps and central, southeastern, and eastern Europe with a decrease over the southwest. Multiannual changes refer also to changes in the pattern of the annual cycle. Comparison of different data sources revealed that although lightning data provide the most objective sampling of thunderstorm activity, short operating periods and areas devoid of sensors limit their utility. In contrast, reanalysis complements these disadvantages to provide a longer climatology, but is prone to errors related to modeling thunderstorm occurrence and the numerical simulation itself.
The Etesians (northern sector winds), which blow over the Aegean Sea during summer, affect human activities in the area. The numerous islands of the Aegean and especially Crete (a mountainous island in the southern Aegean oriented perpendicular to the surface flow) seem to play an important role in the modification of the wind field during the Etesians. The Crete mountain ranges, surrounded as they are by water, are an excellent example of a major isolated topographic feature which significantly modifies the regional airflow and pressure; however, this modification can hardly be defined due to the lack of observing stations over the sea. For this reason, the available land surface and ship synoptic observations are used, together with ERS scatterometer wind data in order to identify the regions over the Aegean where the wind reaches its maximum intensity, and to assess the influence of Crete on the wind field. Moreover, numerical modelling is used to provide some further insight on the orographically disturbed wind flow. Sensitivity tests performed with the hydrostatic model BOLAM show that the interaction of the Etesian wind flow with the mountains of Crete produces deceleration of the Etesians up to almost 120 km upstream, the leftward deflection of the air as it approaches the mountains, and the associated intensification of the flow east of the island.
During the period 21-22 January 2004, an explosive cyclogenesis event occurred over the Aegean Sea. The minimum observed pressure was 972 hPa, a value which is among the three lowest observed over the entire Mediterranean Sea during the last 40 years. This paper is devoted to the investigation of the conditions that contributed to the rapid development of this low-pressure system through analysis of both observations and model results. It was found that the rapid development of the cyclone was associated with a two-trough system that, under the influence of a very intense upper-level jet, merged into a single trough and then acquired a negative tilt. Sensitivity tests with the MM5 model showed that the upper-level dynamic forcing was the main factor that led to the explosive cyclogenesis, while surface sensible and latent heat fluxes contributed to the cyclone deepening mainly during the storm's mature phase.
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