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Mesoscale triggering of the heavy precipitation event in the southern Adriatic area

The event analyzed in this paper is geographically focused on the southern part of the eastern Adriatic region, which is prone to relatively frequent heavy precipitation events in the autumn and winter. This area belongs to the rainiest parts of Europe. The annual amounts of precipitation higher than 5000 mm can be expected in the mountainous hinterland. The aim of this study is to identify the main mesoscale features and mechanisms responsible for the generation of an extreme precipitation event, as a contribution to understanding the factors which cause that climatic maximums of the annual precipitation amounts located over this region. The event occurred during the morning hours on 22 November 2010 over Dubrovnik coast, Croatia and the hinterland mountain range of the southern Dinaric Alps. A peak intensity of 145.5 mm was registered in the four-hour period. The mechanisms responsible for the formation of convection have been analyzed through synop measurements, satellite data and numerical experiments performed with the WRF model, which was set up at the convection-resolving resolution in the innermost domain. The analysis of meteorological products derived from the satellite data provided valuable information especially over the sea, where no other observation data were available. The numerical simulations highlighted the essential role of southerly low-level jet stream (LLJS) in the transport of warm and moist air towards the affected area. The convergence of two branches of low-level marine air favored convection triggering over the coast and sea. Persisting for about four hours, the convergence line contributed to large precipitation amounts in the Southern Adriatic area. It was found that the development of the precipitation system is due to the positive temperature and PV advection associated with very strong large-scale ascent over the southern Italy and southern Adriatic. The maximum of the upward motion extended vertically from the surface throught the entire troposphere, indicating an extremely strong process. In addition, numerical sensitivity experiments suggested that the orography of Dinaric Alps has an essential role for precipitation maximum over the mountainous hinterland, but also that the orography was not the crucial factor for the heavy precipitation near Dubrovnik. This study has highlighted the need for a dense network of observations, especially radar measurements, to validate the simulated mechanisms and improve the numerical forecasts through data assimilation.

heavy precipitation; convection; convergence line

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