Abstract
The aim of this study was to identify the main mesoscale features and mechanisms responsible for the generation of an extreme precipitation event as a contribution to improving the modelling of processes that produce HPEs. The event occurred during the morning hours on 22 November 2010 over the Dubrovnik coast in Croatia and the hinterland mountain range of the southern Dinaric Alps and caused severe flash floods and landslides and consequent interruption of traffic and electricity supply as well as other infrastructural damage. The analysis is geographically focused on the southern portion of the eastern Adriatic region, which is prone to relatively frequent heavy precipitation events that occur mostly in autumn. This area is one of the rainiest in Europe with expected annual amounts of precipitation greater than 5,000 mm in the mountainous hinterland. The mechanisms responsible for the formation of convection were analysed using synop measurements, satellite data and numerical experiments performed with the WRF model, which was set up at the convection-permitting resolution in the innermost domain. Satellite data were used to identify the precipitation systems and to estimate the intensity of the precipitation during the period of interest. The development of the precipitation system was connected to a strong large-scale ascent over the southern Italy and southern Adriatic due to the advection of warm air and cyclonic vorticity advection, which increases with height. The numerical simulations highlighted the essential role of a southerly low-level jet stream in the transport of warm and moist air towards the affected area. The convergence of two branches of low-level marine air favoured convection triggered over the coast and sea. Furthermore, numerical sensitivity experiments suggested that the orography of the Dinaric Alps plays an essential role in the precipitation maximum over the mountainous hinterland, but also that the orography was not the crucial factor in the heavy precipitation near Dubrovnik. This study highlights the need for a dense network of observations, especially radar measurements, to validate the simulated mechanisms and improve numerical forecasts via data assimilation.
Highlights
Natural hazards, among which high-impact precipitation events play an important role, can heavily influence the population as well as the infrastructure and economy of the endangered areas
The analysis is geographically focused on the southern portion of the eastern Adriatic region, which is prone to relatively frequent heavy precipitation events that occur mostly in autumn
The measured rainfall amounts at the Dubrovnik meteorological station were compared with the expected annual precipitation maxima for different return periods estimated according to the generalised extreme value (GEV) distribution based on the long-term data series
Summary
Among which high-impact precipitation events play an important role, can heavily influence the population as well as the infrastructure and economy of the endangered areas. The complex orographic structure in this area favours the lifting of low-level unstable air and initiation of the condensation processes These systems produce heavy short-term precipitation with annual maxima that mostly occur in the period from August to November for durations up to 2 h (Table 1). Approximately 20 % of the annual precipitation total in the Dubrovnik coastal area is due to heavy precipitation that falls on very wet days These events are more frequent and more intense with increasing distance from the coast and increasing altitude (Gajić-Čapka 2010). The measured rainfall amounts at the Dubrovnik meteorological station were compared with the expected annual precipitation maxima for different return periods estimated according to the generalised extreme value (GEV) distribution based on the long-term data series. Their return periods range from approximately 70 years (for the 6-h maximum) to 160 years (for the 4-h maximum)
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