engleski

Wind patterns associated with the development of daytime thunderstorms over Istria

This study investigates the impact of the combined large-scale wind and thermally induced local wind on the moist convection development (cumulonimbus clouds) over the northeastern (NE) Adriatic. Former analyses revealed that the NE Adriatic, particularly the Istrian peninsula, is the area with (i) the highest frequency of thunderstorms in Croatia, typically during three wind regimes on a large scale (from the southwest (SW), northeast (NE) and northwest (NW)), and (ii) frequent appearances of sea breeze along the coast (every other summer day on average). The highest density of lightning strikes was observed in the NE mountainous part of the peninsula. Therefore, the three selected cases (one for each type of dominant large-scale wind) were analyzed using the available near-surface and remote measurements. They were simulated also by WRF high-resolution numerical model and examined by the sensitivity tests. In all cases, the near-surface wind patterns consisted of sea breezes along the coastline that generated a narrow eastward-moving convergence zone along the Istria. When the large- scale SW wind (as an onshore wind) dominated in the upper troposphere, the thunderstorm event was the shortest and weakest with only a minor impact on the sea breeze. This was confirmed by the sensitivity tests without microphysics. The origins and locations of storm cells were completely controlled by the low-level convergence zone and the upward advection of low-level moisture at the sea breeze front. In the other two examined cases with offshore large-scale winds from the NE and NW, the mountain range hastened the beginning of convection and affected its intensity. Except for the low-level convergence zone, the advection of large-scale wind influenced the lifetimes and movements of the initial convective cells. Thus, while the local front collision with the NE wind advection caused the thunderstorm to propagate southward, the convergence zone and fronts interaction determined the afternoon northwestward storm movement against the NW large-scale wind.

sea breeze; synoptic conditions; cumulonimbus; thunderstorms; Istria

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