Analysis of orographic influence on hail parameters in NW Croatia

摘要

Deep moist convection often yields hail, which is a subject of this work, as a significant precipitation form in many parts of the extra-tropics. Located in the midlatitudes of the Northern Hemisphere, Croatia is exposed to frequent occurrence of thunderstorms and hail, especially in its continental part. In the second half of 20th century, to reduce the risk of damage in agricultural production, a hail suppression system was established in the area. With the goal to obtain objective hailstone measurements for research studies there, hailpads were installed in 2001 on all hail suppression and main meteorological stations. In addition, in 2002, a dense network of hailpads (polygon) was built in the northwest of the continental part of Croatia. In this study, hail suppression station data (observers records: occurrence and duration of hailfall), hailpad polygon data and lightning data are used to test the hypothesis about the qualitative influence of orography on the spatio-temporal distribution of hail. We use an objective approach for testing the hypothesis, namely, the Empirical orthogonal function (EOF) analysis of the polygon data. No influence of cloud seeding on the EOF analysis is expected. The results of this analysis have confirmed the influence of orography on the distribution of hail. That may help in hailstorms nowcasting and an optimized agricultural planning.