The present paper analyzes 549 severe weather events reported to the ESWD (European Severe Weather Database) that caused large hail in the territory of Romania. Values of atmospheric instability indices have been analyzed for these episodes using data from Bucharest and Budapest sounding stations. For a period of 140 days with episodes of large hail, 24 instability indices were analyzed to describe the atmospheric conditions of the main daily convective activity. The mean values for most indices characterize an unstable atmospheric environment. Of the indices that measure potential instability, VT (vertical totals index) and TT (totals index) had values that described a conductive atmospheric environment for the development of hailstorms. In addition, the interquartile values of LIV (lifted index using virtual temperature) had values lower than zero. For SWEAT (severe weather threat index) and CAPEV (convective available potential energy index using virtual temperature), only the values in the 75th percentile describe a very unstable environment (according to the literature). Strong linear correlations were registered between several pairs of indices such as CAPEV-LIV and SWEAT-SI that can be used for the operational forecast of hail.
ABSTRACT:Increasing temperature values in altitude, as a deviation from normal law, generates stability of air masses. The most typical situations occur outside the Carpathian chain. In particular cases, when the maximum barometric prevail on large European territories, absolute thermal inversions occur, which are characterized by large thickness, intensity, duration and amplitude. These types of thermal inversions block water vapour at lower levels and produce changes in atmospheric moisture regime by forming mists and low stratiform clouds. Geostatistical analysis of this climatic risk phenomena for aeronautical activity was carried out by processing temperature and moisture database from 7 aerological stations and analysis of synoptic maps. The results revealed a strong connection between the two meteorological phenomena, with the highest frequency in winter.
Temperature inversions are characterized by negative vertical thermal gradients (Donciu, 1953, Ţâştea, 1965, Bogdan, 1971, Neacşa, Frimescu, 1981). The most frequent manifestation is in the depression areas because in addition to radiative cooling and better possibility to store cold air invasion, it also takes place the accumulation and of air flow of cold air due to gravity. The area south of the Carpathians, shows depression features, gaining titles like: carpathianbalkanic bucket (Bălescu, 1962), carpathian-balkanic sink (Ion-Bordei, N. 1988). In this space the inversions occupy the entire area, proof being the values from low altitude stations similar to those from mountain peaks, while the middle part of the slope remains wormer.
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