부산대학교 도서관

Abstract: The first part of this study deals with the available information (date, time, location) of 57 thunderstorms with tornado or with waterspout (9 of which have evolved near the Black Sea coast and 4 are formed over dams). These are about all cases that have been reported in Bulgaria between 1956 and 2010. About half of all tornadoes (49%) that occurred in land are formed over mountainous or hilly terrain and usually move over the near river valleys. Almost all tornado events occur during the warm half of the year with maximum in June, when about 25% of all tornado storms take place. There are only 4 cases from the cold half of the year — 1 in December, 1 in February, and 2 in March. The second part of the paper gives some basic analysis of two severe thunderstorms, connected with the development of tornado events. The typical synoptic situation associated with the formation of a tornado is a slow-moving cold atmospheric front with meridional stretch. It should have bigger than usual temperature gradient between the cold and the warm air masses. A less common synoptic scale structure that leads to the formation of tornado is an upper-air summer low centred over the Black Sea. The first analysed storm is a “winter” case. The tornado occurred in South-central Bulgaria on 24 March 2004. The other one is a typical “summer” case. It occurred in the north–west of the country on 2 June 2009. The data and information that are used in these analyses come from classic meteorological observations, radar images, records from post-event in-situ field investigation as well as various media sources. The characteristics of these tornadoes give us reason to classify them as moderate (class F2 according to the Fujita scale). [Copyright &y& Elsevier]