National Institute of Meteorology and Hydrology - Bulgaria
Based on synoptic and thermodynamic characteristics, a brief overview of the most severe wind and hailstorms over Bulgaria in the last 3 years (1999-2001) is presented. Synoptic- and meso-scale factors for generating two of the most severe convective wind storms are discussed. The first one is a case of local tornado on 15 May 1999 over the most southern part of Bulgaria, close to the Mediterranean coast that caused damages of property valued above 250 000 US dollars and has already described in the literature. A detailed study of the second extremely severe wind storm on 22 May 2001 about 30 km south of the Bulgarian capital Sofia) is presented in this paper. The high-wind-speed vortex blew off house roofs, rooted out trees in the National mountain Park Vitosha. The case is associated also with hail and caused damages of property valued about 300 000 US dollars.
Using numerical model analysis fields and satellite imagery, evidence is presented which illustrates the role of synoptic scale factors for generation the severe storm. On the large scale, the circulation pattern is associated with a deformation middle-level field over the North Atlantic, Western Europe and Mediterranean. A zone of convergence in temperature and mass fields is formed over the Balkans in a narrow area between a polar trough over the north-eastern Europe and a Mediterranean cyclone. Within this convergence zone, there is an interleaving of warm, moist Mediterranean air advected in the region of the Mediterranean cyclone warm front and cold dry air coming from north-west in the eastern flank of a high-level ridge that triggers deep convection along the boundary between the polar and subtropical high-level frontal systems. It is shown that synoptic- and meso-scale elements of the development may be revealed or confirmed by satellite and radiosonde data.
The nearest upper air sounding (at Skopje airport) and aerological data from Sofia station up bellow 700 hPa is used for air mass analysis in the vicinity of the storm (due to an extremely strong thunder storm the radiosonde was damaged during the Sofia sounding). The obtained result shows very high values of atmospheric instability indices (the energy Ei, TTI and KI), updraft, the difference of wind velocity at 300- and 700-hPa levels and cloud water content.