Severe wind/hail storms over Bulgaria in 1999-2001 period: synoptic- and meso-scale factors for generation

A brief over-view of severe wind/hail storms over Bulgaria in the last 5 years (1997-2001) is presented. Synoptic and thermodynamic analyses of 27 cases during 1999-2001 (April-September) were performed. Among others, two events during this period produced extremely damaging winds. The first of them was associated with a tornado close to the Mediterranean coast on 15 May 1999 and has been already described in the literature. This paper illustrates the role of synoptic scale factors for generation of a second severe windstorm (that occurred near the Bulgarian capital Sofia on 22 May 2001) by using numerical model analysis fields. On the large scale, the circulation pattern was associated with a deformation middle-tropospheric field over the North Atlantic, Western Europe and Mediterranean. A zone of convergence in temperature and mass fields was formed over the Balkans, where there was an interleaving of warm, moist Mediterranean air advected in association with a warm front and cold dry air coming from the northwest in the eastern flank of an upper-level ridge. This synoptic circulation pattern produced 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 radiosounding data as well as by satellite and radar imagery. Data from the upper-air sounding of Sofia at 1200 UTC (available up to 650 hPa level) and other proximity soundings were processed and analysed. The obtained results show very high values of atmospheric instability indices (the energy of instability E-i = 3785 J/kg, TT = 59 degreesC, Z((ULC)) = 12.6 km). The updraft was up to 21.9 m/s and the difference of the wind velocity between 300- and 700-hPa levels was up to 12 m/s. Cloud water content reached up to 12.9 g/kg. (C) 2003 Elsevier B.V. All rights reserved.