P13A.18 The evolution and structure of a tornadic supercell storm in Finland on 12 June 1998

Thursday, 9 August 2007
Halls C & D (Cairns Convention Center)
Jenni Teittinen, Finnish Meteorological Institute, Helsinki, Finland

One of the most intense tornadoes in Finland during the last 10 years occurred in the afternoon of 12 June 1998 in southeastern Finland. Based on a ground survey, the damage path was 8.6 km long and its maximum width was 330 m. The tornado was rated F2 on the Fujita scale. The Finnish Meteorological Institute 5.3 cm C-band Doppler radar in Anjalankoski was used as the primary data source in this study. The storm structure and evolution related to the damage path is studied in detail.

During the time of the tornado development, an upper-level trough extended from the Northern Atlantic south to Italy yielding southerly flow over Eastern Europe and Finland. The low pressure over Scandinavia and Finland had brought moist unstably stratified air from the Mediterranean to southern Finland. The tornadic thunderstorm developed in the warm sector, near a quasi-stationary warm frontal boundary. Based on the soundings, the surface-based convective available potential energy (CAPE) was 1371 J kg-1 and the low-level (0-1 km) wind shear was weak. Local mesoscale factors may have increased the existing vertical shear, positive relative vorticity and CAPE.

The first reflectivity echoes of the tornado producing thunderstorm were observed 35 minutes before the tornado formation. This multicell storm had two cells, of which the southern became tornadic. The first signs of a supercell storm and the storm becoming severe, could be observed only 5 minutes before the estimated tornado formation. The storm was splitting and had several updrafts, but the southern cell of the multicell storm already showed supercell features. The tornado developed before a hook echo could be observed in radar pictures. During the tornado, the hook echo was distinct in the right rear flank of the southeastern cell. Comparison of the emergency reports along the path of the tornado and simultaneous radar reflectivity at different times show that the tornado was situated at the right rear flank of the storm. The BWER collapse and echo top lowering could be seen after the tornado formation. The hook echo was at its most intense stage after the tornado dissipation. After the tornado, the storm became multicellular again. The lifetime of the storm was over 3 hours, but after the tornado, no damage was reported.

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