Radar observations of a tornadic severe frontal rainband

On the afternoon of 26 August 2005, severe convective storms caused vast wind damage in western Finland: 382 wind damage or flood reports and 9 tornadoes. Almost all of the observed severe weather occurred along a rainband associated with a cold front. The storm environment was characterized by nearly saturated low level air having only marginal buoyancy and 18 m/s 0-1 km bulk shear. During the event, the operational nowcasting radar products did not indicate many features that would specifically attract the attention of a forecaster for severe weather potential. The storms were shallow, not very intense in reflectivity and there was no lightning. Moreover, another rainband with lightning, more intense radar echoes and higher storm tops preceded the frontal rainband capturing the forecasters' attention. This study concentrates on radar analysis of the case by using data from two Doppler radars and one polarimetric radar.

At the time of severe weather, the severe frontal rainband moved 15 m/s to the northeast. The low-level reflectivity gradient at the leading line was sharp and several weak echo channels were observed behind the line, as well as a rear-inflow jet with a 22 m/s velocity maximum. The convective line broke into reflectivity gaps that occurred at about a 15 km distance from each other and were characterized in PPI pictures by rainband bulges of lower reflectivity surrounded by higher reflectivities. Several mesovortices were detected along the leading line, similar to straight-line wind damage and tornado producing vortices found in previous studies. The core diameter of the mesovortex that passed close to radar was 2.5 km at a height of 1 km with differential velocity of 28 m/s.

As the severe frontal rainband approached the polarimetric radar, the rainband seemed to lose its shape and the reflectivities decreased. It had wavelike patterns at two separate scales. At larger scale, gaps in low-level reflectivity, and at smaller scale, reflectivity waves and small reflectivity hooks developed ahead and south of the apex of the approximately 50 km wide bowing segment. The Doppler velocity data had three small-scale vortices over the sea in the leading line. These vortices were co-located with the reflectivity hooks. The northern vortex, which was only 4 km from the radar was very close to the location where three waterspouts occurred. This cyclonic vortex had a 2 km core diameter at 200 m height and 24 m/s differential velocity. In addition, differential reflectivity ZDR hooks were co-located with the radar identified vortex locations.