Microphysics of a tornado-producing supercell observed with a dual-polarization C-band radar

On 28 August 2005, a supercell thundestorm moved over the Helsinki metropolitan area producing two successive tornadoes, classified as weak (F1). The storm travelled trough Helsinki Testbed mesoscale observational network which allowed collection of unique observational material to this case. The radar measurements were from two C-band radars: a dual-polarization and a Doppler radar which the tornado passed within 4.5 km and within 5.2 kilometres, respectively. To compare the radar measurements to the tornado tracks, multiple approaches were employed to collect data on the tornado paths and estimate wind speed, including damage survey and analysis security camera footage.

Although few tornadic supercells observed with dual-polarization C-band have been documented before, this is the first study in Europe on supercell and tornado by using dual-polarization radar. This study shows that even weak tornado debris cloud can be seen with C-band dual-polarization radar. A unique feature, an arc- shaped reflectivity structure consisting of large raindrops above the debris cloud, was also observed.