The origin of severe winds in a tornadic bow-echo storm over northern Switzerland

Abstract

A severe bow-echo storm over northern Switzerland is investigated. Wind damage occurred along a track 15 km long and some 100 m wide. Damage data, meteorological data from a ground micronet. and Doppler radar data are analyzed. Volume-scan radar data in the direction of the approaching storm are available every 2.5 min. The storm reached a weak-evolution mode when the damage occurred. Updraft impulses followed each other in time steps of typically 5 min. The damage track can be attributed to a strong radar-observed vortex of 2-7-km diameter. The vortex developed at a shear line that was formed by the downdraft outflow of an earlier thunderstorm cell. Most of the damage was collocated with the strongest Doppler winds but some of the damage occurred beneath the strongest signature of azimuthal shear. A weak tornado was observed in that shear region. The two extremes in Doppler velocity, associated with the vortex and referred to as inflow and outflow velocities, are analyzed separately. Early strengthening of the vortex at 2-4-km altitude was due to an acceleration of inflow velocity, caused by the rising updraft impulses. Subsequent strengthening at low layers (0-2 km) could be related to acceleration of both the inflow and outflow velocities. At this stage, the diameter of the vortex decreased from about 7 to less than 2 km. The low-level intensification of the vortex is attributed to vortex stretching. Later on, the vortex and inflow velocity at low layers weakened but the outflow velocity remained strong.