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

A severe bow-echo storm over northern Switzerland is investigated. Wind dam age occurred along a track 15 km long and some 100 m wide. Damage data, met eorological data from a ground micronet, and Doppler radar data are analyze d. Volume-scan radar data in the direction of the approaching storm are ava ilable every 2.5 min. The storm reached a weak-evolution mode when the damage occurred. Updraft i mpulses followed each other in time steps of typically 5 min. The damage tr ack 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 outf low of an earlier thunderstorm cell. Most of the damage was collocated with the strongest Doppler winds but some of the damage occurred beneath the st rongest signature of azimuthal shear. A weak tornado was observed in that s hear region. The two extremes in Doppler velocity, associated with the vortex and referr ed to as inflow and outflow velocities, are analyzed separately. Early stre ngthening of the vortex at 2-4-km altitude was due to an acceleration of in flow velocity, caused by the rising updraft impulses. Subsequent strengthen ing at low layers (0-2 km) could be related to acceleration of both the inf low and outflow velocities. At this stage, the diameter of the vortex decre ased from about 7 to less than 2 km. The low-level intensification of the v ortex is attributed to vortex stretching. Later on, the vortex and inflow v elocity at low layers weakened but the outflow velocity remained strong.