Supercell storms in Switzerland: case studies and implications for nowcasting severe winds with Doppler radar

Abstract

Three severe hail- and windstorms, which occurred in Northern Switzerland, have been investigated. The storms produced hailswaths of 70-170 km in length and 12-25 km in width. Compact tracks of severe wind damages within the hailswaths are documented from public reports. These tracks have a length of 12-25 km. The damage was produced by straight-line winds rather than by tornadoes. Volume-scan Doppler radar data of the storms are available in time steps of 5 min. Radar signatures, such as low-level convergence and shear, mid-level vorticity, and high-level divergence, were attributed to the damage tracks at the ground. The low-level radar signatures allow the deduction of the time of occurrence of the damage tracks with a precision of some minutes.

Striking similarities in the evolution of the three stormes were found. The storms developed in the foothills of the Alps and the Jura mountains and propagated towards the plains of the Swiss midland. The storms can be classified as ‘high-precipitation’ supercell storms, known as producers of severe straight-line winds in the USA. Meso(anti)cyclonic vortex signatures were seen 40-55 min in advance of the heavy wind damage at the ground. The damage tracks were associated with explosive secondary cellular growth aloft. The physical explanation of this behaviour is that the first cells with mid-level rotation produced a gust front outflow that was accelerated to damaging strength at the time when the secondary cellular growth was initiated. The operational implication is that the nowcasting of severe and damaging winds can be improved by considering mid-level rotation in an early stage of the evolving storms.