1Institute for Atmosphere and Climate, IACETH - Switzerland
2Meteobüro und Bildagentur - Switzerland
Severe thunderstorms in Switzerland are affected by the complex orography of the Alps. We know quite well the geographic distribution of hailstorms, we know how the Alps modify the wind environment and, hence, the type and structure of supercell storms. We are beginning to learn the geographic distribution of heavy wind and tornadoes. However, we do not know enough about storms producing heavy wind and tornadoes in the vicinity of the Alps: how they develop and evolve, their internal dynamics, and the factors leading to the damaging wind gusts.
The tornadic storm from 5 June 2000 was the latest in a series of storms that have been observed for 10 years with Doppler radars, radiosoundings, and data from a dense ground network. A key instrument is the ETH-Doppler radar: yielding VAD wind profiles and sector volume scans of the storms. We document the meteorological environment of the storm, its evolution seen with radar and the ground data, and the detailed internal structure seen with high-resolution Doppler radar volume scans before and at the time when the tornado occurred.
The results demonstrate that this storm fits reasonably well the storm spectrum as observed in the last ten years: the storm began as a supercell storm (producing large hail) but evolved rather quickly into a bow-echo storm. Convective cells developed rapidly along the bow axis. One of the cells produced the tornado. A similar evolution has been observed in a couple of severe storms in the past. The wind hodographs of all these storms are similar and apparently modified by the Alps: low-level wind is weak from NW, lower mid-level wind is stronger from south, then veering with height to SW. As a result, the overall wind shear between low and mid-levels (6 km) is lower than in "classical" supercell environments. Storm speed relative to the ground becomes faster, and "high-precipitation" supercell storms and bow-echo storms are favored compared to "classical" supercell storm environments. A conceptual model for these storms is outlined. We believe that this is an important result for a better operational nowcasting of severe storms in the vicinity of the Alps.