Ville de Lausanne Award - 2025 - Monika Feldmann

For having identified, analyzed and modelled mesocyclones in Switzerland from operational weather radar data, and having developed tools for the monitoring by forecasters at MeteoSwiss of those phenomena with potential societal impacts.

Severe convective storms, particularly supercells, pose major hazards but remain poorly understood in complex terrain. Unlike hailstorms, supercells in Switzerland have not been comprehensively studied despite the availability of high-quality radar observations. This thesis provides the first systematic assessment of supercell characteristics in the Alpine region using automatic detection and tracking of rotation in thunderstorms in the Swiss radar network.

Processing improvements to Doppler velocity data enabled the creation of a supercell catalog, allowing analysis of their frequency, spatial patterns, and temporal occurrence. Results reveal pronounced clustering in the Northern and Southern Prealpine regions, especially near lakes in Southern valleys, while increasing altitude reduces supercell rotation strength. Comparisons with severe rain- and hailstorms show that supercells represent the most intense convective storms, with similar frequency clusters across storm types.

Idealized numerical modeling experiments further examined topographic effects, demonstrating that valleys with lakes enhance supercell development even under conditions suppressing convection in flat terrain.