Supercell Thunderstorms in the Alpine Region - From Weather Radar Observations to Idealized Modeling

The high-impact nature and increasing occurrence of severe weather phenomena pushes forward research on their occurrence and improving our understanding thereof. Supercell thunderstorms are the focus of much severe convective research, as they represent one of the most hazardous convective storm types. The behavior of supercell thunderstorms in complex terrain is still poorly understood. In Switzerland there are no previous comprehensive studies concerning supercells, in contrast to hailstorms, which have been studied more extensively. The Swiss radar network offers high-quality observations with a dense spatio-temporal coverage that provides a good foundation for convective studies.This thesis establishes a general understanding of supercell characteristics for the Alpine region. By employing automatic detection in radar data, a large set of events provides the basis for observational analyses. These inform modeling scenarios that aim to achieve an understanding of the underlying meteorological phenomena in complex terrain.This first entails improving the processing of Doppler velocity data from the Swiss operational radar network and implementing a systematic detection and tracking algorithm for rotation in convection to identify supercells. Making use of the radar data archive, a catalog of past events is generated, allowing for a description of the overall frequency, spatial distribution and temporal occurrence patterns. Revealing frequency clusters in both the Northern and Southern Prealpine regions, analyses into topographic influence show a detrimental effect of increasing altitude on rotation strength in supercells. To gain contextual understanding, a comprehensive comparison against severe rain- and hailstorms indicates that the detected supercells represent the most intense fraction of observed thunderstorms. Additionally, the frequency clusters in the Prealpine areas prevail across the different convective classes. Supercells particularly cluster in the proximity of the lakes in the Southern Prealpine valleys. The decreasing peak intensity with increasing topographic complexity is also found across these types of severe convection. By setting up idealized scenarios in a numerical model, we investigate the effects of slopes, valleys and lakes as moisture sources on supercell development and intensification. Especially valleys with a lake are able to sustain supercellular development in initial conditions, where they no longer develop in the absence of topography.The comprehensive assessment of supercell behavior and the comparison to other, more studied, types of severe convection, provide a fundamental description of supercell characteristics in the Alpine region. The modeling experiments help understand the meteorological drivers behind the topography-influenced frequency and intensity distribution of supercells. As systematic supercell observation in mountainous regions is rare, this constitutes an important contribution to research on severe convection in complex topography.