Supercell thunderstorms in complex topography - how mountain valleys with lakes can increase occurrence frequency

Abstract

This study investigates the effects of lakes in mountainous terrain on the evolution of supercell thunderstorms. With a newly developed radar-based, mesocyclone-detection algorithm, a recent study has characterized the occurrence and evolution of supercell thunderstorms in the Swiss Alpine region. That study highlights the influence of orography on both storm intensity and occurrence frequency. To disentangle the different influential factors, an idealized modeling framework is established here using the mesoscale model CM1. The modeling scenarios are based on a high-CAPE environment with unidirectional shear, where a warm bubble serves to initiate the convection. Mimicking the environment of the southern Prealps in central Europe, scenarios with a high mountain ridge, valleys and lakes are explored. The effect on the supercells of the slopes, high-altitude terrain and moisture sources emphasizes the highly localized nature of terrain effects, leading to a heterogeneous intensity lifecycle with transitory enhancement and weakening of the supercell. The dynamic and thermodynamic impact of mountain valleys with lakes increases the range of atmospheric conditions that supports supercellular development through horizontal vorticity production, increased storm relative helicity and higher moisture content. This influence results in a systematic location dependence of the frequency, intensity and lifetime of supercells, as also found in observations.