The dual effect of urban areas on supercell storms

Abstract. The effect of urban land use on convective storms with deep and persistent rotating updrafts (supercell storms) is systematically investigated using ensembles of idealized numerical simulations. A supercell is simulated over a flat domain containing a circular city surrounded by croplands. It is triggered upstream of the city so that it subsequently moves into the urban area. Twenty-five experiments with eleven ensemble members each are conducted in which city size, urban fraction, building height, and building density are varied. The results show a statistically significant weakening of the supercell with increasing city size. A similar trend is observed when varying building density and urban fraction, although these effects are not statistically significant lower than zero. The weakening of the approaching storm is primarily driven by the urban dry island (UDI), which substantially reduces convective available potential energy. While the initial storm can be suppressed by the city, the urban heat island (UHI) generates a pressure minimum that can trigger a new supercell downwind of the city. Compared with UHI-induced vertical velocities, the building-generated vertical velocities are negligible. This study provides a benchmark that expands our understanding of the complex interactions between the urban environment and deep moist convection, emphasizing the role of the UHI and UDI in influencing storm dynamics and highlighting the potential dual effects of urban land use on supercell storms.