Terrain and urban impacts on summer extreme precipitation over Henan, China: A synoptic and simulation analysis

Abstract

The impacts of terrain and urban surfaces on regional hourly extreme precipitation events (RHEPE) in Henan Province of China under specific synoptic conditions remain unclear. In this study, we first identify the dominant synoptic patterns associated with the summer RHEPEs in Henan and surrounding areas by employing spectral clustering methods, revealing that 86 % of RHEPEs are induced by the low vortexes and 14 % by the tropical cyclones, with the latter contributing to the most extreme RHEPEs. Then, based on numerical experiments and moisture budget decomposition, we quantify the effects of terrain and urban surfaces on RHEPE under the most extreme synoptic pattern featured by tropical cyclones. Results suggest that the terrains cause remarkable increases in precipitation primarily along the windward slopes of the Funiu Mountains and Taihang Mountains by 34 % and 26 %, respectively, this can be explained by the enhanced dynamical component of moisture convergence due to the blocking effect of terrains. Compared to the terrain, the impacts of urban surfaces are much smaller (only 1 %) and spatially limited. The much larger surface roughness in the urban areas reduces the horizontal wind speed and causes weakened wind convergence, leading to decreased precipitation over the urban areas. Meanwhile, when traversing the urban areas, the horizontal winds bifurcating and rejoining in the downwind areas increase the low-level convergence and precipitation there. Findings of this study may deepen our understanding of the influences of terrain and urban surfaces on the RHEPE and associated mechanisms under the extreme synoptic pattern.