A Typical Weakly Forced Mountain-To-Plain Extreme Precipitation Event Exacerbated by Urbanization in Beijing

The rainstorms that initiate over mountains, propagate to, and suddenly intensify in nearby plains pose great challenges for weather forecasters and the public. Being in a mountain-plain transition zone, the city of Beijing is prone to rainfall extremes induced by weakly forced mountain-to-plain storms. Despite efforts to understand their possible mechanisms, it remains unclear whether and how these mountain-to-plain precipitation extremes are affected by urbanization in Beijing. Here we use a high-resolution weather prediction model to hindcast a weakly forced mountain-to-plain extreme rainfall event impacting Beijing on 23 June 2011. We incorporate spatially varying urban canopy parameters into the model to assess their effect on rainfall simulation and find no significant improvement in model performance. By comparing simulations under current and pre-urbanization scenarios, we find that urbanization intensifies the regional average rainfall within Beijing's Sixth Ring Road by similar to 23%, with an increase in peak hourly rainfall rate reaching 27 mm hr-1. The higher surface roughness and enhanced vertical motions in urban areas facilitate the transport of warm and humid air from lower to mid-upper layers, fueling the development and maintenance of convective storms. The dragging effect of urban canopy leads to more moisture accumulation and slower-moving convections, resulting in more rainfall falling in urban areas. Our study underscores the key role of local urbanization in shaping the mountain-to-plain rainstorms in Beijing, which may help improve the forecast capacity for these weather extremes and related urban resilience planning. This study aims to investigate whether and how urbanization affects mountain-to-plain rainstorms in Beijing, which pose considerable challenges for weather forecasters and urban residents. A high-resolution weather forecast model is employed to simulate a typical weakly forced mountain-to-plain extreme precipitation event that hit Beijing on 23 June 2011. Incorporating spatially varying urban parameters into the model has a limited effect on the model's ability to reproduce this rainfall case. The findings reveal that urbanization increases rainfall totals in urban areas by nearly 23%, because of the higher urban land surface roughness and drag forces that enhance vertical motions and lower-level water vapor convergence. Moreover, the urban canopy of buildings and roads acts as a barrier, causing more moisture to accumulate and retard the advancement of rainstorms, resulting in more rainfall in urban areas. This understanding could potentially enhance the capability of weather forecasting and disaster planning for these extreme rainfall events, thereby contributing to the strategy development of urban resilience. Gridded urban canopy parameters have a limited effect on model's skill in simulating a typical mountain-to-plain rainfall event in BeijingUrbanization-induced rainfall increase is linked to urban surface dynamics under a weak urban heat islandThe high urban surface roughness and drag force of urban canopy collectively contribute to the precipitation intensification in urban areas