Influence of the urban canopy on the numerical simulation of the “720” rainstorm process in Beijing

Based on high-resolution underlying surface data and revised urban parameters, a heavy rainfall process that occurred on July 20, 2016 in Beijing was simulated using the Weather Research and Forecasting Model, version 4.0 (WRF4.0). Sensitivity experiments by changing the land-use type and terrain height, and coupling a slab urban canopy model (UCM) with modified parameters, were carried out to investigate the effects of the urban canopy on this rainstorm process in Beijing. The simulation results confirmed that the urban canopy of Beijing had significant impacts on the heavy rainfall, and its impacts on the rainfall could mainly be attributed to the internal structure and related processes of the urban canopy. The urban canopy increased the convergence of water vapor flux in the urban area, leading to strengthened rainfall in the urban area. In addition, employing the UCM also had an influence. The experiment uncoupled with the UCM suggested that the urban heat island effect of Beijing was relatively weak; its barrier effect of the urban canopy played a leading role that blocked and delayed the movement of rain bands, which divided the airflow and increased the amount of rainfall outside the urban area. The experiment coupled with the UCM took into account the parameters of building height, albedo, and anthropogenic heat, which helped improve the accuracy of rainfall simulation. Its urban heat island phenomenon was obvious, which benefited the convergence and upward movement of urban airflow and promoted the movement of the front and the total rainfall in the urban center.