Effect of urban expansion on summer rainfall in the Pearl River Delta, South China

The Pearl River Delta (PRD) is one of the most economically dynamic regions of China and has experienced rapid urbanization during the past decades. in this study, the effect of urbanization on summer rainfall events in the PRD region is investigated using the advanced Weather Research and Forecasting (WRF) model. Two comparative urban scenarios (2015 and 1991) were designed to test the sensitivity of summer rainfall to urbanization. The results indicate that the WRF model reasonably reproduces the general synoptic weather pattern, particularly for air temperature and relative humidity. Further comparative analysis reveals that extreme rainfall significantly increased in urban areas, while the total summer rainfall slight decreased. Urbanization causes changes in the surface energy balance, resulting in more sensible heat flux and less latent heat flux. The decreased latent heat flux dampens the moisture availability and results in the decline of the maximum convective available potential energy, which lead to the decrease of the total summer rainfall. In contrast, the increased sensible heat flux enhances the urban surface temperature and boundary layer circulations (e.g., elevated planetary boundary layer height and higher level of free convection and lifted condensation level) and carries more water mixing vapor into the atmosphere over urban areas, enhancing the frequency and intensity of extreme rainfall. The results suggest that urbanization plays an important role in the urban change of summer rainfall and causes more extreme rainfall events, thereby inducing flooding and property losses associated with large economic costs.