Impact of urbanization on summer rainfall in Beijing-Tianjin-Hebei metropolis under different climate backgrounds

The Beijing-Tianjin-Hebei region experienced the most rainfall in 1994 and the least rainfall in 1997 during the last 20 years. Utilizing the Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model (UCM), we investigate the possible effects of urbanization on summer precipitation under different climate backgrounds using the two extreme years. By comparing the results of control and sensitivity runs, we find totally different effects in the 2 years. In 1994, the rainfall and rainfall frequency decrease in most areas due to urbanization, and decreases in the rainfall intensity occur in urban areas of Beijing, Tangshan, and Shijiazhuang. In 1997, the rainfall, rainfall frequency, and intensity are reduced in southwest of BeijingTianjin- Hebei, while the change is opposite in northeast. Urbanization alters the diurnal distribution of rainfall, the energy budget, and the water vapor content in the atmosphere. Due to the decrease in city evaporation and transpiration, the surface latent heat flux is reduced. The water vapor mixing ratio in urban area decreases apparently from surface to 850 hPa, while it increases from 850 to 600 hPa. Overall, the reduction of water vapor mixing ratio in 1994 is more than that in 1997, which implies that the dry island effect caused by urbanization is stronger in the wet year than that in the dry year. Results also show that the inhibition (enhancement) of deep convection may explain the modification of precipitation.