Impact of Urban Surface Characteristics on Summer Rainfall in the Beijing-Tianjin-Hebei Area

Utilizing daily precipitation data from 24 meteorological stations and results from the Weather Research and Forecasting (WRF) model/Urban Canopy Model (UCM), the impact of urban surface characteristics on summer rainfall in the Beijing-Tianjin-Hebei area was investigated. Results indicated that precipitation at most sites in this region has reduced during the last 30 years, and those sites whose precipitation has reduced the most are mainly centered in the Beijing-Tianjin-Tangshan metropolis. Urbanization is one of the possible factors affecting the precipitation in the Beijing-Tianjin-Hebei area. Comparison of the model results from the control run and sensitivity run indicated that rainfall and rainfall frequency clearly decreased in the Beijing-Tianjin-Tangshan metropolis due to the urban surface. Meanwhile, an increase in rainfall and rainfall intensity was apparent downwind of the urban agglomeration; precipitation above 50 mm changed significantly due to the urban surface, and the contribution to the total could be more than 60%. The percentage of rainfall above 50 mm declined by 6%-20% in the Beijing-Tianjin-Tangshan metropolis, while it increased by 8% downwind. The diurnal structure of rainfall changed due to urbanization: precipitation in Beijing and Tangshan mainly reduced due to urbanization, and the increase downwind occurred mainly in daytime. The findings of this study suggest that the inhibition or enhancement of deep convection, as influenced by changes in latent heat flux and convective available potential energy due to the urban surface, may explain the changes in precipitation.