Numerical Investigation and Uncertainty Analysis of Eastern China's Large-Scale Urbanization Effect on Regional Climate

Eastern China has experienced rapid urbanization during the past four decades, and it is necessary to understand the impacts of the urbanization on the regional climate. Previous simulations with either regional climate models (RCMs) or general circulation models have produced inconsistent and statistically non-significant urbanization effects on precipitation during the East Asian summer monsoon. In the studies with RCMs, reanalysis data were used as the lateral boundary conditions (LBCs) for both urban and non-urban experiments. Since the same LBCs may limit the urbanization effect, in this study, the Weather Research and Forecasting (WRF) model nested within the Global Forecast System (GFS), both of which were coupled with an urban canopy model, were used to explore the urbanization effect over eastern China. The WRF's LBCs in the runs with/without urbanization were provided by the corresponding GFS runs with/without urbanization. The results showed a significant decrease in precipitation over North China, mainly due to a marked decrease in evaporation and the divergence induced by the reduced latent heating in the mid and upper atmosphere, from the experiment with urbanization. Meanwhile, to the north and south of the large-scale urbanization areas, especially to the south of the Yangtze River, precipitation increased significantly due to large-scale urbanization-induced circulation change. With the same LBCs for the WRF runs with/without urbanization, the urbanization effects were limited only to urban and nearby areas; no significant change was found to the south of the Yangtze River, since the same LBCs hampered the effects of urbanization on large-scale circulation. In addition, this study demonstrated that the urban fraction may be a key factor that affects the intensity of the urbanization effect within the urban areas.