Numerical Simulations Of The Spring Persistent Rains Over Eastern China Affected By The Land-use Change

The climatology of the spring rainfall over regions south to the Yangtze River (SYR) as well as the change of the rainfall during the period, which corresponds to the rapid increase in the land use over east China around 1993 due to the urbanization, are first examined in this study. Data used include the NCEP/NCAR (National Centers for Environmental Prediction-National Center for Atmospheric Research) reanalysis, the CMAP (Climate Prediction Center Merged Analysis of Precipitation), the CFSR (Climate Forecast System Reanalysis), and observed daily precipitation over 753 stations in China, provided by the National Meteorological Information Center of China. Numerical experiments are then performed, by using the NCAR climate model CAM5.1, to investigate the impacts of the urbanization and the land-use change over east China (20°-50°N,100°-125°E) on the spring rainfall over the SYR. The main conclusions are as follows.(1) It is revealed from the data analyses that, corresponding to the urbanization and the rapid increase in the land use over east China after 1993, there is a decrease in the amount of the spring rainfall, which is due to the later onset but earlier ending of the rainfall as well as the weakening in the rainfall intensity. Analyses also show that there is a significant change in the atmospheric circulation that is associated with the rainfall.(2) The model CAM5.1 possesses the ability to well simulate the seasonal variability of the large-scale atmospheric circulation over East Asia and the rainfall over the SYR. The results from the model are in general consistent with that from the observations.(3) The numerical experiments indicate that because of the rapid urbanization over the east after 1993, the increase in the land use makes the spring rainfall over the SYR onset 3 pentads later than normal and end 1 pentad earlier than normal. While with the shortening of the duration, the rainfall intensity also weakens. These are consistent with the observations.(4) The analyses of the dynamic and thermodynamic processes in the sensitivity experiments suggest that the extensive use of the urban land over the east can change the thermal condition of the surface, leading to the strength of the sensible heating of the atmosphere over the east, and thus the warming of the air there. The uplifting of the pressure levels in the lower troposphere over the east can reduce the difference in the geopotential height between the region southeast to the Tibetan Plateau and the western Pacific Ocean, which results in the weakening of the southwesterly airflow. Along with the weakening in the airflow, which is consistent with the observations, the convergence of the lower level atmosphere and the ascending motion over the SYR also weaken. The decrease of the water vapor contained in the atmosphere finally leads to the decrease of the spring rainfall over the SYR.