Numerical Simulation Of Climate Effects Of Urbanization In Different Regions Of Eastern China

To explore effects on summer climate from the land-use changes in different metropolitan areas and possible mechanisms, the atmospheric general circulation model (LMDZ), developed in the French dynamic meteorology laboratory, is used to simulate East Asia climate change induced by land-use changes in the Pearl River Delta, Yangtze River Delta and Beijing-Tianjin metropolitan area.(1)In both global and regional models, results show that changes of the underlying surface type in the Pearl River Delta, Yangtze River Delta and Beijing-Tianjin metropolitan area significantly reduce the surface latent heat flux. In order to balance the surface energy budget, the ground temperature is increased and the sensible heat flux and surface effective long-wave radiation are enhanced. The surface energy budget is re-equilibrated with an elevation of the surface temperature. Although a local temperature increase is favorable to a thermal low pressure and induces significant upward motions in the lower layers of the atmosphere, the decrease of evaporation makes the local water vapor significantly reduced, which results in a decrease of precipitation. In fact the change of moisture conditions is the main contributing factor for the precipitation reduction.(2) Both global and regional models show main changes in areas of modified underlying surface type. The temperature response has a significant local characteristic, while rainfall response reveals some subtle differences. In fact, the geopotential height field in high layers shows negative anomalies in the north and positive anomalies in the south, the Western Pacific Subtropical High extends westward and strengthens, so the precipitation reduction is not limited to local areas, but extended to a large zone in the eastern part of the domain. This is particularly true in the three-area combined experiment.(3) The impacts of urbanization from different regions are different. In the global model, changes of total heat flux in the Yangtze River Delta and Pearl River Delta are much larger than those in the Beijing-Tianjin metropolitan area, the local warming is also more than doubled in the first two areas. The change of precipitation is mainly affected by changes in evaporation and water vapor transport. Although in the three-area combined experiment, water vapor divergence is simulated in the Beijing-Tianjin metropolitan area, and convergence in other two regions, precipitation decreases in all three areas, due to significant decrease of regional evaporation. In the regional model, changes in surface heat fluxes in the Yangtze River Delta are the largest and those in the Beijing-Tianjin metropolitan area the lowest. Among the different areas, there is a good relationship between the regional surface temperature change and changes in surface heat flux. The change of precipitation in the Pearl River Delta is close to that in the Yangtze River Delta. The three-area combined experiment shows smaller changes than the individual experiments do in the Yangtze River Delta and Pearl River Delta. But the Beijing-Tianjin metropolitan area shows opposite results.