Numerical Simulation Of The Climate Effects Of Urban Expansion And The Regulation Roles Of The Taihu Lake In The Yangtze River Delta Region

Climate simulations of Yangtze River Delta (YRD) region for 10 summers from 2000 to 2009 using the WRF Model are conducted with three different set of land use categories:U. S. Geological Survey (USGS), Moderate Resolution Imaging Spectroradiometer (MODIS) and modified MODIS in which Taihu Lake is changed into cropland, corresponding to pre-urbanization, post-urbanization with Taihu Lake and post-urbanization without Taihu Lake. Based on the model outputs, regional temperature, local circulation and precipitation in YRD region are analyzed in the perspective of the progressive urbanization, together with the regulation of Taihu Lake. The conclusions are as follows:(1) After the urban expansion, urban temperature raises significantly, especially in night. The frequency of high temperature also increases, while diurnal temperature range decreases. Taihu Lake plays a certain role in regulating temperature in vicinity (e.g. Suzhou City), especially weakening the daytime temperature rise.(2) The urban heat island (UHI) effect is stronger at night than daytime. After the urban expansion, UHI effect strengthens with higher frequency. More distinct amplitude of UHI effect happens at night. The UHI effect in Suzhou City tends to be restrained by Taihu Lake, where the probability of strong UHI is reduced.(3) Lake-land breeze in Taihu Lake is generated because of the thermal contrast between water body and surrounded land. The land temperature is higher than water in the afternoon and lake breeze forms. Since the weak thermal contrast at night, no significant breeze happens.(4) A vertical thermal circulation forms between Taihu Lake, Suzhou City and Shanghai City with distinct diurnal variation. After the urban expansion, the circulation is enhanced in the afternoon. The circulation between Taihu Lake and Suzhou disappears after Taihu Lake is changed into cropland.(5) More precipitation in YRD is produced after the urban expansion, while the convective rainfalls in lake-surrounded cities such as Suzhou, Wuxi and Changzhou are remarkably reduced because of the existence of Taihu Lake.(6) After the urban expansion, the precipitation starting at noon lasts longer so that it brings more convective rainfall. Less convective precipitation starting at noon happens in YRD because of the existence of Taihu Lake.