长江三角洲城市群对夏季降水影响机制的模拟研究

利用耦合了单层城市冠层模型UCM的中尺度模式WRF,对长江三角洲城市群夏季城市化效应进行了5 a(2003—2007年)高分辨数值模拟,并作了长江三角洲地区有无城市的对比试验。结果表明,城市化使得长三角城市群及其邻近地区,夏季近地层水汽混合比呈明显减少趋势,而850~700 h Pa层的水汽混合比有所增大。通过对比有与无城市各等级降水日所对应的2 m水汽混合比,得出小雨降水日对应的2 m水汽混合比差异与总降水日对应的差异最为接近。通过分析环流场、散度场和垂直速度场发现,水汽混合比的垂直变化是由于城市群的存在使得近地层辐合、850~700 h Pa层辐散的配置增强,以及在城市群上空增强了的垂直上升运动,从而增强城区对流活动,水汽的垂直输送也更为活跃,由此可能导致对流性降水的增加。

Simulation on mechanism of urbanization effects on summer precipitation over the Yangtze River Delta

The Weather Research and Forecasting(WRF)model coupled with a single-layer urban canopy model(UCM)is used to simulate numerically the summer urbanization effects with high resolution during 2003-2007 over the Yangtze River Delta. The comparison of two experiments with urban land cover(URB) determined by MODIS satellite observations and without urban areas(NOURB) shows that the water vapor mixing ratio near the surface in urban and adjacent areas decreases obviously, while at higher level (850-700 hPa), it increases. By comparing URB and NOURB experiments of water vapor mixing ratio at 2 m in rainy days at each level, the difference in light rainy days is closest to that in all summer days. The results of the analysis on the circulation, divergence field and vertical speed show that the changes in vertical structure of water vapor mixing ratio is due to stronger lower-level convergence and higher-level(850-700 hPa) divergence caused by urbanization, which enhances convection in urbanized areas and makes the vertical transportation of water vapor more active, so as to bring more convective precipitation.