【作者】 敖翔宇；

【导师】 任雪娟；

【作者基本信息】 南京大学 ， 气象学， 2012， 硕士

【摘要】 利用耦合了单层城市冠层模型UCM的中尺度模式WRF,对长江三角洲大城市群夏季和冬季城市化效应进行了5年(2003-2007)高分辨数值模拟,通过长江三角洲地区有无城市的对比试验,分析了长江三角洲地区城市化对夏季和冬季日降水特征的影响,并试图寻找城市化效应对长江三角洲地区夏季降水特征影响的可能机制。模式结果分析表明,WRF模式能较好地再现长三角地区2003-2007年五年平均夏季和冬季降水的空间分布,比较成功地模拟出了降水中心的位置及强度。城市化使得长三角地区夏季降水日数减少了1-5天,这种降水日数的减少主要是由于城市化使得小雨日数减少引起。城市化增强了长三角大部分地区夏季日降水强度。城市化对长三角地区冬季日降水特征影响和夏季类似,但不如夏季明显。城市化对长三角地区四个典型城市群：宁镇扬(NZY)、苏锡常(SXC)、上海(SH)和杭州湾(HZW)城市群的夏季降水日变化也存在一定影响,并表现出区域差异性。对于长三角整个大城市群(YRD),城市化对夏季降水量、降水强度日峰值出现时刻以及降水强度日峰值大小无明显影响,而使得降水量日峰值减少。城市化使得苏锡常地区夏季降水量日峰值略有增加,宁镇扬和上海地区降水量日峰值都减小,而杭州湾城市群区降水量日峰值出现时刻延后。城市化使得四个典型城市群夏季降水强度日变化曲线形态发生改变,使得上海地区夏季降水强度日峰值出现时刻延后,使得杭州湾城市群区夜雨增强。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,而使得850hPa-700hPa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2m水汽混合比,小雨降水日所对应的2m水汽混合比差异与有降水日对应的差异最为接近。通过对环流场、散度场和垂直速度场的分析发现,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、850hPa-700hPa层辐散配置增强,以及在城市群上空存在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,从而有可能导致对流性降水的增加。更多还原

【Abstract】 In this paper, the Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model (UCM) is used to simulate the urbanization effects in summer and winter from2003to2007over the Yangtze River Delta area with high resolution. Two experiments were designed:(1) urban land cover determined by MODIS satellite observations(URB) and (2) no urban areas(NOURB). By comparing the two experiments, the impacts of urbanization on summer and winter daily precipitation are analysed, and the possible mechanism is also investigated.The simulation results show that the WRF model can well reproduce the summer and winter precipitation over the Yangtze River Delta area. The simulated spatial distribution and the intensity of the summer and winter precipitation over a period of5years (2003-07) agree well with the observation. Urbanization results in significant decrease in the summer rainfall days by1-5days over the Yangtze River Delta area. It is mainly caused by the reduction of the minor rainfall days. The summer daily rainfall intensity is increased over most part of Yangtze River Delta area. The impacts of urbanization on winter daily precipitation over the Yangtze River Delta area is similar to that of summer, but is weaker than summer.The analysis of the summer rainfall diurnal cycle of the four typical urban subgroups in Yangtze River Delta area (denoted as NZY, SXC, SH and HZW), shows that there exist some regional differences for the urbanization effects. For the whole Yangtze River Delta city group, urbanization has no significant effects on the occurrence times of the summer rainfall amount and rainfall intensity daily peak, but results in a decrease of daily peak of the rainfall amount. Urbanization results in a slight increase of daily peak of the rainfall amount over SXC area, and a decrease over NZY and SH area. The occurrence time of the rainfall amount daily peak is delayed over HZW area. Urbanization also changes the shapes of the summer rainfall intensity diurnal variation over the four typical city subgroups. It causes a delay of the occurrence time of the rainfall intensity daily peak in SH, an increase in the nocturnal rainfall intensity in HZW.The mean near surface water vapor mixing ratio in urbanized and adjacent areas decreases obviously; while the higher level (850hPa-700hPa) water vapor mixing ratio in urbanized areas increases in summer. By comparing2m water vapor mixing ratio of each level rainy days in URB and NOURB experiments, the difference in light rainy days is closest to that in rainy days. By analyzing the circulation and divergence field, we can see that the cause of changes in vertical structure of the water vapor mixing ratio is that urbanization results in stronger lower-level convergence and higher-level divergence, and further enhances convection in urbanized areas, the vertical transportation of water vapor becomes more active, then possibly results in more convective precipitation.更多还原