Simulation Study Of Urbanization Effects On Summer And Winter Daily Precipitation Over The Yangtze River Delta

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.