A Numerical Study Of Urban Environment Impacts On Clouds And Precipitation In Beijing

With the acceleration of urbanization process in China, the city meteorological and environmental issues become increasingly prominent. More and more urban rainstorm and flood disasters occur, so carrying out the research about the impact mechanism of urban environment on the cloud and precipitation is urgently needed. A heavy rainfall on August 14,2011 were simulated and sensitively tested by using Weather Research and Forecasting Model(WRF) to study the urban environment(including urban aerosol and urban surface) on the influence of clouds and precipitation and the relative importance of urban environmental factors in Beijing. Results show that:(1)with the increasing of aerosol concentration, the regional average cumulative rainfall increases gradually in the case of low aerosol concentration, and the regional average cumulative rainfall decreases after CCN concentration reaches about 60 cm-3. Compared with the condition of urban slightly polluted, the regional average cumulative rainfall for urban polluted by aerosols is reduced by nearly 17%. The urban polluted aerosols increase the number of cloud condensation nuclei and cloud droplets, then the cloud droplet effective radius is decreased and the efficiency of cloud droplets merging below the zero layer is also reduced, so the conversion from cloud water to rainwater is suppressed and raindrops is decreased. Thence,the evaporation of raindrops in the cloud is weakened, which is not favorable to the development and maintain of strong downdrafts below the zero layer. With the weakening of cold pool and maximum wind speed near the surface, the low-level convergence uplift airflow weakens, which is not favorable to the vertical transport of water vapor and conversion of precipitation, and results in the decrease of precipitation. Therefore, raindrop evaporation is a key factor to affect the precipitation. Urban aerosol impacts the thermal and dynamic fields by cloud and microphysical process, which leads to the change of the surface precipitation process.(2) urban surface has a significant impact on precipitation process for the Beijing area,including the northern mountainous area where precipitation intensity decreased and the regional average cumulative rainfall increased by 2.19%, precipitation intensity increased and the regional average cumulative rainfall increased by 15.28% in the north of downtown,precipitation intensity significantly decreased and the regional average cumulative rainfall decreased by 21.79% in the center of downtown. Urban surface weakens the average wind speed for the northern mountainous area and the north of downtown, slowing the movement for convection system to the urban area, so precipitation system has a long time to maintain a steady precipitation, leading to the regional cumulative rainfall increased. With the development of convection, urban surface is not conducive to surface cold pool and maximum wind speed enhance, low-level convergence and divergence weakened leading to ground precipitation intensity decreased. After precipitation system arriving in the north of downtown,urban surface is conducive to form a strong cold pool and wind speed on the surface, low-level convergence and divergence enhanced leading to ground precipitation intensity increased.Urban surface is not conducive to the formation convection movement of low-level convergence in the center of downtown, so precipitation system bypass the center of downtown to the southeast direction and significantly reduces precipitation and weakens precipitation intensity.(3) in the process of precipitation system moving to Beijing urban area, urban aerosols and urban surface as a whole has a largest relative role on precipitation decreased by 24.64% in the Sixth Ring; but urban aerosols have a largest relative role on precipitation decreased by 26.36%in the north of downtown and urban surface has a largest relative role on precipitation decreased by 20.53% in the center of downtown. In the process, the role of urban aerosols is gradually weakening, but the role of urban surface is gradually enhancing. Urban aerosols play a leading role in Beijing area and Sixth Ring and urban surface plays a leading role in the center of downtown. The effect on precipitation of urban surface in Beijing area and Sixth Ring is less than 0.1% of urban aerosols, but urban surface is four times as much as the impact of urban aerosols on precipitation in the center of downtown.The study shows that Beijing urban environment have a more significant impact on cloud and precipitation processes in summer, and we must consider urban surface and anthropogenic heat and urban aerosol in fine weather forecast. In addition, the research has a certain guiding significance on the governance of urban atmospheric environment and the construction of urban planning.