Numerical Simulation Of Urbanization Impact On A Winter Precipitation Case In Beijing

With the increase of urban population density,the vulnerability of urban infrastructure becomes more obvious.Especially in winter,transport and commercial operation will be seriously affected by rain and snow,and even bring serious economic losses.Therefore,it is urgent to carry out the research on the impact mechanism of urbanization on cloud and snow and the mechanism of its formation.A snowfall in Beijing from 5:00 to 17:00 on March 17,2018 were simulated by using Weather Research and Forecasting(WRF)to study the main influence mechanism for urbanization on snowfall.Results show that,The snowfall(rain)process in Beijing on March 17,2019 is mainly affected by the high altitude groove which moving east.The system is relatively weak and it shows a weak precipitation.The system is characterized by the interaction of gaseous,liquid and solid phase particles,forming a complex mixed phase cloud microphysical process.In the process of snowfall(rain),snow mainly comes from the condensation growth of water vapor and the collision growth of snow with cloud water and ice crystal.Graupel particles mainly come from the automatic conversion of snow and the collision growth of graupel and cloud water.Rain water below the zero layer mainly comes from the melting of snow and graupel particles,among which snow melting contributes a lot,while above the zero layer mainly comes from the transformation and collision of cloud water.In Beijing's main urban area,Urbanization changes the precipitation type,reducing snowfall and increases rainfall,this is mainly due to the fact that the low-level warming effect strengthens the melting process of snow and produces mixed precipitation.The closer to the city center,the more prone to mixed precipitation.Urbanization also has an impact on the spatial and temporal distribution of total precipitation.In the beginning of precipitation process,the “urban dry heat island” effect formed by urbanization is not conducive to the horizontal and vertical transport of water vapor and the formation of clouds,and the total precipitation reduced.With the development of the precipitation process,the “urban heat island effect” makes the cloud microphysical process become a mixed process of cold clouds and warm clouds,which is conducive to the vertical transport of water vapor and the development of clouds,and the total precipitation increased.The “urban effect” generated by urbanization has an impact on low-level atmospheric temperature and cloud microphysical processes.While the non-adiabatic process of the cloud microphysical process affects the lower atmospheric temperature and atmospheric stratification,affecting energy and water vapor transport,which has an impact on cloud and precipitation.