Numerical Simulation Of A Beijing Winter Snowstorm And Its Cloud Microphysical Processes

In this paper,a winter snowfall event in Beijing during 22?23 November,2015 was simulated by the WRF mesoscale numerical model.The simulation results were verified by comparing with conventional precipitation data,sounding data,S-band radar data and dual polarimetric cloud radar observation data.On these bases,cloud microphysical processes,distribution of hydrometeors,source and sink terms of ice-phase particles on the cloud radar station and strong precipitation area were further analyzed.Meanwhile,the 3D WRF-based surface precipitation equation was used to examine complicated surface precipitation processes.The following conclusions are drawn:Firstly,the snowfall in Beijing belongs to the reflux weather,which occurred in the favorable weather situation where the blocking high south of Lake Baikal and the western Pacific subtropical high formed a high-pressure dam and the West China inverted trough was strong;the easterly flow at the top of the inverted trough brought sufficient water vapor from the sea,which provided water vapor conditions for the occurrence of storms in Beijing and the Huang-Huai area,accompanied by the eastward movement of cold air.So a snowfall event happened.Secondly,comparing the simulated cumulative precipitation,hourly precipitation in large precipitation area and hourly precipitation at polarimetric cloud radar station with the observation,the model can well simulate the temporal evolution of precipitation,precipitation center and precipitation level;through the observation and simulation comparison of radar echoes,the model can recreate the echo intensity and distribution,generation and disappearance time of snowfall clouds.Hence,WRF model with Thompson scheme has a good simulation effect on this snowfall.Then,by comparing the simulated reflectivity and particles distribution over the polarimetric cloud radar station with observation results of Ka-band vertical-pointing radar,the simulated reflectivity and the temporal and spatial evolution of hydrometeors in cloud radar stations are highly consistent with the observations,which to some extentindicates that WRF model and Ka-band radar can be mutually assisted to analyze the distribution of hydrometeors.Furthermore,according to the numerical simulation results,the evolution characteristics of main source and sink the polarimetric cloud radar station and large precipitation area can be obtained.Next,through the analysis of the temporal and spatial distribution of hydrometeors and their source and sink terms in the high precipitation area of this snowfall,the cloud water mainly distributes in the middle layer of 1.5?4 km in the early and late periods of the storm;the distribution of snowflakes is in good agreement with the echo,mainly in the range of 0?9 km.The three main source terms of snowflakes are the depositional growth of snow,the accretion of cloud water by snow and ice crystals automatically converting into snow,and the third source term is about one order smaller than the first and the second source term.And there is sublimation of snow in the lower layer;ice crystals mainly occur at the altitude of 6?9 km,forming a weak echo zone at the upper level,mainly coming from the depositional growth of cloud ice and consumed by the process of automatically converting into snow,the source and sink basically offset each other;graupel mainly exists at the lower level of 0?3 km,with very small magnitude,originating from accretion of cloud water by snow,accretion of cloud water by graupel and rain collection of snow.And there is graupel sublimation in the lower layer.Finally,water vapor budgets is consistent with the overall trend of surface precipitation,and plays a major role in the process of surface precipitation,while cloud-related processes contribute relatively little.However,cloud-related processes also play an important role in surface precipitation,especially in the strong stage of storms.The contributions of 3D moisture diffusion and surface evaporation term to total water vapor budget are small,and the contributions of water vapor local change rate and3 D moisture advection are different in different periods.Cloud-related processes are more complex.At the early stage of snowfall(00:00?05:00),the total cloud-related budget depends on the interaction of flux convergence and local change of liquid-phase hydrometeors;during the development stage(07:00?10:00),the cloud-related budget is mainly dominated by the local change of ice-phase hydrometeors;during the dissipating stage(after 10:00),both liquid-phase and ice-phase hydrometeors affect the total cloud budget,and processes are completely complex.