Numerical Simulation And Data Assimilation Of A Squall Line In The Yangtze-Huaihe River Valley Under Weak Synoptic-scale Forcing

Squall line often produces thunderstorm,short-term heavy rainfall,hail and so on,which causes serious casualties and property losses.Therefore,the study of squall line has been one key issue in mesoscale convective systems.In this paper,the effects of different cloud microphysical,boundary layer parameterization schemes and radar data assimilation on the simulation of a squall line,which occurred under weak synoptic-scale forcing in the north of Jiangsu Province on July 26 2016,are investigated.Firstly,five typical cloud microphysical schemes and five boundary schemes in the WRF(Weather Research and Forecasting)model were used to simulate the squall line.Then,the simulation results are used to analyze the reasons for the differences between the different schemes.Next,some radar data assimilation sensitivity experiments have been carried out by using the WRF-3DVar assimilation system to find proper len scaling and var scaling as well as time interval,which produce the best results for analyzing the influence of assimilation radar observation on the squall line simulation.Finally,explore the reason for improvement by analyzing radar composite reflectivity,dynamic field,moisture and temperature field.The results of different cloud microphysical parameterization schemes show that the simulations of this squall line with different cloud microphysical schemes are quite different.Only the NSSL 1-momlfo scheme can reproduce this squall line well,including the location,intensity of the strong convective echo region and stratiform cloud region,while the simulated radar echo with other four schemes significantly differ from the real observation.It is found that the NSSL scheme shows a stronger updraft,more concentrated cold pool and the distribution of hydrometeors is concentrated in the horizontal narrow area,with more rainwater in the middle and lower layers and more snow particles and graupel particles in the middle and upperlayers.Meanwhile,the NSSL scheme produces the most latent heat,resulting in a strengthened updraft.The results of different boundary layer parameterization schemes show that GBM,YSU and UW scheme can reproduce the intensity of the strong convective echo region and stratiform cloud region well.Among them,the simulation of GBM scheme has the least false echo,which is the closest to the observation including the location and the vertical structure of radar echo.Meanwhile,the simulated radar echos with other four schemes significantly differ from the real observation.GBM scheme provides sufficient water vapor and unstable energy in the development of squall line.And in combination with strong updraft and low-level instability,it transports a lot of water vapor and energy upwards.At the same time,it maintains updraft with low-level convergence and high-level divergence.The radar data assimilation experiments show that this squall line is more sensitive to the len scaling.When len scaling and var scaling are 0.3 and 0.6respectively,the result of forecast is the best.During the 1-h cycle assimilation,assimilation with 30-min time period produces a better result than with 20-min and10-min.The assimilation directly improves the wind analysis fields at different altitude,which affects the low-level positive vorticity,convergence and the high-level divergence indirectly.Further,it can promote the development of cyclonic circulation and updraft.Meanwhile,the assimilation can improve the mix ratio of hydrometeors at different altitudes in the area where the squall line occurred and increase the temperature gradient at the front of the squall line as well.Further analysis of the forecast fields shows that the assimilation improves the wind field forecasting,moisture field and temperature field in the middle and low level,and then improve the forecast of the cyclonic circulation,vertical updraft and cold pool indirectly.All those factors result in an improved forecast of squall line.In a word,the prediction of the squall line can be influence by cloud microphysical parameterization schemes,boundary layer parameterization schemes and radar data assimilation.Among them,the cloud microphysical parameterization schemes have the most significant impact on the analysis and forecast of the squall line,followed by the radar data assimilation and the planetary boundary schemes.