Analysis Of A Simulated Squall Line Based On Doppler Radar Date Assimilation And Sensitivity Experiments To Graupel Particle

Squall line is a line-shaped mesoscale convective system(MCS)consisting of many active thunderstorm cells that often produces severe weather such as thunderstorm,rainstorm,strong winds,large hail and even tornados.It always brings huge disaster to people?s life and property.Therefore,researching and forecasting squall lines always attract special attention.For lack of observation data,a numerical simulation is always used to investigate the squall line;the simulation can be improved by assimilating Doppler radar data and optimizing the microphysics parameterization(such as hydrometeor particle).Based on this consideration,firstly,Doppler radar data were assimilated with the mesoscale model WRF(Weather and Research Forecasting)and the corresponding 3DVar assimilation system to optimize the initial condition,and then by using simulated results,the dynamic,thermodynamic field and cloud microphysical property of the squall line formed in Hubei on July 26,2011 were analyzed and important hydrometeor particle mostly affecting the simulation results was found out.Finally,four sensitivity tests were made to investigate how the important particle influenced the simulated squall line.The radiosonde observations showed that the squall line was formed under the environment condition of weak low-level vertical wind shear and strong convective instability.The Doppler radar observations showed two strong convective lines respectively in the directions of Northeast-Southwest and Northwest-Southeast,the first one moving slowly during its organization period,the northwest side of the second one disappearing rapidly and the southeast side merging into the first one.The simulated results indicated: during initial period,the dynamical condition is not beneficial to the formation and development of new convective cells,but the strong convective instability energy provides strong thermal lifting condition.Later on,low-level ambient vertical wind shear and cold pool become in favor of development of the squall line,but dynamical lifting condition is still weak and the convective instability energy still plays the vital role.With the increase of dynamical lifting and the strong thermal lifting,the system gradually becomes mature.Finally,the convective instability energy releases,the strength of cold pool becomes stronger than the low-level vertical wind shear obviously,and the squall line disappears.In addition,the analysis on cloud microphysical property shows that precipitation in both the strong convective and stratiform zones of the squall line comes from the graupel melting,the graupel particle contribute to the formation of precipitation mostly.Four sensitivity simulations with different graupel parameters all generally capture the basic characteristics,and notable differences among the simulations still exist in the morphology and evolution of the squall line at cloud scale.The feedback between latent budget and the inner vertical circulation is very sensitive to the graupel parameters in both the strong convective and stratiform region: decreasing(increasing)the average diameter of the graupel will increase(decrease)the total latent heat releasing and absorbing,and then increasing(decreasing)the adiabatic heating and cooling rate in cloud will increase(decrease)the temperature and pressure gradient in the squall line system with increasing(decreasing)the inner circulation.However,in the strong convective region,when the average diameter of graupel becomes large enough,the above conclusion is not completely true.Besides,the sensitive of precipitation to the graupel is different in the strong convective region and stratiform region;as the average diameter of graupel increases,the surface precipitaion will increase in the convective region but decrease in the stratiform region.