Impact Of Radar Data Assimilation On The Simulation Of Squall Lines

Squall lines are often accompanied by severe weather phenomena such as strong wind,hail,and heavy precipitation.As the frequency of severe convective weather in China increases,in-depth research on mesoscale convective systems is particularly important.Radar data assimilation can improve the forecast of severe convective weather,but different assimilation schemes give different results.Therefore,our study uses the mesoscale numerical forecast model WRF,the ARPS 3DVAR assimilation system and the complex cloud analysis module to explore the impact of radar data assimilation on the domestic squall line system.In this paper,focusing on a squall line process in southern China occurred on March 4,2018,the Advanced Regional Prediction System(ARPS)Three-Dimensional Variational data assimilation system(3DVAR)was used to assimilate Doppler radar radial velocity,and cloud analysis was used to process reflectivity data.Considering different assimilation intervals,frequencies and cloud analysis parameter adjustments in cloud analysis,adopting 1 h assimilation window,designing different assimilation schemes,and using the Global Forecast System(GFS)analysis field as the background field,we studied the influence of radar data assimilation on triggering the squall line system and its development mechanism.The results show that when the assimilation interval is too short,false echoes are generated due to the imbalance of the model thermodynamic variables,while if the assimilation interval is too long,the system triggering and strengthening features are generally weak.12 min.interval assimilation is used since it gives the best initial field.Furthermore,the higher the frequency of assimilation,the better the precipitation forecast results.In addition,ARPS cloud analysis can greatly improve the initial field and reduces the mode spin up time.Among different parameters,humidity adjustment,temperature adjustment,rainwater adjustment,and water vapor adjustment have a greater impact on the dynamic process of the system and the initial field distribution of the hydrometeors,while the adjustment of parameters related to vertical velocity has less impact.Next,six cloud microphysical schemes of the ARPS assimilation system are selected for sensitivity analysis.The results show that the simplified WSM6 scheme can more closely simulate the distribution of water in the squall line process.The Kessler scheme is due to better analysis.The mixing ratio of water vapor and cloud water is improved,and the precipitation forecast during the period of strong development of the system is improved.Finally,based on the optimal cyclic assimilation scheme settings and cloud analysis adjustment parameter settings obtained above,the ARPS 3DVAR system and complex cloud analysis module were used to assimilate a strongly-forced squall line process that occurred in the same area on May 11,2017.The forecast results show that the cyclic assimilation scheme has significantly improved the cloud microphysical field and dynamic field of the "05·11" squall line system,and significantly improved the precipitation forecast for this event.Comparing the specific impact of radar observation data assimilation on the squall line process generated under two different forcing degrees,the results show that radar data assimilation has a more obvious impact on the squall line process generated under strong weather scale forcing.It simulates the negative change of the perturbation potential temperature in the area where convection occurs,and predicts the low-altitude shear that triggered the system.Cloud microphysical field mainly generates more rainwater particles,which makes the final precipitation forecast closer to the observation.In comparison,radar data assimilation has little effect on the squall line system under weak weather scale forcing,mainly enhancing the horizontal wind field,but has a small effect on the temperature field.Also,the initial analysis field contains too much ice-phase water hydrometeor,which indirectly affects the distribution of the dynamic field of the squall line system,and then causes the squall line system to produce a descend motion at the back of the bow echo.