Impact Of Assimilating Radar And Surface Observations On The Forecast Of A Nocturnal Squall Line In The Yangtze-Huaihe River

To deeply explore the influence of assimilating radar and surface station observations on the forecast of the severe convection generated on the stable boundary layer at night,the study first analyzed the evolution characteristics and environmental background field of a nocturnal squall line occurred in the Yangtze-Huaihe River on July 11,2014,by using radar,precipitation,sounding observation and reanalysis data.And then,the influence of different radar data assimilation methods on the forecast of the squall line is analyzed by the WRF-3DVar assimilation system.Finally,the surface data assimilation was added to analyze the difference in the influence of assimilating radar and surface observation data on the convection initiation and maintenance forecast of the squall line.The main conclusions are as follows:This squall line evolved from a linear convection generated in the nocturnal stable boundary layer,which is mainly affected by the low-level shear line,southwest vortex,subtropical high and high-low level jet.The precipitation caused by the squall line is mainly distributed in the junction of Henan,Hubei and Anhui.The heavy precipitation center is located in Xinyang City,Henan Province.It has the characteristics of long duration,wide range,concentrated period,large hourly rainfall intensity and accumulated rainfall.In the environmental background field of the nocturnal squall line,the water vapor conditions are sufficient,and there are low-level convergence and highlevel divergence,with strong vertical ascending motion.Radar reflectivity and radial wind assimilation both have a significant impact on the initial field and forecast field.In the initial field,the influence of direct assimilation of reflectivity on the wind field is greater than that of indirect assimilation.When the reflectivity and radial wind are assimilated simultaneously,the radial wind plays a leading role.The influence of assimilation reflectivity on the temperature field and humidity field is greater than that of assimilation radial wind,and assimilation reflectivity and radial wind simultaneously can effectively combine the adjustment effect of both.In the forecast field,the position and intensity of the squall line in reflectivity directly assimilation are close to the observation,while assimilation of radial wind alone,indirect assimilation of reflectivity,and joint assimilation of reflectivity and radial wind produce some biases in the vertical ascending motion and fail to forecast the squall line.The squall line and precipitation belt generated by the direct assimilation of reflectivity and simultaneous assimilation of radial wind are the closest to the observation.The assimilation of radar and surface data can improve the prediction of the convection initiation time,height and vertical ascending motion during the early stage of the squall line formation by adjusting the thermodynamic structure,circulation patterns,water vapor conditions and hydrometeor mixing ratios.The surface data assimilation can directly improve the near-surface temperature,and thus more accurately describe the thermal structure of the boundary layer.Assimilating radar data can produce strong vertical ascending motion and water vapor conditions,thereby enhancing radar reflectivity and precipitation intensity,while assimilating surface data mainly improves reflectivity and precipitation position prediction by reducing false echoes and precipitation.Simultaneous assimilation of the radar and surface data can effectively combine the advantages of the two data,and the predicted radar reflectivity,precipitation intensity and location are closest to the observation.Moreover,during the mature stage of the squall line,radar and surface data assimilation can enhance the intensity of the surface cold pool,thereby causing the outflow of the cold pool to converge with the southerly airflow and trigger new convective cells,thus promoting the maintenance and development of the squall line.Specifically,radar data assimilation or assimilating the two data simultaneously can produce a stronger cold pool than only assimilating surface data,which is more conducive to the maintenance and development of the squall line.