Assimilation of radar and surface observations for the analysis and prediction of a squall line

This paper examines the impact of radar and surface observation assimilation on the analysis and prediction of a squall line during the field experiment of OPACC in East China using a Four Dimensional Variational Doppler Radar Data Assimilation System (4DVAR) scheme. Different from the previous studies that usually interpolate the surface observations in background field, an improved assimilation scheme for the surface data is proposed where surface temperature and wind observations are corrected using standard lapse rates and power-law wind profiles. The ideal experiment with single point assimilation shows this new surface observation assimilation scheme can produce better analysis within the boundary layer. Moreover,the assimilation and forecasting experiments for the squall line show the assimilation of both radar data and surface data can produce the best analysis among all experiments. The assimilation of radar data mainly improves the structure analysis of the squall line, while the additional assimilation of surface data helps improve the wind, temperature and pressure analysis below 1 km. With the improved analysis, the subsequent structure and evolution of the squall line is predicted more accurately. The detailed analysis of the squall line structure will be presented.