Assimilating Surface Observations in a Four-Dimensional Variational Doppler Radar Data Assimilation System for the Analysis and Forecast of a Squall Line Case

Assimilating Surface Observations in a Four-Dimensional Variational Doppler Radar Data Assimilation System for the Analysis and Forecast of a Squall Line Case

Kun ZHAO*1, Xingchao CHEN1,2, Juanzhen SUN2, Bowen ZHOU1, and Wen-Chau LEE2

1.Key Laboratory for Mesoscale Severe Weather/MOE and School of Atmospheric Science, Nanjing University, Nanjing 210046; 2.National Center for Atmospheric Research, Boulder, Colorado 80301, USA

 

       This paper examines how assimilating surface observations can improve the analysis and forecast ability of a four-dimensional Variational Doppler Radar Analysis System (VDRAS). Observed surface temperature and winds are assimilated together with radar radial velocity and reflectivity into a convection-permitting model using the VDRAS four-dimensional variational (4DVar) data assimilation system. A squall-line case observed during the Observation, Prediction and Analysis of Severe Convection of China (OPACC) field campaign in Eastern China is selected to investigate the performance of the technique. Three experiments---assimilating radar data only, assimilating radar data with surface data blended in a mesoscale background, and assimilating both radar and surface observations with a 4DVar cost function---are conducted to examine the impact of the surface data assimilation. Independent surface and wind profiler observations are used for verification. The result shows that the analysis and forecast are improved when surface observations are assimilated in addition to radar observations. It is also shown that the additional surface data can help improve the analysis and forecast at low levels. Surface and low-level features of the squall line---including the surface warm inflow, cold pool, gust front, and low-level wind---are much closer to the observations after assimilating the surface data in VDRAS. Based on the VDRAS analysis, the triggering and evolution mechanism of the squall line will also be presented.