Radar data assimilation with the updated WRFDA-4DVAR system

The 4-dimensional variational data assimilation system of Weather Research and Forecast model (WRFDA-4DVAR) is further developed to assimilate radar observations. The development includes 1) the inclusion of cloud water and rain water as hydrometeor control variables and their error covariances; 2) the development of tangent-linear and adjoint of a Kessler warm-rain scheme; 3) a new observation operator for radar reflectivity. The performance of the updated WRFDA-4DVAR system was estimated with a convective case occurred on June 13 2002 during IHOP_2002 (International H2O Project) experiment. The impacts of radar radial wind and radar reflectivity on analysis and short-term numerical prediction of the IHOP case were assessed. Compared to the control experiment from Eta model 40 km analysis and 3DVAR analysis, radar data assimilation with 4DVAR method initiated the convection system in the first hour and improved the hourly precipitation forecast skill. Assimilating the reflectivity and radial velocity observations in 4DVAR enhanced the moisture and produced a cooling region near the surface in the convective region, which helped to maintain the convective system.