Assimilation of Doppler radial winds into a 3D-Var system: Errors and impact of radial velocities on the variational analysis and model forecasts

A pre-processing approach to the assimilation of Doppler radar radial wind data into a high resolution Numerical Weather Prediction (NWP) model is described. We discuss the types of errors which might occur in radar radial winds. The construction of simulated high resolution radar data containing such errors is described. Examples of these data are presented and compared with actual radar data from the Chilbolton S-band radar located in central southern England. A new approach to specifying the radial velocity observation error is proposed based upon the radial gradient of the velocity along the pulse volume. The; production of 'super-observations' for input to a 3D-Var assimilation systemis discussed. Impact of the assimilation of Doppler velocities on the 3D-Var analysis and on the model forecasts, for two case-studies, are shown.