Impact of assimilation of Doppler radial velocity on a variational system and on its forecasts

An approach to the assimilation of Doppler radar radial winds 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 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 across the pulse volume. The variation of this error with range is derived for a specific case. The production of "super-observations" for the input to a 3D-Var assimilation system is discussed in this paper. The impact of the Doppler radial winds and their errors in the variational assimilation system is also investigated.