Assimilation of radar wind data in the HIRLAM 3D-Var

A 3-dimensional variational data assimilation (3D-Var) scheme has been developed for the HIgh Resolution Limited Area Model (HIRLAM). The HIRLAM 3D-Var minimizes a cost function measuring the distance of the analysis to the background field and to the observations, respectively. Each source of information is weighted according to their estimated error covariances. Radar wind observations can be input for HIRLAM 3D-Var either as radial wind superobservations or VAD vertical wind profiles.

The background quality control of radial wind data requires a priori information of the background error standard deviations in radial wind space (i.e. in observation space). The statistics are estimated using an ensemble of perturbed background states. The derived background error standard deviations in radial wind space are also used to assign radial wind observation error standard deviations such as to give a realistic weight for the radial wind observations with respect to all other sources of information. Parallel data assimilation and forecast experiments are performed in order to estimate the relative impact of radial wind information as compared to the impact of VAD wind profile information.