Variational quality control

Abstract We describe the implementation of variational quality control in the operational four‐dimensional variational data assimilation scheme at the European Centre for Medium‐Range Weather Forecasts (ECMWF). the quality control takes place during the iterative solution of the variational analysis problem itself, rather than in a separate step prior to the main analysis. This ensures that the quality control is consistent with the analysis in terms of error statistics, background and model constraints, in particular. All observational data are used and quality‐controlled simultaneously. the probability of gross error is computed for each observation, given the preliminary analysis at each iteration. the weight of each observation is smoothly decreased with increased likelihood for gross error. the formulation, the implementation details and results from data assimilation and forecast experiments are presented.