The ECMWF implementation of three‐dimensional variational assimilation (3D‐Var). III: Experimental results

In this third and final paper of a series, we assess the performance of the three‐dimensional variational data assimilation scheme, in the light of the results from the extensive pre‐operational programme of numerical experimentation. Its performance is compared with that of the previous operational scheme at the European Centre for Medium‐Range Weather Forecasts, which was based on Optimal Interpolation. The main features of the new scheme are illustrated, in particular the effects of non‐separable structure functions and the improved data usage. TIROS‐N Operational Vertical Sounder cloud‐cleared radiances, for example, are used directly without a separate retrieval step. Scatterometer data are assimilated in the form of ambiguous winds with the ambiguity removal taking place within the analysis itself. Problems encountered during the tests are discussed and the solutions implemented are explained.

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