Design and testing of the Met Office variational data assimilation scheme


Année de publication

The Meteorological Office has developed a variational assimilation for its Unified Model forecast system, which contains a grid-point model, run operationally in global, mesoscale, and stratospheric configurations. Key characteristics of the design are: development path from 3-dimensional to 4-dimensional scheme; global and limited area configurations; variational analysis of perturbations; carefully designed, well conditioned "background" term. The background term is implemented using a sequence of variable transforms, to independent balanced and unbalanced variables, vertical modes, and spectral coefficients. The coefficients used are based on statistics from differences of 1- and 2-day forecasts valid at the same time. The covariance model represents many of the features seen in the covariances of forecast differences. The limited trials carried out at low resolution without OPS showed good results for VAR. The initial results at high resolution are slightly worse but close to those for the AC scheme, and changes to the use of satellite data and background error covariances can improve the results so that they are slightly better than the AC scheme in terms of NWP index. However ...

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