The Met. Office global three-dimensional variational data assimilation scheme

The Met. Office has developed a Variational assimilation for its Unified Model forecast system, which contains a grid-point model that is run operationally in global, mesoscale, and stratospheric configurations. Key characteristics of the design are: a development path from three-dimensional to four-dimensional variational assimilation; global and limited-area configurations; variational analysis of perturbations; and a carefully designed, well conditioned background term. The background term is implemented using a sequence of Variable transforms to independent balanced and unbalanced variables, to vertical modes, and to spectral coefficients. The coefficients used are based on statistics from differences of one- and two-day forecasts valid at the same time. The covariance model represents many of the features seen in the covariances of forecast differences. The three-dimensional Variational data assimilation (3D-Var) system was implemented in the operational global forecast system on 29 March 1999. In parallel trials, the 3D-Var system gave a 2.7% improvement in a composite skill score (verified against observations and weighted according to the importance of each field).