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

TLDR

The Met Office has developed a variational assimilation for its Unified Model forecast system, operational in global, mesoscale, and stratospheric configurations. The design incorporates a progression from 3D to 4D variational assimilation, global and limited‑area configurations, perturbation analysis, and a well‑conditioned background term implemented through variable transforms to independent balanced and unbalanced variables, vertical modes, and spectral coefficients derived from one‑ and two‑day forecast differences. The covariance model captures many features of forecast‑difference covariances, and the 3D‑Var system achieved a 2.7 % improvement in a composite skill score. Abstract The Met.

Abstract

Abstract The Met. Office has developed a variational assimilation for its Unified Model forecast system, which contains a grid‐point mode) that is run operationally in global, mesoscale, and stratospheric configuration. 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).