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The Met. Office global three‐dimensional variational data assimilation scheme
Author(s) -
Lorenc A. C.,
Ballard S. P.,
Bell R. S.,
Ingleby N. B.,
Andrews P. L. F.,
Barker D. M.,
Bray J. R.,
Clayton A. M.,
Dalby T.,
Li D.,
Payne T. J.,
Saunders F. W.
Publication year - 2000
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712657002
Subject(s) - data assimilation , covariance , mesoscale meteorology , grid , computer science , meteorology , term (time) , forecast skill , variational analysis , mathematics , mathematical optimization , statistics , geography , geometry , physics , quantum mechanics
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).