An extended specification of flow-dependent background error variances in the Météo-France global 4D-Var system



A variational ensemble assimilation has been developed at Météo-France to provide errors ‘of the day’ to the operational 4D-Var assimilation of the Arpège model. In the reference Arpège system (operational until April 2010), ensemble-based variances are used for vorticity and for the associated balanced parts of mass and divergence, while variances of the unbalanced parts remain static and horizontally homogeneous, and humidity variances are calculated with an empirical flow-dependent formula. This article presents some diagnostic and impact studies to examine the effects of extending the specification of flow-dependent ensemble-based variances to all variables in the minimization. The extended flow-dependent variances derived from the ensemble are first compared with the reference system values. It is observed that the flow-dependence of local variances of all variables tends to be strengthened with the extended specification. This is particularly noticeable for surface pressure variances in the vicinity of mid-latitude storms and tropical cyclones. The impact of using variances ‘of the day’ for all analysis variables is examined through analysis/forecast experiments over a 1-month period. The global impact of this extended specification is neutral to positive. Results suggest that the new variances are likely to produce localized (in space and time) positive impacts, most likely connected with dynamically active systems. This is supported by a case study of an intense storm that hit the northern part of France on February 2009. It is observed that the extended flow-dependent information improves the 48 h surface pressure forecast, by correcting both position and intensity errors. Copyright © 2011 Royal Meteorological Society