Modelling of flow‐dependent ensemble‐based background‐error correlations using a wavelet formulation in 4D‐Var at Météo‐France. (19th May 2015)

• Record Type: Journal Article
• Authors: Berre, Loïk; Varella, Hubert; Desroziers, Gérald
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 141:Number 692(2015:Oct.)
• Page Start: 2803
• Page End: 2812
• Publication Date: 2015-05-19
• Subjects: wavelet -- ensemble -- error correlations -- flow dependence; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.2565
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract : The use of a wavelet formulation and ensemble data assimilation is considered in this study, in order to specify flow‐dependent background‐error correlations for the Météo‐France global 4D‐Var system. This wavelet model is calibrated by using a temporal series of recent perturbed backgrounds, which is sampled in a sliding way. This approach is compared to a homogeneous and static spectral formulation of error correlations. Diagnostic studies are conducted in order to illustrate the representation of heterogeneous and flow‐dependent correlation features, corresponding for instance to smaller horizontal length‐scales in the vicinity of midlatitude lows and to larger vertical correlations in vertically unstable regions. The positive impact of the flow‐dependent wavelet formulation on the forecast quality is then shown during two contrasting three‐week periods. The impact of an increased ensemble size when using this approach is also illustrated, in terms of length‐scale dynamics and forecast quality.