On the use of EDA background error variances in the ECMWF 4D-Var

A hybrid assimilation system which uses sample statistics from an Ensemble of Data Assimilations (EDA) to estimate background error variances has been implemented at ECMWF. We show that the new system is beneficial in terms of deterministic forecast skill, provided that random and systematic errors in the estimation of variances are properly accounted for. The mechanisms through which EDA sample variances influence the deterministic analysis are clarified. An interesting aspect is that the use of flow-dependent variances alone is able to introduce a significant degree of flow dependency in the analysis increments. Further possible improvements and extensions to the current methodology are also discussed. Copyright (c) 2012 Royal Meteorological Society