Title: The Met Office's hybrid ensemble-4D-Var scheme

Authors: Adam Clayton (Met Office, Exeter, UK)
Dale Barker (Met Office, Exeter, UK)
Neill Bowler (Met Office, Exeter, UK)
Peter Jermey (Met Office, Exeter, UK)
Andrew Lorenc (Met Office, Exeter, UK)
Rick Rawlins (Met Office, Exeter, UK)
Mike Thurlow (Met Office, Exeter, UK)

The Met Office has developed a hybrid DA scheme, where perturbations from the MOGREPS ensemble are used to supplement the background error covariance matrix used in 4D-Var, and the 4D-Var analysis is used to recentre the MOGREPS ensemble. The ensemble perturbations are localised in VAR using the "alpha control variable method" applied to transformed variables so as to minimise the introduction of imbalance.

Early trials demonstrated a clear benefit of hybrid-3D-Var over 3D-Var, as found at other centres. Demonstrating a similar benefit from hybrid-4D-Var has been harder; after careful tuning of 3-dimensional localisation, and weighting of ensemble and climatological covariances, full resolution trials now demonstrate that the hybrid gives significant improvement in a range of scores. Final pre-operational trials are underway with the expectation of implemention in our operational global NWP system before the Workshop.

GMAO Head: Michele Rienecker Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: May 27 2011