Hybrid 4D EnVar for the NCEP GFS: Implementation plans and potential for outer loops
Beyond the initial implementation of hybrid 4D EnVar, various enhancements are still in their early phase such as improved data selection, the role of the static error covariance, and outer loop configuration. Analogous to the outer loop configuration in 4DVAR and so-called Quasi-Outer Loop of the EnKF, an outer loop can be run as part of 4D EnVar whereby the non-linear model is re-run using the increment at the beginning of the assimilation window. This may prove to be particularly important for dual-resolution configurations that use low-resolution ensemble members within high-resolution deterministic assimilation. Results from toy model experiments will be presented to demonstrate the potential impact of such a configuration within the 4D EnVar paradigm.