Hybrid 4D EnVar for the NCEP GFS: Sensitivity experiments and plans for operational implementation
By taking the existing 3D EnVar algorithm in GSI and allowing for four-dimensional ensemble perturbations, coupled with the 4DVAR infrastructure already in place, a 4D EnVar capability has been developed. The 4D EnVar algorithm has a few attractive qualities relative to 4DVAR, including the lack of need for tangent-linear and adjoint model as well as reduced computational cost. Results for the GFS from an observing system simulation experiment (OSSE) show that analysis error was reduced when going from 3D- to 4DEnVar.
The results from various sensitivity experiments using real data will be presented. In particular, emphasis will be placed on the use of an outer loop as is done within traditional 4DVAR. Additionally, balance and noise related issues will be explored and addressed through the use of a variety of dynamic constraints. Lastly, plans will be discussed regarding the operational implementation 4D EnVar as well as potential long term development path for the NCEP GDAS/GFS.