Development and Testing of a Generic Model for a 4DVAR version of GSI

The development of a 4DVAR data assimilation system for the GSI analysis has been underway for some time through collaboration between EMC/NCEP/NOAA and DAO/NASA. This paper describes development of a generic (“inner loop”) model for a 4DVAR version of GSI that will be implemented at NCEP, including its tangent linear and adjoint versions. This development relies on the available numerical infrastructure contained within GSI code suite, primarily on the model for calculation of time tendencies, based on high order compact differencing that is used in the 3DVAR version of GSI for formulation of dynamical constraints and FOTO.

The paper describes dynamical part (“core”) of the model, time-stepping scheme, strategy for splitting in time, compact differencing schemes applied, strategy for polar filtering, method used do minimize artificial generation of gravity wave noise and the PBL scheme included in the model including its time stepping algorithm. A preliminary comparison of results comparing the nonlinear model with the tangent linear and adjoint versions of the model and the full operational forecast system will also be discussed. Initial results from the 4dvar system will also be presented.