Use of NCEP operational global ensembles in a GSI-based hybrid ensemble analysis for the North American Mesoscale (NAM) model runs

The hybrid ensemble variational method has been installed as an option in the NCEP Gridpoint Statistical Analysis (GSI) system. Extensive testing is underway for the global application of the hybrid ensemble but they are reported elsewhere. This paper will describe that approach and the results of its initial application for the limited area regional NAM using the new NEMS-NMMB model

The ensemble used is a direct interpolation of 80 members of the NCEP global ensemble forecast system (GEFS) to the domain of the NAM which is run at 12 km resolution. The hybrid ensemble option in GSI has a dual resolution feature which allows the ensemble perturbations to be represented at a resolution appropriate to the ensemble forecast model (currently T190L28 for the GEFS). Because the effective resolution for T190 with quadratic grid is about 70km, the extra cost of running GSI for NAM with the hybrid ensemble dual resolution option should be minimal.