The ECMWF operational implementation of four-dimensional variational assimilation. III: Experimental results and diagnostics with operational configuration

The first two papers of this series describe the development of the operational four-dimensional variational assimilation (4D-Var) configuration implemented at the European Centre for Medium-Range Weather Forecasts (ECMWF). The basic features are a 6-hour incremental 4D-Var set-up with two minimization steps, using very simplified physics in the first minimization and a more complete set of linear physics in the second. This paper describes the validation of this configuration. Prior to implementation, 12 weeks of experimentation showed a consistent improvement relative to 3D-Var. After an additional 6 weeks of encouraging parallel operation with the then current operational suite, 4D-Var with physics was introduced in operations at ECMWF in November 1997. The difference in scores is statistically significant, and the fast-growing components of the 4D-Var analysis errors are shown to be smaller than their 3D-Var counterparts. The performance of this new operational assimilation system is studied for the month of January 1998, for which the 4D-Var analyses exhibit more realistic baroclinic waves than the 3D-Var, especially in the Pacific area. A case-study illustrates the improvement one can expect in forecast terms in the mid latitudes. The 4D-Var system improved the forecast skill in the Tropics in general. Observing-system experiments show that the current 4D-Var operational system benefits from the assimilation both of satellite data and conventional observations.