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

Abstract 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.