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

Année de publication

2000

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

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