Impact of ERS scatterometer data in the ECMWF 4D-Var assimilation system. Preliminary studies

ERS scatterometer data have been assimilated operationally in the ECMWF forecasting system since January 1996. The current data assimilation system makes use of a three-dimensional variational analysis scheme (3D-Var). The impact of scatterometer ocean surface winds, assimilated using 3D-Var, on the forecasts remains limited although larger impacts are found than in the previously operational optimal interpolation analysis. It seems that the fixed error structure assumed by the 3D-Var scheme, especially in the vertical, does not allow the effect of the surface wind observations to propagate up through the atmosphere to upper levels. ECMWF is currently developing a 4D-Var assimilation scheme which extends the 3D-Var by including time as another dimension in the minimisation. This means that observations can be used closer to measurement time, and the structure functions can change dynamically, consistent with the meteorological conditions. It is expected that scatterometer data in particular will have more of a positive impact with 4D-Var. Results will be presented that shows the positive impact of ERS scatterometer data within the 4D-Var assimilation system. This is proven by parallel analysis and forecast cycles carried out with or without scatterometer data. Comparison with the operational runs will also be presented.