Impact of ERS scatterometer winds in ECMWF's assimilation system

The impact of ERS scatterometer winds on the European Centre for Medium-Range Weather Forecasts (ECMWF) three-dimensional variational (3D-Var) and four-dimensional variational (4D-Var) assimilation systems is investigated. ERS scatterometer winds are found to be of consistent high quality in comparison to other surface wind observations. Both 3D-Var and 4D-Var systems assimilate the data well and a small, but significant, positive impact is found on medium-range weather and wave forecasts. ERS scatterometer winds are not affected by rain or clouds, so they give valuable wind information near intense cyclones, tropical cyclones and in baroclinic regions. It is shown how the 4D-Var method allows for dynamically consistent propagation of the surface wind information in the vertical. Four case studies that highlight important aspects of ERS scatterometer wind assimilation are presented. The main drawback for the ERS scatterometer instrument is the rather narrow swath compared with most other satellite instruments.