Comparison of ECMWF and satellite ocean wind speeds from 1985 to 1992

We compare wind speeds derived from analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) from 1986 to 1992 with wind speeds retrieved from three space borne instruments: the Geosat altimeter, the Special Sensor Microwave/Imager (SSM/I) microwave radiometer and the European Remote Sensing (ERS-1) scatterometer. The comparison reveals imperfections in both ECMWF and SSM/I wind speeds. The ECMWF wind speeds are systematically underestimated with respect to the satellite wind speeds in the tropical Pacific and the Atlantic Ocean before May 1989. After the changes to the ECMWF model in May 1989, the differences vanish in the Atlantic and are reduced by about 50 per cent in the Pacific Ocean. However, the differences between ECMWF and satellite wind speeds are greater than those observed between the satellite wind speeds themselves: yearly differences between the satellite and the ECMWF wind speeds are greater than 2m s(-1) in tropical regions whereas intercomparison of satellite wind speeds reveals differences within +/-1m s(-1). In September 1991 a new version of the ECMWF model was introduced which greatly altered the wind analyses, both in the tropics and in the southern Pacific Ocean. At the global scale, the mean ECMWF-SSM/I wind difference is close to Om s(-1) before the September 1991 changes and increases to 0.4m s(-1) following the changes, meaning that the ECMWF wind speeds are 5 per cent less than the SSM/I wind speeds, an observation which may also be made from the ECMWF-ERS1 comparisons. The SSM/I wind speeds appear as underestimates with respect to the other three data sets in regions of high and moderate wind speeds, in particular in the high northern latitudes and in the Arabian sea, which could result from a Wentz algorithm directional flaw.