ECMWF and SSM/I Global Surface Wind Speeds

Abstract

Monthly mean 2.5° ? 2.5° resolution 10-m-height wind speeds from the Special Sensor Microwavelimager (SSM/I) instrument and the European Centre for Medium-Range Weather Forecasts (ECMWF) forecast-analysis system are compared between 60°S and 60°N during 1988?91. The SSM/I data were uniformly processed while numerous changes were made to the ECMWF forecast-analysis system. The SSM/I measurements, which were compared with moored-buoy wind observations, were used as a reference dataset to evaluate the influence of the changes made to the ECMWF system upon the ECMWF surface wind speed over the ocean. A demonstrable yearly decrease of the difference between SSM/I and ECMWF wind speeds occurred in the 10°S?10°N region, including the 5°S?5°N zone of the Pacific Ocean, where nearly all of the variations occurred in the 160°E?160°W region. The apparent improvement of the ECMWF wind speed occurred at the same time as the yearly decrease of the equatorial Pacific SSM/I wind speed, which was associated with the natural transition from El Niña to El Niño conditions. In the 10°S?10°N tropical Atlantic, the ECMWF wind speed had a 4-yr trend, which was not expected nor was it duplicated with the SSM/I data. No yearly trend was found in the difference between SSM/I and ECMWF surface wind speeds in middle latitudes of the Northern and Southern Hemispheres. The magnitude of the differences between SSM/I and ECMWF was 0.4 m s?1 or 100% larger in the Northern than in the Southern Hemisphere extratropies. In two areas (Arabian Sea and North Atlantic Ocean) where ECMWF and SSM/I wind speeds were compared to ship measurement the ship data had much better agreement with the ECMWF analyses compared to SSM/I data. In the 10°S?10°N area the difference between monthly standard deviations of the daily wind speeds dropped significantly from 1988 to 1989 but remained constant at about 30% for the remaining years.