ECMWF AND SSM I GLOBAL SURFACE WIND SPEEDS

Monthly mean 2.5-degrees x 2.5-degrees resolution 10-m-height wind spe eds from the Special Sensor Microwave/Imager (SSM/I) instrument and th e European Centre for Medium-Range Weather Forecasts (ECMWF) forecast- analysis system are compared between 60-degrees-S and 60-degrees-N dur ing 1988-91. The SSM/I data were uniformly processed while numerous ch anges were made to the ECMWF forecast-analysis system. The SSM/I measu rements, which were compared with moored-buoy wind observations, were used as a reference dataset to evaluate the influence of the changes m ade to the ECMWF system upon the ECMWF surface wind speed over the oce an. A demonstrable yearly decrease of the difference between SSM/I and ECMWF wind speeds occurred in the 10-degrees-S-10-degrees-N region, i ncluding the 5-degrees-S-5-degrees-N zone of the Pacific Ocean, where nearly all of the variations occurred in the 160-degrees-E-160-degrees -W region. The apparent improvement of the ECMWF wind speed occurred a t the same time as the yearly decrease of the equatorial Pacific SSM/I wind speed, which was associated with the natural transition from La Nina to El Nino conditions. In the 10-degrees-S-10-degrees-N tropical Atlantic, the ECMWF wind speed had a 4-yr trend, which was not expecte d 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 midd le latitudes of the Northern and Southern Hemispheres. The magnitude o f the differences between SSM/I and ECMWF was 0.4 m s-1 or 100% larger in the Northern than in the Southern Hemisphere extratropics. In two areas (Arabian Sea and North Atlantic Ocean) where ECMWF and SSM/I win d speeds were compared to ship measurements, the ship data had much be tter agreement with the ECMWF analyses compared to SSM/I data. In the 10-degrees-S-10-degrees-N area the difference between monthly standard deviations of the daily wind speeds dropped significantly from 1988 t o 1989 but remained constant at about 30% for the remaining years.