A 10 year intercomparison between collocated Special Sensor Microwave Imager oceanic surface wind speed retrievals and global analyses

To evaluate the scalar ocean surface wind speeds obtained from the Special Sensor Microwave Imager (SSM/I), we compare them over the time period from July 1987 through December 1997 with those from two global analyses: the Na tional Center for Environmental Prediction (NCEP)/National Center for Atmos pheric Research (NCAR) Annual Reanlysis and the European Center for Medium- Range Weather Forecasts (ECMWF)/Tropical Ocean-Global Atmosphere Global Sur face Analysis. We perform a statistical analysis for the whole globe and pr esent time series analyses for selected geographical regions in connection with collocated wind speed difference maps. In order to evaluate further ge ographical biases observed in the SSM/I versus analyses comparisons we use wind speeds from the NASA scatterometer (NSCAT) for the 10 month period fro m September 1996 through June 1997 as a third data source. The value of the standard deviation for all collocated SSM/I - ECMWF wind speed differences is 2.1 m s(-1) and for all collocated SSM/I - NCEP/NCAR reanalyis wind spe ed differences is 2.4 m s(-1). When taking monthly or yearly averages in ea ch pixel, which has the effect of cancelling our small timescale wind speed fluctuations, the values are between 0.8 and 1.2 m s(-1) respectively. Glo bal biases range between -0.05 and +0.55 m s(-1) for the various SSM/I sate llites. Our analysis allows us to identify regional biases for both the SSM /I and analyses winds. The NCEP/NCAR reanalysis wind speeds appear underest imated in the tropical Pacific and tropical Atlantic. ECMWF wind speeds app ear underestimated near the southern Pacific islands NE of Australia. The a nalyses wind speeds are higher than the SSM/I wind speeds near the Argentin ean coast. The SSM/I wind speeds appear high in the extratropical central a nd eastern Pacific and low in certain coastal regions with eastern boundary currents and in the Arabian Sea. The size of some of these biases are seas onally dependent.