Evaluation of NSCAT scatterometer winds using equatorial Pacific buoy observations

Abstract

As part of the calibration/validation effort for NASA's Scatterometer (NSCAT) we compare the satellite data to winds measured at the sea surface with an array of buoys moored in the equatorial Pacific Ocean. The NSCAT data record runs from September, 1996 through the end of June, 1997. The raw NSCAT data, radar backscatter, is converted to wind vectors at 10 meters above the surface assuming a neutrally stratified atmosphere, using the NSCAT-1 and NSCAT-2 model functions. The surface winds were measured directly by the TAO (Tropical Atmosphere Ocean) buoy array which spans the width of the equatorial Pacific within about 8° of the equator. The buoy program and data archive are maintained by the Pacific Marine Environmental Laboratory, at the National Oceanic and Atmospheric Administration, in collaboration with institutions in Japan, France and Taiwan. We also use data from two buoys maintained by the Woods Hole Oceanographic Institution located along 125°W. Since the buoy winds are measured at various heights above the surface, they are adjusted for both height and atmospheric surface layer stratification before comparisons are made to the NSCAT data. Co-location requirements include measurements within 100 km and 60 minutes of each other. There was a total of 5580 comparisons for the NSCAT-1 model function and 6364 comparisons for the NSCAT-2 model function. The NSCAT wind speeds, using the NSCAT-1 model function, are lower than the buoy wind speeds by about 0.54 ms-1 and have a 9.8° directional bias. The NSCAT-2 winds speeds were lower than the TAO buoy winds by only 0.08 ms-1, but still have the same 9.8° directional bias. The wind retrieval algorithm selects the vector closest to the buoy approximately 88% of the time. However, in the relatively low wind speed regime of the TAO array, approximately 4% of the wind vectors are more than 120° from the buoy wind.