Validation of Geosat altimeter‐derived wind speeds and significant wave heights using buoy data

Abstract

This paper presents a comparison of Geosat radar altimeter‐derived wind speeds and significant wave heights with those measured by buoys in the National Data Buoy Center (NDBC) network. Measurements from a subset of 43 buoys moored in coastal regions and deep ocean, including the North Pacific, North Atlantic, and Gulf of Mexico, were examined. Data from other buoys within the NDBC network were not included because of their proximity to land. Seven months of altimeter comparisons were obtained during the Geosat geodetic mission for the periods May–August and October–December 1985. Only Geosat data within 150 km of buoy locations were accessed; this resulted in 1166 wind speed and significant wave height pairs. An error analysis was performed to better understand the differences between altimeter and buoy‐derived results and to establish consistency between the two sets of measurements. The following error sources were identified and quantified in terms of their impact on wind speed and significant wave height uncertainties: (1) inaccuracies due to buoy instrumentation, (2) errors resulting from temporal and spatial separations, (3) errors arising from averaging of the buoy data in time and averaging of altimeter data over its footprint, and (4) altimeter instrumentation errors. Four algorithms relating altimeter radar cross section to ocean surface wind speed were investigated. The Brown algorithm (Brown et al., 1981), and the smoothed form of the algorithm of Brown et al. (Goldhirsh and Dobson, 1985) were found to give the smallest rms errors. These were 1.7 m/s and less for a maximum altimeter track‐buoy separation of 50 km for most cases considered. This uncertainty agreed with the error obtained from the error analysis. Significant wave heights measured by the altimeter were found to be in agreement with buoy‐derived values to within 0.5 m rms. This value was also found to be in agreement with that derived from the error analysis. The measurement goals for Geosat were 1.8 m/s rms for wind speeds from 1 m/s to 18 m/s and 0.5 m rms for significant wave height or 10%, whichever is greater (Frain et al., 1985). These goals were met for the range of wind speeds compared.