ERS-1 scatterometer measurements. I. The relationship between radar cross section and buoy wind in two oceanic regions

Extensive European Remote Sensing Satellite (ERS-1) spacecraft scatterometer measurements were collocated within ±25 km of buoy measurements at midlatitudes and in the equatorial Pacific during 1992-1994. Two different directional functional forms for the geophysical model were fit to these measurements described by cross section maxima aligned and offset, with respect to the mean wind direction. The two models exhibit fits with similar residuals for each of three years in the equatorial region. Thus, neither of these two model functions can convincingly be identified as more accurate than the other unless additional factors are considered. Nevertheless, the offset model results in an average directional offset near 10° for each season during the three-year period; whereas, no such offset was inferred at midlatitudes. The cross section dependence on wind direction also exhibits another unexpected average property: a larger downwind compared to upwind cross section, A₁₀≈-0.25 dB in the equatorial Pacific. This characteristic could be caused by a concentration of the short Bragg waves on the backside of the long wave crests. Both of these unexpected characteristics are likely related to a dominant systematic swell traveling nearly perpendicular to the prevailing easterly winds across the equatorial Pacific Ocean. The possible cross section dependence on wind stress and wind speed is due to long waves when adjusting the buoy measurements to a common height are discussed