Satellite altimeter models for surface wind speed developed using ocean satellite crossovers

A large global compilation of altimeter and scatterometer satellite crossover samples is used to address altimeter wind speed inversion. The emphasis here is altimeter wind error due to sea state impacts. Numerous past studies have suggested that long-scale gravity waves, which do not adjust to wind as quickly as the short gravity-capillary waves, have a measurable influence on interpretation of altimeter returns in terms of wind speed. This hypothesis is affirmed here. TOPEX altimeter operational wind speed estimates are compared with NSCAT-derived surface winds to show a clear correlation between altimeter wind error and the TOPEX-derived significant wave height (SWH) estimate. This observation holds for both the C and Ku-band frequencies of TOPEX. Candidate empirical algorithms using both altimeter backscatter and wave height estimates are then developed to remove the observed average significant wave height signature from altimeter-derived wind speeds. These algorithms are developed for global application and validated using numerous independent sources to confirm that slight but statisticallysignificant global error reductions (e.g. 10-15% reduction in rms error) are obtained. An algorithm for potential operational use is proposed and detailed. Our study indicates that inclusion of the altimeter’s wave height parameter provides limited additional improvement to the point-to-point accuracy of the wind speed estimates. This finding is consistent with the understanding that SWH is a crude descriptor for the complexities of actual gravity wave conditions where some mix of sea and swell is nearly always in evidence. The suite of altimeter algorithms (C and Ku-band) presented here may aid global climate studies and refinement of other satellite sensor or numerical model wind estimates. They should also serve to improve our present understanding of the electromagnetic range bias phenomenon that corrupts altimetric sea surface topography measurements.