Steps towards an Improved Algorithm for Ocean Near Surface Wind Speeds derived from Satellite Radar Altimeter Data
Abstract
Satellite radar altimetry is a valuable tool for the determination of the instantaneous or mean sea surface heights. Beside these quantities, the returning waveform and the backscatter coefficient can be used to estimate the significant wave height (SWH) and ocean near surface wind speeds (ONSWS). Both values have to be calibrated properly for useful operational services, like weather forecasting, ship routing, or storm warnings. For the SWH determination there exist waveform retracking algorithms that can deliver highly accurate values. The ONSWS derivation from satellite radar altimeter data is based on the relationship of the onboard measured radar backscatter coefficient σ 0 and the wind speed. The operational algorithm used is a look-up table which was developed for the GEOSAT altimeter. Even though the successive radar altimeter instruments on Topex/Poseidon and ERS-1 and ERS-2 had different specifications than the Geosat system, the algorithm was at most slightly adjusted. There are many recent efforts to improve the ONSWS determination by different approaches. Most of them are restricted to relatively small geographical areas or oceanographic regimes. We have found that these approaches are reasonable in the framework they were developed for. But they fail, if they are applied to data of global scale or of other regions. In some studies global model wind fields, such as from ECMWF, are used for calibration, but local wind may vary more rapidly than computed from these models. Therefore, in our study we focus on the use of in-situ data for validating any algorithm. We have set up a highly accurate database of buoy wind and wave measurements for this purpose. Further, we identify on a global scale additional oceanographic phenomena, e.g. swell / wind-sea regimes or chlorophyll concentration. These quantities influence the derived backscatter and, therefore, the ONSWS determination both in coastal and offshore regions. First results to incorporate them into a new ONSWS algorithm to retrieve highly accurate input to the initially mentioned applications will be shown.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFMOS51C0495E
- Keywords:
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- 1640 Remote sensing;
- 3360 Remote sensing;
- 4200 OCEANOGRAPHY: GENERAL;
- 4275 Remote sensing and electromagnetic processes (0689)