Abstract Wind Speed and SWH Calibration for Radar Altimetry in the North Sea

Abstract

Satellite altimetry is a valuable tool for the determination of instantaneous sea level or mean sea surface height. Beside these quantities, the returning waveform and the backscatter coefficient can be used for computing the significant wave height (SWH) and wind speed. Both values are only raw estimates and have to be calibrated properly for useful operational services, like weather forecasting, ship routing or storm warnings. The accuracy of the satellite derived wind speed and SWH is degraded due to insufficient in-situ data used for calibration or the large distance between the in-situ sensors and the measurements. Only a few locations worldwide, oil platforms or buoys, exist for calibrations. The limited geographical distribution causes problems due to the different sea state behavior in different regions. In some studies ECMWF wind fields are used for calibration, but the local wind may vary more rapidly than computed from the models. In addition, the wind speed retrieval algorithm as used today was developed for Geosat and has to be calibrated and inter-calibrated between different satellites, e.g. to ERS-2 and TOPEX/Poseidon in order to get comparable values. Also for ENVISAT the algorithm of Witter & Chelton [16] will be used. The German Weather Service (DWD) is operating weather stations onshore and offshore, achieving highly accurate wind speed, wind direction and air pressure data. Moreover, the Federal Agency for Maritime Service (BSH) is operating a network of waverider buoys, estimating the SWH every 3 hours. Both data sets are available since 1992 an