Extreme Waves Observed by Synthetic Aperture Radar

Within the last years a considerable number of large ships have been lost due to severe sea state conditions. The cause of accidents are in many cases believed to be rogue waves, which are individual waves of exceptional wave height or abnormal shape. In particular steep breaking waves can be fatal for smaller ships. Damage is sometimes also be caused by unusual grouping of waves, which can lead to dangerous ship motion. In situ measurements of extreme waves are sparse with most observations reported by ship masters after the encounter. In this paper a global data set of 5 x 10 km sized synthetic aperture radar (SAR) images acquired by the European Remote Sensing satellite ERS-2 every 200 km along the track is used to analyse extreme ocean wave events. As the European Space Agency (ESA) does not provide this dataset as a standard product wave mode raw data were reprocessed to complex SAR images using the processor BSAR developed at the German Aerospace Center (DLR). About 1000 globally distributed SAR wave mode images are available every day. Two dimensional ocean wave fields are derived from SAR images by inversion of the SAR imaging mechanism. Individual high waves are detected in the derived wave fields using a matched filter technique. Grouping of waves is studied using wavelet techniques. Statistics are presented on wave group size and frequency. The inhomogeneity of ocean wave fields is anlysed using a parameter, which describes the shift invariance of the wave spectrum.