Investigations on extreme sea states and hurricanes using Synthetic Aperture Radar data

Within the last 20 years at least 200 super carriers have been lost due to severe weather conditions. In many cases the cause of the accidents is believed to be 'rogue waves', which are individual waves of exceptional wave height or abnormal shape. In situ measurements of extreme waves are scarce and most observations are reported by ship masters after the encounter. A normal Atlantic hurricane season typically has an average of 10 tropical storms, of which six reach hurricane strength with two classified as major. Abovenormal activity has been observed for six of the seven Atlantic hurricane seasons between 1994 and 2001. Here, timely forecasting is essential to take the necessary precautions to protect life and property. In this paper a global set of synthetic aperture radar (SAR) images is used to detect extreme ocean wave events. The data were acquired aboard the European remote sensing satellites ERS-1/2 and ENVISAT. Complex SAR data contain information on both single and significant ocean wave height, wave propagation direction, and wave grouping, as well as on ocean surface winds. For hurricane analysis radar wave mode (5 10 km 2 ) as well as image mode (100 100 up to 500 500 km 2 ) data have been used. Wave mode data were available for the hurricane Fran active during August 23 to September 8, 1996. Due to the small size and distance of 200 km of the imagettes, only a small part of the hurricane is imaged. Progressing swell was imaged and wave parameters from the hurricane have been extracted. The larger coverage of 500 500 km 2 ENVISAT images can reveal the complete storm system. In image mode data, wind speed and direction are extracted. New gradient based algorithms show the turning winds in detail. Extreme Wave events are high single waves in a relatively calm sea state as well as groups of high waves both endangering ships and offshore industry. Single waves can be imaged globally and global two dimensional ocean surface measurements are possible now for the first time using this data. Thus the parameters mentioned provide information about sea states near (in location and time) to known ship accidents and can be used to draw maps of dangerous regions, all using some 34000 images of the 3 weeks in mid 1996. A huge dataset of 10 years ERS data to be processed in future can provide a climatologic atlas of sea states.