Hurricane Winds Measured with Synthetic Aperture Radars

Since 1999 several synthetic aperture radar (SAR) images of hurricanes have been acquired by the Canadian satellite RADARSAT-1 as well as the European satellite ENVISAT. Several of these SAR images have captured hurricanes of category 4 and 5. These SAR images provide a unique opportunity to investigate the utility of SAR data for estimation of hurricane winds as well as for the improvement of hurricane forecasting. Using the SAR wind retrieval algorithm WiSAR, we have obtained good accuracies (root mean square error of 18´ circ and 1.5 ms^-1) for low to moderate wind speed conditions. The algorithm enables one to retrieve wind fields with a resolution of up to 300 m over a swath width of up to 500 km. WiSAR is an algorithm, which has shown to give good results under low and moderate wind conditions. The algorithm extracts wind directions from wind induced streaks imaged by the SAR at scales above 200 m. Wind speeds are extracted from the SAR measured normalized radar cross section (NRCS) utilizing the C-band model CMOD5, which describes the dependency of the NRCS on wind. It will be shown that the algorithm enables to measure wind directions as well as wind speeds of over 50 m s^-1. The SAR-retrieved wind fields are compared to results of a high resolution numerical hurricane model.