Use of SAR cross spectra for wind retrieval from ENVISAT ASAR wave mode data

The new satellite ENVISAT, launched in march 2002, provides 10 x 5 km Synthetic Aperture Radar images (SAR imagettes) every 100 km along the track. This data set continues the data acquisition of the two European Remote Sensing satellites ERS-1 and ERS- 2, that have acquired similar SAR data for more than 10 years. In contrast to the ERS satellites, calibrated intensity images as well as look cross spectra are provided as standard products from ENVISAT. The new data, e.g. enable the extraction of information on wind fields on a global scale. In this study the problem of wind direction estimation is addressed. The idea is to use the spectral information contained in look cross spectra to estimate the propagation direction of the wind sea, which is strongly correlated with wind direction. The normalised radar cross section (NRCS) provided by calibrated imagettes then allows to estimate the wind speed using the CMOD4 model. A new classification method is applied to distinguish between swell and wind sea systems. The method is tested using a reprocessed data set of ENVISAT-like ERS-2 data, which are collocated with scatterometer data [SCAT]. The analysis comprises case studies of hurricanes. The data set includes several hurricanes in the Atlantic Ocean and single events will be investigated. In addition comparison with ECMWF model data will be presented. The benefit the extracted directional information on wind speed estimation is analyzed. In particular it is shown that the method leads to better results than obtained assuming a constant Wind direction.