Coastal high resolution wind fields retrieved from RADARSAT-1 ScanSAR

An algorithm is presented for retrieving high resolution wind fields from the scanning synthetic aperture radar (ScanSAR) data acquired aboard the Canadian satellite RADARSAT-1. The wind directions are extracted from wind induced streaks, e.g. from boundary layer rolls, Langmuir cells, or wind shadowing, which are approximately in line with the mean wind direction. The wind speeds are derived from the normalized radar cross section (NRCS) and image geometry of the calibrated ScanSAR images, together with the local wind direction. Therefore the semi-empirical C-band model CMOD4, which describes the dependency of the NRCS on wind and image geometry, is inverted. CMOD4 was originally developed for the scatterometer operating at C-band with vertical polarization and has to be modified for horizontal polarization, which is performed by considering the polarization ratio according to Kirchhoff scattering. To verify the algorithm, wind fields were computed from several RADARSAT-1 ScanSAR images and compared to collocated results from the Danish high resolution limited area model. To estimate relative errors of wind speed due to uncertainties in wind direction and NRCS, sensitivity studies were performed.