Evaluation of an operational SAR wind field retrieval algorithm for ENVISAT ASAR

The operational algorithm WiSAR is introduced, which enables to extract high-resolution ocean surface wind fields from satellite borne synthetic aperture radars (SARs) on a fully operational basis. WiSAR can be applied to SAR data acquired in C-band at either vertical (VV) or horizontal (HH) polarization in transmit and receive from the European satellites ERS-1/2 and ENVISAT as well as the Canadian satellite RADARSAT-1. SAR wind field retrieval is a two step process. In the first step wind directions are extracted from wind induced streaks that are visible in the SAR images at scales above 200 m and that are assumed to be approximately in line with the mean surface wind direction. The orientations of these streaks are derived by a method based on investigation of local gradients of the SAR intensity image. The SAR retrieved wind directions are used in the second step, where wind speeds are derived from the normalized radar cross sections of the SAR data under consideration of the wind direction and local SAR imaging geometry. Therefore, the empirical model CMOD4, is used, which was developed for the C-band VV polarized scatterometer aboard ERS-1/2. CMOD4 has been extended to HH polarization considering the polarization ratio and its dependency on incidence angle. To show WiSARs applicability it is applied to a set of 32 ENVISAT ASAR data from the North Sea. The resulting wind fields are compared to the results of the operational numerical model of the German Weather Service.