Ocean wind field retrieval using ENVISAT ASAR data

In the last years several algorithms for high-resolution ocean surface wind field retrieval from space borne synthetic aperture radar (SAR) have been developed. These algorithms were specially designed for the C-band SARs aboard the European satellites ERS-1 and ERS-2 operating at vertical (VV) polarization in transmit and receive and the Canadian satellite RADARSAT-1 operating at horizontal (HH) polarization. This paper shows the application of the algorithms to data of the advanced SAR (ASAR) from the European satellite ENVISAT. The wind retrieval algorithm consist of two parts. In the first wind directions are extracted from wind-induced streaks visible in most SAR images at scales between 200 and 1600 m using two different methods. In the second part wind speeds are derived from the normalized radar cross section (NRCS) and image geometry of the calibrated SAR images, together with the local wind direction resulting from the first step. For the wind speed retrieval the semi empirical C-band scatterometer model CMOD5 is used. CMOD5 was originally developed for the scatterometer aboard ERS-1 and 2 operating at VV polarization and consequently requires modification if applied to HH-polarized SAR data. In case of HH-polarization the CMOD5 model is extended by considering the polarization ratio. It is shown that the quality of ENVISAT ASAR data is very well suited for measuring high-resolution ocean wind fields, which are especially important in coastal zones.