Wind speed retrieval from RADARSAT-2 quad-polarization images using a new polarization ratio model

This paper presents a first attempt to analyze C-band RADARSAT-2 measurements of the normalized radar cross sections (NRCS) in quad-polarization acquisition mode (HH, VV, HV, and VH) over the ocean. NRCS in copolarizations and cross-polarizations are found to be different; the latter is independent of radar incidence angles and wind directions, but is quite linear with respect to wind speeds. We also investigate the properties of the polarization ratio, denoted PR, and show that it is dependent on incidence angle and azimuth angle as well as wind speed. It also correlates well with wave steepness and significant wave height. Moreover, the polarization difference shows a linear relationship with wind speed. Two new analytical models are proposed to estimate PR; one is a function of incidence angle only, while the other has additional dependence on wind speed. Comparisons are presented with theoretical and empirical PR models from the literature; the new PR model which includes wind speed dependence is shown to best compare with observed RADARSAT-2 data. An assessment of this PR model is given using different CMOD algorithms and RADARSAT-2 images. Results show that the wind speeds retrieved from this PR model and CMOD5. N are in good agreement with buoy measurements (standard deviation, 1.37 m/s). This joint GMF-PR approach constitutes a promising hybrid model for wind speed retrievals from HH-polarized RADARSAT-2 images.