Impact of Sea State on Wind Retrieval From Sentinel-1 Wave Mode Data

Abstract

High-resolution wind field is essential to investigate local winds variability over the global ocean. Space-borne synthetic aperture radar (SAR) can provide wind observations up to hundreds of meters. Although SAR wind retrieval algorithms have been widely studied, improvements are still required. Sea state has been proven to be a primary factor in impacting wind speed retrieval. In this study, we analyzed data from Sentinel-1 wave mode, which alternately operates at two incidence angles (23.8 $^\circ$ and 36.8 $^\circ$ ) over open ocean. Accuracy of wind speed retrieval is assessed against collocated in situ buoys. Wind speed residual is found to be closely related to significant wave height $H_s$ at both incidence angles. A comparable dependence of wind speed residual on sea state is observed at 23.8 $^\circ$ with results of advanced SAR since they operate at the same incidence angle. While a weaker sea state impact on the retrieved wind speed is obtained at 36.8 $^\circ$ , consistent with the scatterometer analysis. In addition, the wind speed residual is similarly dependent on azimuth cutoff as $H_s$ . We, therefore, demonstrate the possibility to compensate the sea state impact by including azimuth cutoff in the geophysical model function of the normalized radar cross section.