Study on Retrievals of Ocean Wave Spectrum by Spaceborne SAR in Ice-Covered Areas

The sea ice in the Arctic is retreating rapidly and ocean waves may accelerate the process by interacting with sea ice. Though Synthetic Aperture Radar (SAR) has shown great capability of imaging waves in ice, there are few attempts to retrieve the ocean wave spectrum (OWS) by SAR in ice-covered areas. In this study, based on the previously developed nonlinear inversion scheme, i.e., the Max Planck Institute (MPI) scheme, and the Sentinel-1 SAR data acquired in the Barents Sea, ocean wave spectra were retrieved by using the different combinations of modulation transfer functions (MTFs) in the MPI scheme: (1) using the same MTFs as those used in open water; (2) by neglecting both the hydrodynamic and tilt modulations; (3) by neglecting the hydrodynamic modulation but involving a newly fitted tilt modulation over ice for HH-polarized SAR data. We compared the simulated SAR image spectra based on the retrievals with the observational SAR image spectra to quantify their respective performances. The comparisons suggest that neglecting hydrodynamic modulation can significantly improve the retrievals. The remaining tilt modulation can further improve the retrievals, particularly for range-travelling waves. This study enhances the understanding of the principles of SAR imaging waves in ice and provides basics for retrievals of ocean wave spectra by SAR data in ice-covered areas.