Surface Wave Parameters Retrieval in Coastal Seas from Spaceborne SAR Image Mode Data

Abstract— We have developed a retrieval scheme of surface wave parameters (wave height and wave propagation direction) from ERS Synthetic Aperture Radar (SAR) image mode data in coastal seas around Japan. In this study, we investigated the energy of simulated SAR spectrum at different wind speed using nonlinear wave-SAR imaging mechanism, derived the criteria to differentiate swell from wind waves, and accordingly processed SAR images containing swell and wind waves in different way respectively. SAR spectra are converted to surface wave spectra of swell-dominated or wind-wave dominated cases. The SAR spectrum and SAR-derived wind speed are used for the derivation of surface wave spectrum. The wind-wave dominated case and swelldominated case are differentiated by the wind speed of 6 m/s, and processed in different ways because of their different nonlinear degree. It is indicated that the cutoff wavelength for retrieval of the wind-wave dominated spectrum is proportional to the root of significant wave height, which is consistent with the results of previous studies. We generated 66 match-ups coupling the SAR sub-images and the in-situ surface wave parameters, which were measured by the wave gauges installed in near-shore seas. Among them, the number of swell-dominated case is 57, and the number of wind-wave dominated case is 9. The SAR-derived and in-situ significant wave heights agree well with the bias of 0.24 m, the RMS error 0.92m and the correlation coefficient of 0.66. The averaged absolute deviation of wave propagation directions is 18.4±, and the agreement tendency does not depend on the wave height. It is proven through the comprehensive validation that the developed SAR surface wave spectrum has high accuracy in the Japanese coastal seas.