Wave Age Retrieval from SAR Images in Japanese Coastal Waters

In this study, we present an approach to retrieve wave period, phase speed at the peak of the spectrum and thus the wave age from SAR image mode precision image (PRI) data around the coast of Japan. The in-situ NOWPHAS wave gauge data is used for matching up the SAR images and for validation. The wind speed at 10m height is acquired by inverting the relationship between the radar cross section and the wind speed (CMOD-IFR model). The peak wavelength can be directly derived from the SAR image spectrum. Using the dispersion relation of surface wave, the two dimensional wave number spectra can be transformed to the frequency spectrum which determines the peak wave period. The correlation coefficient (cc) between the SAR-derived wave period and in-situ wave period is 0.73. The surface waves appeared on SAR images along the coast of Japan, usually has the dominant phase speed larger than 8m/s. The phase speed of the waves off the coast of Pacific ocean side is larger than that of Japan sea side due to the statistic long travelling distance. The multi-peak of SAR spectra gives the large error, which should be dealt with carefully.