Application of Velocity Bunching Model to Estimate Wave Height of Ocean Waves using Multiple Synthetic Aperture Radar Data

The purpose of this study is to investigate the images of ocean waves produced by synthetic aperture radar (SAR) through the image modulations based on the normalized radar cross section (NRCS) and wave orbital motions, leading to the estimation of ocean wave height. The former is known as the tilt or NRCS modulation and the latter as velocity bunching. In general, the dominant contribution to the image modulation of range travelling waves is the NRCS modulation, and that of azimuth travelling waves is the velocity bunching mechanism. In this study, these two wave imaging mechanisms are investigated using the airborne Pi-SAR (Polarimetric-Interferometric SAR) X-band VV-polarization images of ocean waves around the Miyake Island, Japan. Two images in a same region were produced at approximately 20 minutes interval from two orthogonal directions. One image shows the images of dominant range travelling waves, and the other shows a different wave pattern of azimuth traveling waves. This difference can be caused by the different image modulations of NRCS and velocity bunching. In this study, 18 sub-images are extracted from the two sets of Pi-SAR data, and the directional wave spectra of these sub-images in the orthogonal look directions are compared. We have estimated the dominant wavelength from the images of range waves, and the wave phase velocity from the dispersion relation; the image intensity is also computed by using the velocity bunching model and compared with the data. The comparison of the results strongly suggests that the latter images of azimuth waves are produced by velocity bunching. Furthermore, it is shown that the wave height of 1.4 m estimated from the velocity bunching model is similar to the height of 1.5 m simulated using the MM5 numerical weather model and data assimilation by JWA (Japan Weather Association).