Laboratory and Theoretical Studies on Doppler Spectra From Bound and Free Waves

There exist bound gravity-capillary waves that move at the speed of the dominate waves on the ocean surface, apart from free gravity-capillary waves that satisfy the linear dispersion relation. The radar Doppler spectra are hypothesized to be the weighted sum of two parts, one is attributed to free waves, and the other is attributed to bound waves. Based on the BoundFree wave composite surface model (BF), a theoretical model to predict the Doppler bandwidths of bound and free waves is proposed in this paper. According to the nature and the phase distribution of bound and free waves, their Doppler bandwidths can be derived from ocean wave spectra. Meanwhile, the nonlinear motion of partial intermediate scale waves is considered. Besides, the experiments on microwave backscatter from surface waves are carried out in the wind-wave tank. General Gaussian curve fitting is used to separate and obtain the measured Doppler offsets and bandwidths of bound and free waves in the data processing. Compared with the experimental measurements at X-band and Ku-band, the model shows its applicability to predict the Doppler bandwidths of bound and free waves.