The Doppler spectral characteristics of electromagnetic backscattered echoes from dynamic nonlinear surfaces of finite-depth sea is investigated with the second-order small-slope approximation (SSA-II). The revised nonlinear hydrodynamic choppy wave model (CWM) combining with an experiment-verified shoaling coefficient is utilized to model the finite-depth sea wave profiles, and the simulated surfaces of finite-depth sea show steeper crests and more flat troughs as depth decreases. First, Comparison of the Doppler spectra for linear sea surfaces and nonlinear choppy sea surfaces shows that nonlinear hydrodynamic effect greatly enhances the Doppler shift and the Doppler spectrum bandwidth, and the predicted results agree well with the rigorous numerical model data. The Doppler spectra of backscattered echoes from finite-depth sea with different depths are further evaluated. At small incident angles, the Doppler shifts and the spectra bandwidths are much lower for shallower sea, and the opposite situation can be gradually observed for increased incident angles. This indicates that the nonlinear wave-wave interactions among waves occur more frequently in finite-depth sea and the long waves will be suppressed while shorter wind waves will be boosted in shallower water. Moreover, the dependence of the Doppler spectral characteristics on polarization is also discussed.
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