Radar Backscattering from Short Waves Riding on Long Gravity Waves Propagating in the Same and the Opposite Direction

Abstract

To understand the microwave radar response from ocean, it is useful to determine the time evolution of the Fourier components of the spectra of short waves riding on long gravity waves. The pseudo spectral numerical method we have developed treats the complete set of hydrodynamic equations and allows us to consider unsteady wave groups propagating on longer gravity waves. Experiments using two wave makers have been conducted in the large I.M.S.T. wind wave tank to valid the numerical, results. Numerical and experimental results show that the envelope of the short wave train is unsteady relative to the long wave. The frequency modulation phenomenon of short packet is experimentally observed and the recurrence phenomenon numerically exhibited. As main results, we find that the period of the time evolution of the backscattered power, given by an electromagnetic model based on the Bragg resonance, from unsteady short wave packet is close to that of the long waves.