Sea-Surface Backscattering and the Soliton Mechanism

Sonar and radar measurements both indicate that there are fundamental problems with the present wind-wave models of the sea surface at higher wavenumbers i.e., backscattering strengths at high frequencies and small grazing angles are much greater than predicted and the Doppler spectra show no evidence of the dispersion expected in the gravity-capillary regime. Quasi-linear wave theories do not account for these effects. Laboratory wind-wave flume measurements also support this conclusion. Photographs reveal that wavefronts of ripples tend to steepen with increasing fetch. Spectra of wave-gauge data also show a corresponding growth of coherent harmonics. Nonlinear amplitude effects can account for the non-dispersive behavior of waves in this regime. With further increase in fetch, the wave-gauge data also show the growth of subharmonics followed by rapid degeneration to a continuous spectrum. This suggests that a second mechanism is involved in sea-state development namely, chaotic behavior arising from surface-instability and wave-wind interaction. The proposed program could have far-reaching implications. The primary goal is to predict sonar and radar scattering phenomena in total detail from knowledge of surface-wave statistics however, the study reveals a potential chaotic mechanism for wave generation that could be crucial to developing a more comprehensive predictive model of sea-state development.