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Wind-generated solitons: A potentially significant mechanism in ocean surface wave generation and surface scattering

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2 Author(s)
Middleton, D. ; New York, NY USA ; Mellen, R.

It is proposed that wind-generated solitons (hydraulic "jumps"), moving nondispersively on the thin wind-driven drift layer of wind-excited (ocean) wave surfaces, are: a) important products of the process whereby wind-wave surfaces are generated; b) potentially significant in low-angle high-frequency (for example, X -band radar) scattering from such surfaces; and c) a plausible mechanism responsible for the observed anomalous (vis-à-vis conventional theory) large backscatter returns in underwater acoustics when high frequencies and small grazing angles ( \phi < 20\deg ) are employed, in the absence of significant near-surface bubble layers [1]. When the local atmospheric conditions are such that significant bubble densities are created (principally by breaking waves), the resulting bubble layer will dominate and mask these solitons, in underwater acoustic scattering. They should, however, remain effective at low angles and high frequencies in electromagnetic (EM) or radar scattering from above the surface, unless surface spray is heavy. A qualitative mixed linear-nonlinear model of ocean-wave generation is briefly described, a key element of which is the production of soliton ensembles. These are created, and destroyed, on the moving gravity-wave "facets" of the developing wind-wave surface, along with accompanying capillary components. Unlike these gravity-capillary waves, which are dispersive, the solitons are nondispersive, i.e., move at constant speed, on the thin (moving) wind drift layer generated upon the upper part of the water surface. A variety of experimental data and evidence [2]-[7], [10]- [14], [17] is cited and discussed in support of the proposed soliton mechanism. Included are recent results for acoustic backscatter at small grazing angles in the frequency range 5-20 kHz [5], based on a Poisson model for the soliton statistics [1], which yields a theoretical soliton wavenumber spectrum of the form W_{2}(k|0) = A{1 +- (bk)^{2}}^{-2} . The inferred parameters of the soliton structure are consistent with experimental observations of the acoustic baekscatter [5]. They also suggest the critical role of the near-surface locally turbulent winds. These are inadequately parameterized by mean wind speed ( \bar{U}_{a} ) alone: higher moments (var \bar{U}_{a} , etc.) are needed to account for the often markedly different wave surface states and resulting spectra, backscatter levels, etc., for the unsaturated nltragravity and capillary wavenumber regions (1/2-6 rad. cm{-1} ), which are consistently observed in practice [2], [3], [11], [13]. Finally, it is emphasized that, at this stage, the aim is to demonstrate feasibility and plausibility, not a full proof of the proposed mechanisms.

Published in:

Oceanic Engineering, IEEE Journal of  (Volume:10 ,  Issue: 4 )