Acoustic and microwave signatures of breaking waves

Journal of Geophysical Research, 1996

Experiments were performed to exploit the dispersive properties of unsteady surface waves and to induce breaking by using a modified chirp pulse technique to focus the wave energy at a specific location in the Naval Research Laboratory deep water wave channel. The experiments have resulted in a highly resolved archive of breaking events ranging from wave steepening and incipient breaking to spilling and to plunging. The potential energy density, the crest front steepness, the horizontal asymmetry, and other geometric properties of an incipient breaker vary only within a moderate band about their mean values over the extent of these experiments. Thus the properties of an incipient unsteady breaker are well defined. The application of the phase-time or Hilbert transform method to the data set provides new insights into the local properties of the unsteady wave breaking. Recently, spectral and piecewise-linear algoritbans for two-dimensional potential flow were developed and used by Schultz et al. [1994] to compare the onset of breaking for several methods of energy input to the unsteady wave system. The computations show that steep plunging waves occur when energy input rates are large. The various energy input methods exhibit similar breaking trends in the limit as the energy input rate becomes small in that incipient spilling breakers form when the potential energy is approximately 52 to 54% of the energy for the most energetic Stokes wave, with the formation of a singularity immediately before the crest. 16,515 16,516 GRIFFIN ET AL.: EVOLLrHON OF DEEP WATER BREAKING WAVES Background The fundamental experiments for studying twodimensional wave breaking (apart from wind-generated waves which are discussed briefly later) fall into three main categories: (1) the focusing of essentially two-dimensional waves in the lateral direction [Van Dom and Pazan, 1975; Ramberg et al., 1985; Ramberg and Griffin, 1987]; (2)the towing of a submerged object, such as a hydrofoil to produce steady breakers [Duncan, 1981, 1983]; and (3)the focusing of variable-length waves from a modulated or chirped wave maker to produce unsteady breakers [Dornmermuth et al., 1988; Duncan et al., 1987; Rapp and Melville, 1990; Peltzer et al., 1993; Duncan et al., 1994; Sletten and Savtchenko, 1996] or the overturning of an irregular wave train [Ochi and Tsai Bonrnarin, 1989] to produce unsteady breakers.

Remote Sensing

A method for obtaining two dimensional fields of wave breaking energy dissipation in the surfzone is presented. The method relies on acquiring geometrical parameters of the wave roller from remote sensing data. These parameters are then coupled with a dissipation model to obtain time averaged two dimensional maps, but also the wave breaking energy dissipation on a wave-by-wave basis. Comparison of dissipation maps as obtained from the present technique and a results from a numerical model, show very good correlation in both structure and magnitude. The location of a rip current can also be observed from the field data. Though in the present work a combination of optical and microwave data is used, the underlying method is independent of the remote sensor platform. Therefore, it offers the possibility to acquire high quality and synoptic estimates that could contribute to the understanding of the surfzone hydrodynamics.

Journal of Geophysical Research, 1999

Radar backscatter measurements from stationary breaking waves were used to examine how the surface roughness generated by wave breaking affects radar backscatter at moderate incidence angles. Stationary breaking waves were generated by submerging a stationary hydrofoil in a uniform flow. X band radar backscatter measurements were made at numerous streamwise positions along the stationary breaking waves from an incidence angle of 45 ø for horizontal transmit and receive polarization (HH) and vertical transmit and receive polarization (VV) looking both upwave and downwave. The radar returns increased substantially, and the HH-to-VV polarization ratio approached unity near the breaking crests. This radar signature is consistent with those observed in the field. Detailed optical measurements of the breaking surfaces revealed that the observed radar returns near the breaking crests were the result of increased incoherent backscatter from the small-scale surface roughness generated by the breaking waves, although surface tilt effects also modified the radar return. Scattering models based upon the small perturbation solution performed well in the wake of the stationary breaking crest, but they significantly underestimated the HH-to-VV polarization ratio near the breaking crest. More advanced scattering solutions such as the integral equation method produced more accurate results in regions containing the largest surface roughness. These findings suggest that incoherent backscatter from surface disturbances produced by deep water breaking waves may be the source of the high radar returns and small polarization ratios observed from the ocean at moderate incidence angles.

Radiophysics and Quantum Electronics, 2012

We performed laboratory experiments to study peculiarities of radar scattering of microwave radiation by strongly nonlinear (breaking) gravity-capillary waves on the water surface. It was shown that the scattering from strongly nonlinear waves within the centimeter and, partially, decimeter wavelength range is due to the effects of their "micro-breaking" and the bound (spurious) capillary ripples excited on their profile. The phase velocity of the ripples coincides with the phase velocity of the generating waves. The scattering by meter waves, which at high amplitudes are characterized by strong breaking with tipping of the crest, is determined mainly by the quasi-linear capillary ripples, whose phase velocity is determined by the dispersion relation for free surface waves. In the case of the waves within an intermediate range, which have lengths from several decimeters to one meter, both the spurious and free capillary ripples contribute to the scattering.