Two-scale treatment of low-grazing-angle scattering from spilling breaker water waves

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Abstract

[1] A two-scale scattering analysis is applied to two series of profiles that represent the measured time evolution of the crests of spilling breaker waves that were mechanically generated and measured in a wave tank. The backscattering at microwave frequencies and low-grazing-angle (LGA) illumination were considered. A deterministic implementation of the perturbation theory that is the foundation of any two-scale scattering model (TSM) is used that allows an instantaneous comparison of TSM cross sections with reference “exact” cross sections found using a numerical moment-method-based electromagnetic approach. TSM proved unable to model the scattering from steep features that appear on the crest both before and after the onset of breaking, even in cases where the horizontal-to-vertical polarization ratios are as low as −10 dB. This shows that the “fast” scattering (with Doppler speeds greater than or equal to the velocity of the breaker) associated with these features is not due to Bragg scattering in the traditional sense in these cases despite the low polarization ratios. TSM is more accurate after the steep features subside and the turbulent “scar” roughness is primarily on the front face of the breaker. However, accuracy is lost as the wave propagates and the scar roughness moves to the crest top and back face of the wave. It appears that a scattering coefficient based on the spectral content of the roughness at the Bragg-resonant condition can provide only a rough estimate of the actual LGA scattering coefficient even after the major steep features have disappeared.

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