Diffraction, reflection, and interference during near‐grazing and near‐normal ocean surface backscattering Available to Purchase

Theory for the impulse response of a spherical wave backscattered from a plane strip has been used with a computer model of a randomly rough long‐crested ocean surface to reveal the relative contributions of diffraction, facet reflection, and interference at near‐grazing and near‐normal incidence and to incorporate acoustical shadowing in a natural way. The major findings for near‐grazing incidence are that backscatter is due to diffraction at troughs and crests; the backscattering strength is −54 dB at 5 m/s and only 2 dB more at 20 m/s, both flat for grazing angles 1° to 10°; and it decreases about 3 dB for an octave increase in frequency, independent of wind speed. At normal incidence the decreasing value of the backscattered coherent intensity with increasing roughness is due to interference. It is related to the transform of the probability density function of the surface height until the roughness parameter g (=4k²σ²) reaches approximately 5, where there is a deep minimum due primarily to interference of the highly coherent diffracted contributions. At normal incidence the backscatter is dominated either by diffraction or reflection, depending on the sphericity of the wave front.