Low-grazing-angle scattering from 3-D breaking water wave crests

The low-grazing-angle (high incidence angle) microwave back scattering from three-dimensional (3-D) surfaces approximating the crests of plunging-breaker water waves has been numerically found using the multilevel fast multipole algorithm. The test profiles were synthesized from a series of two-dimensional (2-D) profiles that represent the time evolution of a breaker. The results show that the strong cancellation of the vertically polarized backscattering (VV) midway through breaking that was previously identified with the 2-D profiles is maintained when the azimuthal variation of the 3-D wave is included. An optical-based model confirms that this is due to interference between back reflection from points on the forming jet and cavity of the wave. This is matched by constructive interference at horizontal polarization (HH), leading to HH-to-VV backscattering ratios that are much larger than unity. This effect remains when the azimuthal look angle is moved from directly upwave. When looking upwave, the interference disappears later in breaking when the fully formed jet shadows the cavity region. However, it returns when looking sufficiently away from directly upwave so that the cavity again becomes visible. The HH-to-VV ratios are typically greater than unity at this time, although HH cancellation can appear very late in the breaking when the round-trip distances to the jet and cavity specular points are sufficiently different.