LGA sea-spike backscattering from plunging breaker crests

The low-grazing-angle (LGA) microwave backscattering from the crest region of an evolving plunging breaker wave has been calculated using a numerical electromagnetic technique. The high frequency backscattering from crests that are not yet sufficiently steep to give quasi-specular reflection can be modeled as diffraction from a near-discontinuity in the surface curvature at profile inflection points and predicted using the geometrical theory of diffraction. Reflection from surface sections whose radii are smaller than the electromagnetic wavelength can yield "sea spikes" even in the absence of multipath. This effect is predicted by an extended geometrical optics approach. Including the front face of the wave to yield multipath reflection points further increases the magnitude of the sea spike by up to 10 dB. Low frequency backscattering from the crest appears to be more diffractive than quasi-specular when the crest features are very small electromagnetically. This backscatter can be predicted using a two-scale approach provided that the surface can be tilted to eliminate multi-valued sections.