Directional characteristics of sea-wave scattering observed at low-grazing angles

The mathematical and statistical description of waves on the sea surface has evolved over more than two centuries starting with Laplace (1776), Gerstner (1802), Airy (1845), Stokes (1847), Kelvin (1887), and Helmholtz (1888). Most of the major developments in this century are summarized by Apel (see J. Geophys. Res., vol.99, no.C8, p.16269-91, 1994) who has derived an improved model for the ocean surface wave vector spectrum of equilibrium sea states. Part of the motivation for this paper is Apel's observation that "...in spite of the incorporation of an anisotropic angular distribution of wave energy, the observed azimuthal variation of radar scatter is not captured..." The data set analysis summarized in this paper show that wave group processes are responsible for directionally organized sea surface radar back scattering patterns, which are long lived and consequently extend over large areas. This includes crosswind traveling wave systems. A property of wave group process structure is the steepening of waves as they pass through the center of the group. The mid-group sharpening of the waves, which can include crest spilling or breaking, significantly increases the radar scattering cross section observable with low grazing angle radars, usually most noticeable with horizontal (HH) polarization. Therefore, low grazing angle (LGA) radar provides a vastly improved means of seeing wavegroup phenomena over conventional oceanographic methods.