Numerical calculation of electromagnetic scattering from measured wind-roughened water surfaces
The electromagnetic backscattering from one-dimensionally rough, perfectly conducting surface profiles has been numerically calculated using a periodic-surface moment method. The profiles were derived from direct measurements of the upwind/downwind slope of wind-roughened water surfaces. Significant asymmetry appears between the upwind- and downwind-looking backscattering. The asymmetry at moderate incidence (about 30 to 55) is consistent with the predictions of the combined tilt and hydrodynamic modulation of the Bragg-resonant small-scale waves by the large-scale waves. At larger incidence the large-scale surface curvature leads to even greater asymmetries. The scattering calculated from profiles numerically generated from an estimate of the surface height spectrum accurately predicts many of the features of the scattering from the actual surface itself, including the HH to VV ratio and dependence on incidence angle, but fails to show the upwind/downwind asymmetry. Direct evaluation of the first-order field-perturbation equations that are the basis of most two-scale scattering models accurately predicts the upwind/downwind asymmetry at incidence angles up to 70 and greater at horizontal polarization, but only to 55 at vertical polarization. The failure of the two-scale model at higher incidence appears to be due to inherent limitations in the small-perturbation approach of the model itself rather than due to any assumptions used in the determination or use of a scale-separation wave number threshold.