Near-nadir radar backscatter from a well-developed sea


  • Roman E. Glazman


Recent advances in understanding dynamics and statistical geometry of wind-generated gravity waves necessitate a re-examination of the radar return as a function of external factors. The Kirchhoff approximation for the case of well-developed seas is analyzed. In this peculiar case, the equilibrium range in the wave number spectrum (approximated by αk−(4−2μ) where μ > 0 can be interpreted as a fractal codimension of the surface) corresponds to a cascade pattern in the surface geometry. Its high wave number cutoff (the internal scale h), determined by the dissipation of energy due to wave breaking, is shown to be a major factor of the radar backscatter. This intrinsic scale is evaluated (h ∼ 0.4 m), and both the geometrical and the physical optics terms are related to major parameters of wind-wave dynamics. The range of validity of the Kirchhoff approximation and the relative importance of the diffraction correction are analyzed. Finally, the radar cross section σ0 of well-developed seas is compared with that of poorly developed seas (when μ = 0). The great qualitative difference shown in the wind speed dependence of σ0 for these two regimes is pointed out as a source of a considerable error trend recently discovered in satellite altimeter wind measurements.