Near‐nadir radar backscatter from a well‐developed sea

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Abstract
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.

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