On the theory of electromagnetic scattering from a raindrop splash


  • L. B. Wetzel


Electromagnetic scattering from a raindrop splash on a water surface is examined in terms of the splash structures that have been disclosed by high-speed photography. Of the three basic scattering features, the “crown,” the “stalk,” and the “ring wave,” the first two are modeled as dielectric cylinders, while the third is treated by a perturbation approximation. Cross-section predictions based on heuristic application of these models are found to be in good agreement with laboratory measurements of individual splash-scattering events. A statistical model in which surface slope and stalk height distributions are taken into account is proposed for extending these results to natural rain on a real water surface. Unfortunately, there is at present insufficient experimental data to allow a reasonable test of the theory. Nevertheless, a few qualitative conclusions can be drawn from the formalism: the major scattering feature is the stalk, and while the vertically polarized returns will have only a weak dependence on rain rate, the horizontally polarized returns will depend strongly on both the rain rate and the shape of the stalk (or drop) size distribution curves.