Elsevier, Journal of Quantitative Spectroscopy and Radiative Transfer, 2-6(63), p. 643-666
DOI: 10.1016/s0022-4073(99)00042-4
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Scattering of visible light by oscillating equilibrium raindrops is studied in the ray optics approximation, the geometric optics part of the approximation being of main interest. For two drop sizes, d=2.0 and 6.0mm, full scattering matrices are analyzed in detail in the whole 4π solid angle in 19 different orientations. The raindrop shape is given as a product of the so-called equilibrium and oscillation parts. The former part is assumed to have a fixed orientation, while the latter is assumed isotropic. Five different oscillation cases are studied: the non-oscillating case and four cases with varying degrees of oscillations. According to the model results, most of the novel features seen in the scattering phase matrix elements are caused by the non-spherical equilibrium shape, while the role of the oscillation part is mostly that of smoothing away the features. Total internal reflection is found to be an important mechanism for light scattering by non-spherical raindrops, as most of the strong intensity features in the scattering phase matrices of model raindrops were only weakly polarized, contrary to strongly polarized rainbows in the spherical case. It is shown that using spheres in approximating scattering by realistic raindrops may yield erroneous results even in the case of integrated variables such as the asymmetry parameter. The discovery of a possible new scattering feature for raindrops, the 90° rainbow, also speaks for the importance of total reflection.