Light scattering by Gaussian, randomly oscillating raindrops

Scattering of visible light by oscillating equilibrium raindrops is studied in the ray optics approximation, the geometric optics part of the approxim ation being of main interest. For two drop sizes, d = 2.0 and 6.0 mm, full scattering matrices are analyzed in detail in the whole 4 pi 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 diff erent oscillation cases are studied: the non-oscillating case and four case s with varying degrees of oscillations. According to the model results, mos t of the novel features seen in the scattering phase matrix elements are ca used by the nonspherical equilibrium shape, while the role of the oscillati on part is mostly that of smoothing away the features. Total internal refle ction is found to be an important mechanism for light scattering by non-sph erical raindrops, as most of the strong intensity features in the scatterin g 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 errone ous 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 degrees rainbow, also speaks for the importance of total reflectio n. (C) 1999 Elsevier Science Ltd. All rights reserved.