The paper presents the phenomenon of negative polarization for large randomly oriented irregular particles what occurs as a result of interference between specific reflected waves associated with geometric optical trajectories of light beams for four refraction/reflection events. All calculations were performed for randomly oriented particles with a convex irregular shape. We used our physical optics approximation computer algorithm where the light scattered inside the particle consists of many plane-parallel beams. It is shown that the appearance of a backscattering peak is characteristic of the trajectories of light beams with any number of refraction/reflection events; however, only trajectories with four events (two internal reflections and two refractions at the exit faces of the particle) create negative polarization.
The purpose of this work is to solve an important issue: the light scattering problem for ice crystals of cirrus clouds less than 10 μm and matching the obtained solution with the existing solution obtained within the physical optics approximation. The article presents a solution to the problem of light scattering by hexagonal ice particles of cirrus clouds with sizes from 0.05 to 5.17 μm for a wavelength 0.532 μm, obtained within the discrete dipole approximation. It is found that the obtained solution is in good agreement with the physical optics approximation in the vicinity of scattering angles of 0–10° (the vicinity of forward direction scattering). However, to solve the problem of light scattering in the vicinity of the backward scattering direction, which is important for the interpretation of lidar data, it is necessary to continue the calculations to sizes of the order of 20 μm. The results obtained are necessary for constructing algorithms for the interpretation of lidar data obtained by sounding cirrus clouds.
The Umov effect is the inverse correlation between the maximum of the linear polarization of the light scattered on an object and the geometrically albedo of this object. The importance of studying this effect should be considered in the context of one of the complex and important tasks of remote sensing: determining the concentration of particles in optically thin clouds. Since the intensity of the scattered light depends on two unknown quantities: the concentration of particles in the cloud and the phase function of the particles of the cloud. Then to retrieve the concentration from the measured signal, it is necessary to know the phase function in advance. In real observations, the phase function is, usually, not known. The Umov effect will make it possible to estimate some necessary unknown characteristics of particles in a cloud, which determine the phase function. This paper is devoted to the study of this effect for particles with sizes much larger than the wavelength of the incident light. The report presents a solution to the problem of light scattering by randomly oriented particles of irregular shape for particles with sizes of 100 and 200 microns, for a wavelength of 0.532 microns. The solution was obtained within the both frameworks: the physical optics method and the geometric optics approximation. It was found that if the imaginary part of the refractive index less than 0.001, the Umov effect is observed with good accuracy for particles of size from 100 to 200 microns. However, the Umov effect is violated when the imaginary part of the refractive index is greater than 0.001.
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