Light Scattering by Ice Clouds in the Visible and Infrared: A Theoretical Study

Computations of the intensity and linear polarization for single scattering by ice clouds have been made based on the assumption that the particles in ice clouds can be approximated by long circular cylinders which are allowed to be polydispersive as well as arbitrarily oriented in space. The results of two models of optically thin ice clouds are presented and compared with those for polydisperse ice spheres. The two models for ice cylinders are assumed to be either uniformly or randomly oriented in a horizontal plane. Four different wavelengths, 0.7, 3, 3.5 and 6.05 μ, are employed in the light scattering computations. It is found that, compared to ice spheres, long ice cylinders scatter more light in the region with scattering angles near 90°, at the expense of scattering in both the forward and backward directions. The glory and cloudbows, which occur in light scattered by spherical particles, are either lost (the glory) or largely reduced and distorted (the cloudbows) in the case of cylinders. It is probable that for more irregular particles the cloudbows would also disappear. These differences in scattering by spherical and non-spherical scatterers therefore provide useful information for the differentiation between the ice and liquid phase of cloud particles. The light scattering computations performed for ice cylinders in this paper represent a new theoretical approach in an attempt to understand the radiation scattered by ice crystals. Hence, results of the angular scattering patterns for ice cylinders could be of use in the evaluation of the transfer of visible or infrared radiation through thick ice clouds, especially cirrus.