Solar Radiative Transfer in Cirrus Clouds. Part I: Single-Scattering and Optical Properties of Hexagonal Ice Crystals

We have developed an efficient light scattering and polarization program, based on a ray-tracing technique, for hexagonal ice crystals randomly and horizontally oriented in space. Improvements have been made on the ray-tracing computations through a proper treatment of the δforward transmission by geometric rays and incorporation of the effect of birefringence of ice. Using this program, computations of the scattering phase matrix are made from the observed ice crystal size distributions for four typical cirrus clouds. The results for single-scattering parameters, including the phase function, single-scattering albedo, extinction cross section, and asymmetry factor for five solar wavelengths, are presented and discussed. Moreover, we show that the assumption of equivalent spheres with the same surface areas as hexagonal ice crystals leads to leads to larger asymmetry factors for all wavelengths and smaller single-scattering albedos for near IR wavelengths. The computed phase matrix elements compare reasonably well with experimental scattering results for laboratory ice crystal clouds. In particular, we illustrate that the neutral point (angle of zero linear polarization) is sensitive to the ice crystal shape. Thus, an observation of this position from space could provide a means for determining the aspect ratio of cloud particles. Light scattering computations are also made for horizontally oriented columns and plates of various sizes. Results are used to interpret the optical phenomena produced by cirrus clouds. The present light scattering program for hexagonal columns and plates identifies the positions of halos and arcs, as well as providing relative intensifies for these optical features.