Radiative Transfer on a Linear Lattice: Application to Anisotropic Ice Crystal Clouds

Abstract

The problem of radiative transfer in a horizontally infinite cloud layer possessing anisotropy with respect to volume extinction and other single-scattering properties was solved using the method of discrete space theory. The model was applied to a hypothetical ice crystal cloud composed of long cylinders displaying preferential orientation (in the horizontal) to provide the gross radiative properties of shortwave reflection, shortwave absorption, and longwave emission and reflection. These results were directly compared to clouds with the assumed microstructure of cylinders randomly orientated in three dimensions and of equivalent (by area) spheres. Generally, the gross radiative properties for clouds composed of equivalent spheres are substantially different than those for either of the cylinder models. The relative differences between the three assumed microstructures suggests that equivalent spheres cannot be employed to approximate the gross radiative properties determined for clouds composed of long cylinders. The preferential orientation of the long cylinders does affect significantly the estimates of cloud albedo, shortwave absorption and cloud emission when compared to the three-dimensional randomly orientated case. Thus it may be necessary to incorporate preferential crystal orientation into detailed multiple-scattering calculations which may eventually be employed to develop some parameterization of the gross radiative properties of ice crystal clouds.