Average ice crystal size and bulk short-wave single-scattering properties of cirrus clouds

The bulk single-scattering properties of cirrus clouds required for driving the radiation scheme in large-scale climate models are computed with respect to various size distributions and ice crystal shapes. It is shown that the average ice crystal size, defined as the ratio of total volume to the total projected area, can well-characterize the effect of various size distributions in determining the bulk radiative properties of cirrus clouds. Details of the size distributions are not significant in specifying the radiative properties of these clouds if the effective average size is thus defined. Therefore, the ratio of the total volume to the total projected area is an ideal parameter for describing the bulk single-scattering properties. The effect of ice crystal shape is not critical in the parameterization of the extinction coefficient and single-scattering albedo. However, the various crystal habits must be accounted for in the parameterization of the asymmetry parameter. The resulting parameterization is intended for radiative transfer calculations involving cirrus clouds in large-scale models.