A two-stream radiative transfer model is utilized for this study [Harrington et al., 1999]. The two-stream model solves the radiative transfer equations for three gaseous constituents, H2O, O3, and CO2 and the optical effects of the hydrometeor size spectra. Gaseous absorption is calculated by following the fast exponential sum-fitting of transmissions method proposed by Ritter and Geleyn [1992]. Lorenz-Mie theory is used to compute the optical properties for water drops, while the theory of Mitchell et al. [1996] is used for non-spherical ice crystals. The radiative transfer model responds to the bin microphysical model using a method whereby the optical properties based on bin averages of the appropriate quantities are computed beforehand, and then summed with appropriate weights during the simulation [Harrington et al., 1998].