We have used an SCM to examine the sensitivity of fundamental quantities such as atmospheric radiative heating rates and surface and top-of-atmosphere radiative fluxes to various parameterizations of clouds and cloud microphysics. The single-column model was run at the ARM Southern Great Plains, Tropical Western Pacific, and North Slope of Alaska sites using forcing data derived from operational numerical weather prediction. Our results indicate that atmospheric radiative fluxes are sensitive to the scheme used to specify the ice particle effective radius by up to 30 W m-2 on a daily time scale and up to 5 W m-2 on a seasonal time scale. We also found that the inclusion of ice particle fallout can have a significant effect on the amount and location of high cirrus clouds. On a seasonal time scale, atmospheric fluxes were sensitive to the inclusion of ice particle fallout by 8 W m-2. An unexpected finding was that the variance of the modeled ice particle effective radius at a given level is considerably smaller than that suggested by ARM cloud radar measurements. Our results indicate that this theoretical underestimate of the ice particle effective radius variance can have effects on modeled radiative fluxes comparable in magnitude to those cited above for sensitivity to the mean values of ice particle effective radius.