The Effects of the Nonsphericity and Size Distribution of Ice Crystals on the Radiative Properties of Cirrus Clouds

Kinne, S. and Liou, K.N., 1989. The effects of the nonsphericity and size distribution of ice crystals on the radiative properties of cirrus clouds. Atmos. Res., 24: 273-284. Hexagonal ice crystals and equivalent ice spheres have significantly different single-scattering properties. Although the extinction cross-section for spheres with equivalent surface areas is about the same as that for nonspherical ice crystals randomly oriented in space, equivalent spheres produce more forward scattering and have smaller single-scattering albedos. On the basis of broad-band radiative transfer calculations, in this note we will illustrate that the assumption that ice particles are spheres with equivalent surface areas leads to an underestimate of the solar albedo of cirrus clouds. Furthermore, we show that, for a given optical thickness, small ice particles reflect more solar flux than large ice particles. This implies that the ice crystal size distribution could be extremely important in the determination of the solar albedo of cirrus clouds due to external radiative forcings in climatic perturbation experiments. In the thermal infrared region, absorption by ice crystals predominates and the effects of the nonsphericity and size distribution of ice crystals on infrared radiative transfer appear to be secondary.