Using the observed ice crystal size distribution in contrail cirrus from SUCCESS, we have carried out the scattering and absorption calculations based on a unified theory for light scattering by ice crystals covering all sizes and shapes. We illustrate the effects of ice crystal size and surface roughness on the scattering phase function features for remote sensing applications. The extinction coefficient and single-scattering albedo exhibit a minimum feature at 2.85 &mgr;m, referred to as the Christiansen effect, which is particularly pronounced for clouds consisting of a significant number of small ice crystals. Based on a line-by-line equivalent solar model, we show from spectral curves that cloud reflection increases as ice crystal sizes become smaller, but the cloud absorption increase is only evident for wavelengths longer than about 2.7 &mgr;m. The ice crystal shape has a substantial effect on the cloud reflection and absorption for a given size; more complex ice particles reflect more solar radiation. Finally, we propose a contrail cirrus cloud model consisting of a combination of bullet rosettes (50%), hollow columns (30%), and plates (20%), with sizes ranging from 1 to 90 &mgr;m in association with radiation perturbation studies. ¿ 1998 American Geophysical Union