Hess, Michael and Matthias Wiegner (1993): Modelling cirrus optical properties in: IRS ´92: Current Problems in Atmospheric Radiation (S. Keevallik and O. Kärner, Ed.), A. Deepak Publishing.
Abstract:
Ice crystals in cirrus clouds in many cases have complex shapes or they are agglomerates of various simple crystals. Despite this fact, their single scattering properties usually are modelled assuming spherical or regular hexagonal particle shapes. The assumption of spheres has been shown to be not suited for describing the optical properties of cirrus clouds (e.g. Kinne and Liou, 1989). In this paper, preliminary results of a study concerning the usefulness of hexagonal plates and columns for modelling cirrus optical properties are presented.
Two questions are addressed. The first concerns the visibility of halo phenomena from the surface. If no haloes are seen in the presence of cirrus clouds, this may be due to multiple scattering effects inside the cloud and in the atmosphere below the cloud. In this case, the absence of haloes is no argument against the use of idealized hexagonal crystals in numerical models. The second question is, whether haloes are relevant for energy flux calculations, which would, for instance, affect climate modelling. Haloes are generally produced by hexagonal plates and columns, but not necessarily by more complex particles. If haloes do not affect flux calculations, again the absence of haloes isno sufficient argument against the use of idealized crystal shapes for radiative transfer calculations.