On the limitation of using asymmetry factor for radiative transfer involving cirrus clouds

We investigate the limitation of using the asymmetry factor through the Henyey-Greenstein function in representing the scattering phase function for radiative transfer in ice clouds containing hexagonal ice crystals. Relative accuracy checks for solar fluxes are carried out using three cirrus cloud size distributions. The flux results computed from the Henyey-Greenstein phase function, based on the asymmetry factor, differ significantly from those computed from the phase function for hexagonal ice crystals. For optically thin ice clouds, the relative error can be as large as 70%. The maximum absolute error in solar reflected fluxes is 27 Wm -2. The present flux calculations demonstrate that the phase functions for ice crystals which contain strong forward scattering peak and specific halos and back scattering features cannot be adequately represented by a single parameterization in terms of the asymmetry factor.