Dependence of ice crystal optical properties on particle aspect ratio

The single-scattering properties of randomly oriented hexagonal ice crystals have been extensively used to study the bulk radiative properties of ice clouds. It has been reported in the literature that the asymmetry factors of these particles vary with their aspect ratios in a “V-pattern” with the minimum at a unit aspect ratio. However, this phenomenon was not explained in the previous studies. The present paper reports on an in-depth analysis of the optical properties of hexagonal ice crystals. It is shown that the delta-transmission is primarily responsible for the aforementioned “V-pattern” variation of the asymmetry factor as a function of the aspect ratio. Additionally, the term (1−fδ)g is also partially responsible for the large values of the total asymmetry factor in the case of small aspect ratios, where fδ indicates the ratio of the amount of the energy associated with the delta-transmission rays to the total amount of the scattered energy and g is the asymmetry factor associated with the geometric optics rays.