The Effective Radius in Ice Clouds

Abstract

The effective radius (re) is a measure of the particle size used to calculate the optical properties of clouds. The objective of this study is to derive re from the microphysical composition of ice clouds. All ice crystals are assumed to be hexagonal columns with an aspect ratio depending on their size. Several existing particle size distributions are evaluated. The shape of the spectra is considered to be unsatisfactory for small particles and a new distribution is suggested that includes a Γ distribution for small crystals. The suggested spectrum agrees well with observations, although it is still speculative for small particles due to the limited availability of data. The effective radius for nonspherical particles is not uniquely defined, and several possible definitions for re are tested. Large differences in re arise from the different definitions even if the same assumptions on the shape and the size distribution of ice particles are used. Norming factors help to adjust the differently defined re in order to make re from different sources compatible. Finally, a new parameterization for re is suggested to avoid the expensive explicit computation. The proposed parameterization makes re a function of both the ice content and the temperature. A fair agreement between parameterized and observed re is found.