The definition and significance of an effective radius for ice clouds

Single scattering shortwave properties of ice clouds are frequently derived in terms of the "effective" radius (r(e)) of the ice crystal population. Substantial discrepancies between definitions exist, making interpretation and comparison of various radiative parameterization schemes and satellite retrieval techniques difficult. Each definition of effective radius can be related to more physically based parameters, such as the crystal dimension of median mass. For vertically inhomogeneous clouds, the relative importance of different cloud heights in determining the retrieved r(e) is examined, and a definition of r(e) for vertically inhomogeneous clouds composed of hexagonal columns is proposed. This definition shows reasonable agreement with the r(e) values that would be retrieved using visible and near-infrared channels for some microphysical data acquired in tropical ice clouds near Kwajalein, Marshall Islands, in the mid-1970s. Because only the upper parts of a thick ice cloud are detected by satellite, and because the near-infrared channels demonstrate reduced sensitivity to large crystals, it may not be possible to obtain information about typical particle sizes over the entire depth of thick clouds using current satellite retrievals alone.