A Bulk Parameterization of the Ice Particle Size Distribution and the Optical Properties in Ice Clouds

Abstract

A new parameterization has been developed that assumes that nonprecipitating particles obey the Heymsfield?Platt power-law (H?P particles) and that the precipitating particles obey the Marshall?Palmer distribution (M?P particles). The parameterization defines a critical ice content for the onset of precipitation particles and allows the number of ice crystals, the extinction coefficient, and the effective diameter of the crystals for the cloud layer in the model to be diagnosed. The implementation of the new parameterization in a model unifies the microphysical assumptions used to calculate the optical properties and precipitation. If it is assumed that the number of H?P particles at cloud top is much larger than the number of M?P particles in southeastern Australia frontal systems, then the observed number of ice crystals at cloud top agrees well with the diagnosed number of H?P particles at cloud top. A simulation of the passage of a cold front is used to test the parameterization. The modeled H?P and M?P particle concentrations are compared with microphysical observations of ice crystal concentrations and the modeled optical depths are compared with International Satellite Cloud Climatology Project satellite data. The modeled cloud ice contents and the particle numbers (H?P particles + M?P particles) in the middle-level frontal cloud are consistent with the ice crystal numbers that are observed in these types of clouds. The model simulations show that when the front is in the mature stage of development, the model-derived optical depths are in reasonable agreement with those derived from satellites.