Parameterization of scattering and absorption properties of nonspherical ice crystals at microwave frequencies

摘要

The single-scattering properties of ice crystals are fundamental to the radiative transfer in ice clouds, which thereby are the basis for estimating the optical and microphysical properties of ice clouds. This study computes the scattering and absorption properties of nonspherical ice crystals in ice clouds using the discrete dipole approximation method at frequencies of 89–340 GHz. Six ice crystal habits, including hexagonal column, hollow column, six-branch bullet rosette, droxtal, aggregate, and plate, ranging in maximum dimensions from 2 to 5500 μm, are considered. The single-scattering properties of ice crystals are sensitive to their habits. The sensitivity of the angular distribution of the scattering energy to ice crystal habits is also evident and increases with size of ice crystal. The mean scattering properties of ice clouds with two ice crystal habit distributions are computed by averaging the single-scattering properties over a Gamma particle size distribution. It is found that the mean scattering properties of ice clouds are strongly dependent on ice cloud effective particle size and microwave frequency. The mean scattering properties increase with increasing ice cloud effective particle size and microwave frequency. On the basis of the calculated mean optical properties, parameterizations of the mean scattering efficiency, absorption efficiency, and asymmetry factor as functions of ice cloud effective particle size are developed.