Radar backscattering properties of nonspherical ice crystals at 94 GHz

The millimeter wavelength radar backscattering properties at 94 GHz for six nonspherical ice crystals, which include hexagonal column, hollow, plate, bullet rosette, aggregate, and droxtal with 46 maximum dimensions ranging from 2 to 10,500 mu m, are investigated using the discrete dipole approximation (DDA) method and Lorenz-Mie theory. It is found that the radar backscattering cross sections are sensitive to ice crystal habits and their representations, which use spherical particles with equivalent maximum dimension, volume, projected area, or the ratio of volume to projected area to model nonspherical ice crystals for the Lorenz-Mie theory. The radar backscattering cross sections of the six nonspherical ice crystals from the DDA method are further parameterized as functions of maximum dimensions of ice crystals. The results from the parameterizations agree well with those computed from the DDA method. Moreover, the mean radar backscattering cross sections derived by averaging the results from the parameterizations over gamma distributions for ice clouds are consistent with those from the DDA method. The parameterizations are applied to derive the coefficients for the relationships between equivalent radar reflectivity factor and ice water content for ice clouds consisting of the six nonspherical ice crystal habits or a mixture of habits. The coefficients from the parameterizations are close to those from the DDA method. Both of them are sensitive to ice crystal habits. The pronounced differences among the relationship between equivalent radar reflectivity factor and ice water content for an ice cloud with a mixture of ice crystal habits, which has been extensively used for solar and infrared retrievals of ice cloud properties, and several previous relationships further confirm their sensitivity to microphysical properties of ice clouds.