Single-scattering properties of complex ice crystals in terrestrial atmosphere

Various ice crystal shapes including fernlike geometry, plates with dendritic extensions and with sector-like and broad branches, fractal geometry, and aggregates, have been numerically defined on the basis of available observations. The surface roughness of ice crystals is also accounted for by specifying the facet-tilt distribution in terms of Gram Charlier series for the small facets of which the rough surface consists. These ice crystal geometries along with those defined in our previous studies may approximately represent the ice crystal shapes frequently observed in cirrus clouds. An improved Monte Carlo/ray-tracing method has been developed to compute the single-scattering parameters for these complex ice crystals. The polarization configurations of the localized waves associated with Fresnelian rays are comprehensively accounted for by the improved method in ray-tracing procedure. Complex ice crystals, in particular, those with fernlike structures, scatter more energy in the angular region 2°-20° and in the lateral and backward directions than hexagonal ice crystals. The former ice crystal geometries normally produce smaller polarization values. In particular, a substantial reduction for the negative polarization associated with the backscattering is found as a result of the complex crystal geometries. The surface roughness of ice crystals incorporated into the single-scattering calculation tends to smooth out the scattering peaks corresponding to halos. The roughness also significantly reduces the backscattering. When a substantial roughness condition is imposed, the computed phase function and the polarization configuration of scattered light are essentially featureless. A database of the single-scattering parameters of ice crystals at solar wavelengths covering 0.2-5 μm has been established for various ice crystal shapes and sizes. This database can be useful in the parameterization of the bulk radiative properties of cirrus clouds to account for the effects of ice crystal size distribution and the percentage of various crystal habits. By applying this database to cold and warm cirrus clouds, it is demonstrated that the scattering and absorption characteristics of these clouds depend on both the size distribution and the shapes of ice crystals.