Single-scattering of preferentially oriented ice crystals at centimeter and millimeter wavelengths

The scattering properties of frozen cloud and precipitation particles at centimeter and millimeter wavelengths are fundamental for active and passive microwave remote sensing. So far the scattering parameters (scattering-, absorption-, and backscattering cross sections as well as asymmetry factor) have been calculated for randomly oriented particles only. However, some particle types, especially planar crystals, exhibit a preferred horizontal orientation. In this study the scattering and absorption parameters of four different planar ice crystal types have been modeled and exemplarily calculated from 1 GHz to 300 GHz with a freely available software code implementing the Discrete Dipole Approximation (DDA). It is presented how scattering and absorption of horizontally oriented ice crystals turn out for nadir-looking and inclined satellite sensors. The results show that small deviations around the horizontal orientation, as they are reported by relevant papers on cloud microphysics, have only marginal influence. Among all scattering parameters, the backscattering cross section of oriented crystals can differ most with respect to a random orientation. Especially if the incident wave direction is normal to the basal face, the parameter was found to be up to 20 times higher. Therefore the consideration of the orientation especially can be important for active microwave sensors. The more so if horizontally oriented particles and a nadir looking active sensor are involved. Furthermore the existing compilations for the index of refraction of ice at microwave frequencies have been reviewed.