Circular depolarization ratios of single water droplets and finite ice circular cylinders: a modeling study

Computations of the phase matrix elements for single water droplets and ice crystals in fixed orientations are presented to determine if circular depolarization delta(C) is more accurate than linear depolarization for phase discrimination. T-matrix simulations were performed to calculate right-handed and left-handed circular depolarization ratios delta(+C), respectively delta(-C) and to compare them with linear ones. Ice crystals are assumed to have a circular cylindrical shape where their surface-equivalent diameters range up to 5 mu m. The circular depolarization ratios of ice particles were generally higher than linear depolarization and depended mostly on the particle orientation as well as their sizes. The fraction of non-detectable ice crystals (delta < 0.05) was smaller considering a circular polarized light source, reaching 4.5%. However, water droplets also depolarized light circularly for scattering angles smaller than 179A degrees and size parameters smaller than 6 at side- and backscattering regions. Differentiation between ice crystals and water droplets might be difficult for experiments performed at backscattering angles which deviate from 180A degrees unlike LIDAR applications. Instruments exploiting the difference in the P-44/P-11 ratio at a scattering angle around 115A degrees are significantly constrained in distinguishing between water and ice because small droplets with size parameters between 5 and 10 do cause very high circular depolarizations at this angle. If the absence of the liquid phase is confirmed, the use of circular depolarization in single particle detection is more sensitive and less affected by particle orientation.