Abstract
Abstract. Computations of the phase matrix elements for single water droplets and ice crystals in fixed orientations are presented to determine if circular depolarization δC is more accurate than linear depolarization for phase discrimination. T-matrix simulations were performed to calculate right-handed and left-handed circular depolarization ratios δ+C, respectively δ−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 μm. 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 (δ<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 179° 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 180° unlike LIDAR applications. Instruments exploiting the difference in the P44/P11 ratio at a scattering angle around 115° 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.
Highlights
Aerosol particles in the atmosphere are essential components for cloud formation where they are known to act as cloud condensation nuclei (CCN) (Spurny, 2000)
Following the previous modeling study made by Nicolet et al (2007) for linear depolarization ratios of single ice crystals, the same representation is used for the computed circular depolarization ratios
Some specific orientations of non-spherical ice crystals do not generate linear depolarization, meaning that ice particles can behave like spherical water droplets in terms of light scattering from a linearly polarized laser source
Summary
Aerosol particles in the atmosphere are essential components for cloud formation where they are known to act as cloud condensation nuclei (CCN) (Spurny, 2000). The Zurich Ice Nucleation Chamber (ZINC) follows the design of the CSU chamber but uses two parallel walls instead of two concentric cylinders (Stetzer et al, 2008) It permits activation and growth of ice nuclei (IN) in an ice supersaturated environment to detect ice crystal with sizes from 1 μm in diameter. Linear and circular cases, ice plates ( = 2) have better chances to be detected as ice crystals than isometrical ones considering a volume-equivalent diameter of 2 μm These results confirm that the circular depolarization ratio of single ice particles is in general higher and less sensitive to the actual particle orientation than the linear depolarization ratio. It has a non-negligible interference with liquid droplets as discussed
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