A study of optical scattering modelling for mixed phase Polar Stratospheric Clouds

Abstract. Scattering codes are used to study the optical properties of Polar Stratospheric Clouds (PSC). Backscattering and extinction can be computed with available scattering codes once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs often appear as external mixtures of Supercooled Ternary Solution (STS) droplets, solid Nitric Acid Trihydrate (NAT) and possibly ice particles, making questionable the assumption of a single refractive index and a single morphology to model the scattarers. Here we consider a set of fifteen coincident measurements of PSC above McMurdo Station, Antarctica, by ground-based lidar and balloon-borne Optical Particle Counters (OPC), and in situ observations taken by a laser backscattersonde and an OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows to test the performances of optical scattering models when both spherical and aspherical scatterers of different composition and, possibly, shapes are present.

Here we consider particles as STS if their radius is below a certain threshold value R_th and NAT or possibly ice if above it. The refractive indices are assumed known from the literature. Mie scattering is used for the STS, assumed spherical, while scattering from NAT particles, considered as spheroids of different Aspect Ratio (AR), is treated with T-Matrix results where applicable, and of geometric-optics-integral-equation approach where the particle size parameter is too large to allow for a convergence of the T matrix method. The parameters R_th and AR of our model are chosen to provide the best match with the observed optical backscattering and depolarization. The comparison of the calculations with the measures is satisfactory for the backscattering but not so for the depolarization, and possible causes are discussed. The results of this work help to understand the limits of the application of these scattering theories in modeling the optical response of particles of different composition and morphology.