A study of optical scattering modelling for mixed-phase polar stratospheric clouds

• Published in: Atmospheric Measurement Techniques
• Main Authors: Cairo, Francesco, Deshler, Terry, Di Liberto, Luca, Scoccione, Andrea, Snels, Marcel
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: article; Verlagsveröffentlichung; Kiruna; McMurdo Station; Antarc*; Antarctica
• Online Access: https://doi.org/10.5194/amt-16-419-2023; https://noa.gwlb.de/receive/cop_mods_00064668; https://noa.gwlb.de/servlets/MCRFileNodeServlet/cop_derivate_00063403/amt-16-419-2023.pdf; https://amt.copernicus.org/articles/16/419/2023/amt-16-419-2023.pdf

Scattering codes are used to study the optical properties of polar stratospheric clouds (PSCs). Particle backscattering and depolarization coefficients 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 the assumption of a single refractive index and a single morphology to model the scatterers questionable. Here we consider a set of 15 coincident measurements of PSCs above McMurdo Station, Antarctica, using ground-based lidar, a balloon-borne optical particle counter (OPC) and in situ observations taken by a laser backscattersonde and OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows us to test the performances of optical scattering models when both spherical and aspherical scatterers of different composition and, possibly, shapes are present. We consider particles as STS if their radius is below a certain threshold value Rth and NAT or possibly ice if it is above it. The refractive indices are assumed known from the literature. Mie scattering is used for the STS, assumed spherical. Scattering from NAT particles, considered spheroids of different aspect ratio (AR), is treated with T-matrix results where applicable. The geometric-optics–integral-equation approach is used whenever the particle size parameter is too large to allow for a convergence of the T-matrix method. The parameters Rth and AR of our model have been varied between 0.1 and 2 µm and between 0.3 and 3, respectively, and the calculated backscattering coefficient and depolarization were compared with the observed ones. The best agreement was found for Rth between 0.5 and 0.8 µm and for AR less than 0.55 and greater than 1.5. To further constrain the variability of AR within the identified intervals, we have ...