Reply on RC1

Francesco Cairo

doi:10.5194/amt-2022-28-ac1

Abstract

Mie scattering codes are used to study the optical properties of Polar Stratospheric Clouds (PSC). Backscattering and extinction can be computed 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 use of Mie theory with a single refractive index and with the underlying assumption of spherical scatterers. 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 Mie theory under fairly reasonable corrections when aspherical scatterers are present. Here we consider particles as STS if their radius is below a certain threshold value Rth and NAT or possibly ice if above it. The refractive indexes are assumed known from literature. Moreover, the Mie result for solid particles are reduced by a factor C < 1, which takes into account the backscattering depression expected from the asphericity. Finally, we consider the fraction X of the backscattering from the aspherical part of the PSD as polarized, and the remaining (1-X) as depolarized. The three parameters Rth, C and X 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 for the depolarization, and possible causes are discussed. The results of this work help to understand the limits of the application of Mie theory in modeling the optical response of particles of different composition and morphology.

Highlights

20 Polar Stratospheric Clouds (PSC) appear in the polar stratosphere during winters due to the very low temperatures and the dynamic isolation of air within the polar stratospheric vortex
It is appropriate to ask how much these results depend on the choices made so far on the limits to the BR and βAcross we have imposed on our dataset, and on the value of X previously obtained
We have used an optical model to compute with Mie theory the backscatter and depolarization of mixed phase PSC

Summary

Introduction

20 Polar Stratospheric Clouds (PSC) appear in the polar stratosphere during winters due to the very low temperatures and the dynamic isolation of air within the polar stratospheric vortex. 25 PSCs can either be formed of liquid droplets composed of supercooled ternary solutions (STS) of sulfuric acid, nitric acid and water, or solid nitric acid trihydrates (NAT), the thermodynamically stable form of HN O3 and H2O in the polar stratosphere, or possibly - when temperature is low enough - ice. Initially PSCs were classified as three types based on lidar measurements of the intensity of the backscattered light and the amount of depolarization of the returned signals (Browell et al, 1990). It is believed that the nucleation of NAT could start in droplets of a pre-existing population of STS, but not all liquid droplets may convert into solid NATs, allowing the coexistence of particles of different composition and phases and of intermediate optical characteristics (Peter and Grooß, 2012)

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