New Method for Phase Discrimination of Single Cloud Particles and Application in High Latitude Mixed-phase Clouds.

Mixed-phase clouds (consisting of coexisting supercooled liquid water and solid ice particles) occur widespread in many different regions and thus play a crucial role in understanding our climate as well as climate change on regional and global scales. However, our knowledge and understanding of mixed-phase cloud processes is still incomplete and more research (especially in situ measurements) is needed. One crucial step towards understanding the nature and implications of mixed-phase clouds lies in its phase composition, i.e. the number fraction of liquid and frozen water in the cloud, as it severely affects the cloud’s radiative balance, precipitation and life cycle.

An instrument capable of contributing to answer this question is the Particle Habit Imaging and Polar Scattering (PHIPS) probe that simultaneously measures a) the scattering phase function of singular cloud particles in the angular range from 18° to 170° with a 8° resolution whilst b) imaging the same particle, using two microscopic cameras from two different angles (120° apart), thus giving 3D information about the particle's morphology.

In this work, we present a new method to discriminate between supercooled liquid droplets and ice particles based on the scattering phase measurements from 18° to 170°. The advantage of using the angular scattering function for phase discrimination is, that droplets have an unique angular scattering feature with scattering minimum around 90° and a rainbow feature around 140°. This way, droplets can be discriminated from ice, even to small (<100 µm) particle sizes where traditional imaging methods have restrictions. We will show how the rainbow feature as well as scattering minimum and specular reflection peaks of single particle angular scattering function can be used for phase discrimination.

This discrimination algorithm was developed and tested with a subsample of real cloud particles that were correlated to a stereoscopic image that could be identified as an ice crystal or a droplet with high confidence. Additionally, the algorithm was used to analyse PHIPS data from two aircraft studies focusing on mixed-phase clouds in high latitudes: the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD, May/June 2017) in Longyearbyen (Spitsbergen, Norway) and the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES, Jan/Feb 2018) in Hobart (Tasmania, Australia) and the resulting particle concentrations are compared to results of particle concentrations derived from other in-situ instruments from the same campaigns.