New Insights into the Radiative Properties of Single Atmospheric Ice Crystals from Airborne Imaging and Correlated Angular Light Scattering Measurements

The interaction of solar radiation with ice particles is an important process in the atmosphere, which redistributes solar light before reaching the ground. Therefore, the knowledge of the angular light scattering behaviour of ice particles is crucial for a reliable calculation of the shortwave radiative transfer in climate models and for retrieving cloud bulk properties from satellites observations. Much of the current knowledge of the light scattering behaviour of atmospheric ice particles is gained from optical models using simplified ice particle morphologies. Although particle light scattering models have been significantly improved over the last decade, their results are still questionable especially when it comes to the effects of ice crystal complexity like hollowness and surface roughness. This is mainly because there are no in-situ measurements available on single atmospheric ice particles that would allow for a validation of ice particle light scattering models.

This lack of measurement data was the motivation to develop the Particle Habit Imaging and Polar Scattering (PHIPS) probe. PHIPS is a combination of a stereo microscopic bright field imager and a polar nephelometer working on single atmospheric cloud particles. After the demonstration of its full functionality in the Airborne Research Instrumentation Testing Opportunity 2017 (ARISTO2017) flights, PHIPS participated in two aircraft campaigns targeting mixed-phase and ice clouds in high latitudes, namely the Arctic Cloud Observations Using airborne measurements during polar Day (ACLOUD) campaign in 2017 as well as the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) campaign in 2018. A unique and comprehensive data set of microphysical properties and correlated angular light scattering functions of real atmospheric ice particles has been gained from these campaigns. A catalog of individual ice crystals is currently being compiled that will be of high value for scientists developing and applying single particle light scattering models.

In this contribution the PHIPS technology is introduced, followed by an overview of the acquired single particle data sets. First attempts of interpreting some of the measured single particle light scattering functions from a fundamental light scattering perspective are presented. The presentation concludes with an outlook for future collaboration possibilities.