Angular Resolved Light Scattering as a Sensitive Tool to Infer Ice Crystal Complexity

The interaction of shortwave 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 behavior of atmospheric ice particles is crucial for a reliable calculation of the shortwave radiative transfer in climate models and for retrieving cloud bulk properties from satellites. Much of the current knowledge of the light scattering behavior of atmospheric ice particles is gained from modelling studies, which apply optical models on simplified ice particle morphologies. Although these 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 that occur on different scales, like crystal aggregation, hollowness and inclusions, as well as surface roughness. This is mainly because there are no in-situ measurements available that (i) detect crystal complexity down to the sub-micron or sub-wavelength scale and (ii) measure the angular light scattering on single atmospheric ice particles that would allow for a validation of existing models from the perspective of fundamental optics.

This lack of measurement data was the motivation to develop the Small Ice Detector Mk. 3 (SID-3) and the Particle Habit Imaging and Polar Scattering (PHIPS) probe. While SID-3 measures 2D high-resolution light scattering patterns of single crystals in near-forward direction, PHIPS takes stereo microscopic bright field images of individual crystals and simultaneously acquires the correlated angular light scattering function.

In this contribution the capabilities of the SID-3 and PHIPS measurement methods for inferring ice crystal complexity from angular resolved light scattering measurements are discussed from the perspective of fundamental optics. Over 15 years of laboratory and airborne ice cloud data collected by the SID-3 and PHIPS probes are then reviewed in the context of the occurrence of ice crystal complexity on different scales. The review reveals that ice crystal complexity is a prevalent microphysical feature in natural ice clouds that is likely caused by a low threshold supersaturation for disturbed crystal growth. The consequences of this microphysical feature for the shortwave radiative properties of ice clouds are discussed.