Cirrus clouds are ubiquitous in Earth’s atmosphere, and as high-altitude clouds usually the first solid matter that solar radiation encounters. They have an important role in Earth’s energy balance, yet understanding the magnitude and, in some cases, even the sign of the cirrus cloud radiative effect is still under debate. Cirrus clouds consist purely of ice crystals that have been shown to consist of a myriad of different geometries. Unlike spherical liquid droplets, no analytical solution for interactions between electromagnetic radiation and ice crystals can be derived. However, accurate knowledge of the ice crystal radiative properties is needed both in climate and weather prediction models as well as in remote sensing applications. Frequently, simplified assumptions on ice crystal morphology are made to numerically estimate the ice crystal radiative properties. Here we present microphysical and radiative properties of atmospheric ice crystals from an observational perspective. We give an overview of the current state-of-the-art knowledge of detailed ice crystal morphology based on both laboratory and field studies. The ice crystal morphology is linked to a simultaneously measured ice crystal asymmetry parameter; one of the key optical parameters in radiative transfer. Implications for the global ice cloud radiative effect are discussed at the end.

Bio. Emma is a leader of the group Airborne Cloud Observations at the Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT). She was awarded her PhD in physics by KIT in 2016. As an early-stage researcher she conducted cloud chamber studies at the AIDA cloud chamber and at the CERN CLOUD chamber on microphysical and optical properties of ice and aerosol particles. From 2018 to 2020, she joined the Research Aviation Facility of the National Center for Atmospheric Research (NCAR) as a postdoctoral fellow in the Advanced Study Program where she investigated secondary ice production in the Souther Ocean clouds. In 2020 she was awarded a Helmholtz Young Investigator Group for the time period between 2020 and 2026 to develop an observational-based parameterisation of ice crystal optical properties for climate and weather prediction models.