Low and consistent asymmetry parameters in Arctic and mid-latitude cirrus

Abstract. Cirrus clouds play a critical role in the Earth’s radiation budget, yet their shortwave optical properties remain poorly constrained. The asymmetry parameter (g), which governs the angular distribution of scattered light, is particularly sensitive to ice crystal morphology, a property that varies widely in cirrus. To provide observational constraints on the magnitude of g and to investigate its relationship with ice microphysical properties, we analysed simultaneous in situ measurements of particle morphology and angular light scattering using the Particle Habit Imaging and Polar Scattering (PHIPS) probe. These measurements were conducted during the Cirrus in High Latitudes (CIRRUS-HL) campaign in June and July 2021, which sampled both mid-latitude and Arctic cirrus across a range of cloud types and temperatures down to –63 °C. We found that cirrus in both regions exhibited a consistently low median asymmetry parameter of 0.727. The observed g values were largely insensitive to variations in temperature, humidity, and crystal aspect ratio, and showed only minor variation across ice habits. A systematic decrease in g with increasing particle size was identified, ranging from 0.760 for sub-30 µm particles in mid-latitude cirrus to minimum values of 0.707 and 0.703 for 175 µm particles in mid-latitude and Arctic cirrus, respectively. The measured values are significantly lower than those commonly used in current radiative transfer schemes, suggesting that cirrus clouds may contribute less to net atmospheric warming than often assumed. These results provide improved observational constraints for the representation of ice cloud optical properties in climate models and support efforts to reduce uncertainties in cirrus cloud radiative forcing.