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. In particular, the short wave 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 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. The measurements were obtained during the Cirrus in High Latitudes (CIRRUS-HL) campaign in June and July 2021, sampling both mid-latitude and Arctic cirrus across a range of cloud types and temperatures down to −63 °C. Across both regions, we found consistently low median asymmetry parameters, with a campaign-wide median of 0.738. The observed g values were largely insensitive to variations in temperature, humidity, and crystal aspect ratio, and showed only minor differences between ice habits. In contrast, a systematic decrease in g with increasing particle size was identified, median g ranging from 0.777 for sub-30 µm particles in mid-latitude cirrus to minimum values of 0.719 and 0.713 for 175 µm particles in mid-latitude and Arctic cirrus, respectively. The measured values are substantially lower than those commonly used in current radiative transfer schemes, suggesting that the shortwave warming effect of cirrus clouds may be overestimated in many climate models. These results provide improved observational constraints for the representation of ice cloud optical properties and support efforts to reduce uncertainties in cirrus cloud radiative forcing.