Observational constraints on the ice cloud asymmetry parameter and a new optical parameterisation for cirrus clouds

Cirrus clouds exert a strong control on Earth’s radiation budget, yet their shortwave radiative impact remains one of the largest sources of uncertainty in climate projections. A key quantity governing this impact is the asymmetry parameter (g), which describes the angular redistribution of scattered solar radiation and is highly sensitive to ice crystal morphology and surface structure. However, direct observational constraints on g in natural cirrus clouds remain scarce.

Here, we present simultaneous in situ measurements of ice particle morphology and angular light scattering obtained with the Particle Habit Imaging and Polar Scattering (PHIPS) probe during the CIRRUS-HL aircraft campaign in summer 2021. The dataset spans both mid-latitude and Arctic cirrus clouds over a wide range of cloud types and temperatures down to -63°C. Across all conditions, we find consistently low median asymmetry parameters, with a campaign-wide median of g = 0.738. The observed values show little sensitivity to temperature, relative humidity over ice, crystal habit, or aspect ratio, but exhibit a systematic decrease with increasing particle size. These values are substantially lower than those commonly assumed in current radiative transfer schemes, implying that the shortwave warming effect of cirrus clouds may be overestimated in many climate models.

Motivated by this discrepancy, we introduce an observationally constrained optical parameterisation for ice crystals aimed at improving their representation in climate models. The parameterisation is based on a new physical-optics hybrid approach that explicitly accounts for surface roughness using a physically motivated description, avoiding ad hoc treatments employed in earlier schemes. By fitting this model to the measured scattering properties, we derive an updated parameterisation of ice crystal optical properties suitable for climate applications. Together, these results provide both new observational constraints and a pathway toward more physically realistic representations of cirrus cloud optical properties, helping to reduce uncertainties in cloud radiative forcing.