Morphologically complex ice crystal plate structures in mid-latitude cirrus: combining space-borne lidar, in-situ scattering, and physical optics simulation

Abstract. The scattering properties of ice crystals in cirrus clouds are critical for interpreting lidar observations, yet practical retrievals focus on idealised habits representing fewer than 10 % of natural cirrus, leaving the irregular and aggregated geometries that dominate real clouds poorly constrained. This study combines three independent lines of evidence to identify which habits are most consistent with mid-latitude cirrus observations: lidar and depolarisation ratios from the EarthCARE ATLID high-spectral-resolution lidar, in-situ angular scattering and stereographic imaging from the PHIPS probe during CIRRUS-HL, and physical geometric optics simulations of hexagonal plates, intersecting plate aggregates, and touching plate aggregates with surface roughness. Both datasets cover the same mid-latitude region and the in-situ data provides about 12,000 manually classified plate, side-plane, and aggregate particles across 22 flights. For each habit and roughness level, orientation- and ensemble-averaged scattering properties are simulated for 1000-particle ensembles with sizes drawn from the measured distribution. All three pristine habits produce lidar and depolarisation ratios within the observed range, but any roughening of the single plate habit drives the lidar ratio away from most observations, attributed to loss of retroreflective backscattering. Touching plate aggregates remain consistent across all roughness levels, owing to diffuse scattering from their open structures. The angular scattering function alone does not distinguish habits, but combined with the lidar constraints it identifies smooth intersecting aggregates and roughened touching aggregates as the most plausible dominant backscatterers. The extent to which these ratios can discriminate habit composition is discussed.