The light-scattering properties of atmospheric ice particles are central to climate simulations and remote sensing and have, therefore, been studied using a wide range of theoretical and numerical approaches, ranging from wave-optical methods to geometrical and hybrid optical models. While these models differ in physical depth and computational complexity, they are rarely constrained by direct experimental validation that links particle microphysics to angular scattering behavior. The Particle Habit Imaging and Polar Scattering (PHIPS) instrument addresses this gap by providing aircraft-based, in situ measurements that combine high-resolution stereo-microscopic imaging with correlated angular light-scattering measurements of single atmospheric ice crystals. In this study, PHIPS measurements are analyzed for evidence of sub-wavelength-scale, mesoscopic surface roughness. For this purpose, the measurements are compared with simulations employing two conceptually different surface-roughness representations: the widely used tilted-facet geometrical model and the ABg model, a diffraction-based surface-scattering parameterization implemented within ray-tracing frameworks. The ABg approach yields consistently better agreement with the measured scattering patterns than the tilted-facet model, particularly when roughness scales approach the wavelength of light. These results demonstrate that physically based surface-scattering formalisms are required to accurately simulate the angular scattering behavior of pristine atmospheric ice particles. They further suggest that mesoscopic surface roughness is a common and fundamental surface scale governing ice-particle light scattering, with implications for backscattering and polarimetric characteristics, and demonstrate the capability of PHIPS to experimentally constrain these effects. • Single-particle PHIPS data enable direct experimental validation of ice optical models. • Pristine atmospheric ice crystals require mesoscopic roughness to match observations. • A diffraction-based ABg roughness model outperforms the tilted-facet approach. • Mesoscopic roughness is a fundamental surface scale for ice-particle light scattering.
Schnaiter et al. (Wed,) studied this question.