An observational benchmark of ice crystal light scattering: insights from a decade of airborne in situ measurements with the PHIPS instrument

Accurately representing the optical properties of atmospheric ice particles remains a major challenge for climate simulations and remote sensing. A key limitation is the lack of experimental benchmark data that directly link ice crystal microphysics to angular light scattering, particularly at the level of individual atmospheric particles.

In this contribution I will give an overview of recent advances enabled by the Particle Habit Imaging and Polar Scattering (PHIPS) instrument. PHIPS provides unique aircraft-based, in situ measurements combining high-resolution stereo-microscopic imaging with simultaneous angular light-scattering observations of the same ice crystal. This capability enables a consistent investigation of scattering behavior from single, oriented particles to habit-averaged populations and cloud-averaged ensembles.

Single-particle analyses show that even ice crystals appearing pristine in microscopic images require a finite degree of mesoscopic surface roughness to reproduce their measured angular scattering functions. This demonstrates that sub-wavelength-scale surface irregularities fundamentally control the angular scattering properties of individual atmospheric ice crystals.

For habit-averaged crystal populations, PHIPS observations of atmospheric bullet rosette crystals reveal asymmetry parameters (g) that are substantially lower than predicted by ray-tracing models assuming idealized geometries, implying a significantly enhanced shortwave reflectivity of cirrus clouds. At the cloud-averaged scale, PHIPS measurements from mid-latitude and Arctic cirrus consistently yield low g values of about 0.74, with a systematic decrease toward larger particles.

Together, these results show that ice crystal complexity across scales must be explicitly represented in optical models and establish PHIPS data as a critical benchmark for advancing such models.