Light scattering and microphysical properties of atmospheric bullet rosette ice crystals

Abstract. Cirrus clouds play a critical role in the Earth's radiative budget. The extent of the radiative impact of cirrus clouds is determined by a number of their physical properties, such as aspherical ice crystal composition. One of the most relevant cirrus ice crystal habits is a polycrystalline bullet rosette, where individual bullets are radiating from the same nucleation point. Here, the link between the crystal morphology of atmospheric bullet rosettes and their asymmetry parameter (g) is experimentally investigated using correlated high resolution in situ stereo-images of individual rosettes and their angular scattering functions measured by the airborne Particle Habit Imaging and Polar Scattering (PHIPS) cloud probe. Bullet rosette stereo-images are analyzed for their microphysical properties, including maximum dimension, bullet aspect ratio, number of bullets, projected area, bullet hollowness, derived mass, derived effective density and derived terminal velocity, as well as their optical properties such as g and optical complexity parameter. Results indicate that much lower g values represent real atmospheric bullet rosette crystals than what is expected by numerical calculations assuming solid or hollow bullets with smooth idealized surfaces, indicating higher levels of crystal complexity than have been previously incorporated within bullet rosette ray-tracing models.