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 governed by the physical properties of aspherical ice crystals. One of the most relevant cirrus ice crystal habits is a polycrystalline bullet rosette, where individual bullets radiate from the same nucleation point. Here, the link between the crystal morphology of atmospheric bullet rosettes and their radiative properties in the form of the asymmetry parameter (g) is experimentally investigated using correlated high-resolution in situ stereo images of individual rosettes and their corresponding 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 the 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, indicating higher levels of crystal complexity than have been incorporated within previous bullet rosette ray-tracing studies. Measured g values herein have a direct impact on modeling the shortwave reflection of cirrus clouds, resulting in an increase in scaled optical depth by an average of 53 % in comparison to previously calculated g values. This study will provide a valuable in situ dataset that can be used as a basis for the development of future ice optical models.