PHIPS-HALO Radiative Measurement Applications to Atmospheric Bullet Rosette Ice Crystals

Cirrus clouds provide a substantial amount of coverage in the earth’s atmosphere, resulting in a major impact on the global radiative budget. The extent of the radiative impact is determined by the aspherical ice crystal composition within the cirrus. Thus, a proper understanding of ice crystal single scattering properties is necessary for accurate climatological modeling and forecasting. One of the most relevant cirrus ice crystal habits is a polycrystalline bullet rosette, where individual bullets are radiating from the same nucleation point. Comprehensive studies on the dependencies of ice crystal habit formation have shown that bullet rosettes grow at a range of ice supersaturations with temperatures below -40 °C; environmental conditions frequently found within high altitude cirrus clouds. The Particle Habit Imaging Polar Scattering (PHIPS-HALO) probe is a unique aircraft mounted instrument which simultaneously acquires high-resolution stereo images and single scattering properties of individual cloud ice crystals. The ability to combine observed angular scattering functions with detailed images of their related crystals allows for in-depth analysis of the radiative effects of specific habits in critical atmospheric systems such as cirrus clouds. Here, a detailed explanation of the PHIPS-HALO performance and capabilities is provided with an investigation of single scattering properties of atmospheric bullet rosettes. Bullet rosette stereo-images were taken during a range of flights from the CIRRUS-HL campaign and were analyzed for their maximum dimensions as well as for crystal complexity and visually inspected for number of bullets per rosette, individual bullet aspect ratios and bullet hollowness. These bullet rosette microphysical properties were then associated with environmental conditions and with the simultaneously measured angular light scattering function and resulting asymmetry parameter. Angular scattering functions and asymmetry parameters are discussed for bullet rosettes grouped into subsets by complexity parameter. Results indicate that much lower asymmetry parameters represent real atmospheric bullet rosette crystals than what is expected by theoretical studies assuming smooth surfaces.