Using simple ray-tracing simulations, the cause of the rare Parry arc has been linked historically to horizontally oriented columns that display the peculiar ability to fall with a pair of prism faces closely parallel to the ground. Although we understand the aerodynamic forces that orient the long-column axis in the horizontal plane, which gives rise to the relatively common tangent arcs of the 22° halo, the mechanism leading to the Parry crystal orientation has never been resolved adequately. On 16 November 1998, at the University of Utah Facility for Atmospheric Remote Sensing, we studied a cirrus cloud producing a classic upper Parry arc using polarization lidar and an aircraft with a new high-resolution ice crystal imaging probe. Scanning lidar data, which reveal extremely high linear depolarization ratios δ a few degrees off the zenith direction, are simulated with ray-tracing theory to determine the ice crystal properties that reproduce this previously unknown behavior. It is found that a limited range of thick-plate crystal axis (length-to-diameter) ratios from ~0.75 to 0.93 generates a maximum δ ≈ 2.0–5.0 for vertically polarized 0.532-μm light when the lidar is tilted 1°–2° off the zenith. Halo simulations based on these crystal properties also generate a Parry arc. However, although such particles are abundant in the <i>in situ</i> data in the height interval indicated by the lidar, one still has to invoke an aerodynamic stabilization force to produce properly oriented particles. Although we speculate on a possible mechanism, further research is needed into this new explanation for the Parry arc.
© 2000 Optical Society of America
(010.0010) Atmospheric and oceanic optics : Atmospheric and oceanic optics
(010.2940) Atmospheric and oceanic optics : Ice crystal phenomena
(010.3640) Atmospheric and oceanic optics : Lidar
(290.1350) Scattering : Backscattering
Kenneth Sassen and Yoshihide Takano, "Parry Arc: A Polarization Lidar, Ray-Tracing, and Aircraft Case Study," Appl. Opt. 39, 6738-6745 (2000)