Remote Sensing of Planar Ice Crystal Fall Attitudes

Abstract

The ability of planar ice crystals to assume horizontal orientations during fall is examined as a function of crystal diameter. After reviewing fall velocity measurements for the various planar crystal types, the fall attitude predictions from model experiments are compared to the results of photographic analyses of light pillar displays, an atmospheric optical phenomenon generated by near-horizontally aligned ice crystals. It is concluded that the prediction for stable fall in terms of Reynolds number (Re) through the range 1.0 < 100 is generally valid in the atmosphere. It is further shown that planar crystals with Re * 10 display the most stable fall attitudes, and that crystal diameters >0.1-0.2 mm are required to generate the optical displays. It also appears that the distribution of ice crystal orientations from the horizontal plane is Gaussian in character owing to the effects of turbulence. The implications of these findings for other optical phenomena and the active remote sensing of cloud composition are discussed.