Lidar Backscatter from Horizontal Ice Crystal Plates

Abstract

Some unusual lidar returns from an altostratus cloud are interpreted in terms of reflections from hexagonal ice plates falling with their long axes aligned in the horizontal. Such an explanation is consistent with the observed high backscatter coefficients and low depolarization ratios and also with the temperature range (−12 to −20°C) of the cloud layers, as well as the known fall characteristics of naturally occurring ice platelets. Backscatter efficiencies are calculated for “perfect” ice platelets when illuminated at or near an axis orthogonal to the crystal long axis. It is shown that very high backscatter coefficients can potentially be measured from a cloud of ice plates, depending on the fraction of crystals which are “perfect,&rdquo,; the degree to which the plates' long axes stay horizontal, and the angle of the lidar to the vertical. It is further shown that reflection from a single crystal gives an appreciable signal-to-noise ratio at the receiver and that only a few crystals will be correctly aligned in the horizontal in a typical laser pulse volume, and for realistic particle number densities.