T-matrix computations of zenith-enhanced lidar backscatter from horizontally oriented ice plates

Zenith-enhanced backscattering (ZEE) of a lidar beam by cirrus clouds is a remarkable phenomenon usually explained in terms of specular reflection from large plane facets of horizontally oriented ice plates. Since the standard geometric optics approximation (GO) may be inapplicable in many cases, especially in analyzing infrared measurements, and ignores physical optics effects, we use the recently improved exact T-matrix method to compute the scattering of light by ice plates at visible and infrared wavelengths. Computations for horizontally and randomly oriented thin disks and oblate spheroids with size parameters up to 50 show that while all particles produce a strong Fraunhofer diffraction peak centered at exactly the forward-scattering direction, a strong and narrow ZEE peak can be produced only by horizontally oriented disks but not by horizontally oriented spheroids or particles in random orientation. This finding demonstrates that ZEE can be produced even by particles which are not in the GO domain of size parameters and supports the traditional interpretation of ZEE. Also, we have found that the angular width of the ZEE peak for horizontally oriented disks is equal tb half the width of the Fraunhofer diffraction peak. This result can be used in practice to derive a lower estimate of ice particle sizes from high angular resolution measurements of ZEE. We show that our exact T-matrix computations can explain the peculiar zenith-angle dependence of depolarization observed by Platt et al. [1978] in the visible and can be interpreted qualitatively in terms of the modified Kirchhoff approximation.