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 refl ection from large plane facets of horizontally oriented ice plates. Si nce the standard geometric optics approximation (GO) may be inapplicab le in many cases, especially in analyzing infrared measurements, and i gnores physical optics effects, we use the recently improved exact T-m atrix method to compute the scattering of light by ice plates at visib le and infrared wavelengths. Computations for horizontally and randoml y oriented thin disks and oblate spheroids with size parameters up to 50 show that while all particles produce a strong Fraunhofer diffracti on peak centered at exactly the forward-scattering direction, a strong and narrow ZEE peak can be produced only by horizontally oriented dis ks but not by horizontally oriented spheroids or particles in random o rientation. This finding demonstrates that ZEE can be produced even by particles which are not in the GO domain of size parameters and suppo rts the traditional interpretation of ZEE. Also, we have found that th e angular width of the ZEE peak for horizontally oriented disks is equ al tb half the width of the Fraunhofer diffraction peak. This result c an be used in practice to derive a lower estimate of ice particle size s from high angular resolution measurements of ZEE. We show that our e xact T-matrix computations can explain the peculiar zenith-angle depen dence of depolarization observed by Platt et al. [1978] in the visible and can be interpreted qualitatively in terms of the modified Kirchho ff approximation.