Investigations of the optical properties of cirrus clouds crystals in the case of predominantly azimuthal orientation

Abstract

Orientation probability densities of ice crystals constituting cirrus clouds are required for correct interpretation of the lidar signals. It is shown that in the case of the azimuthal orientation, the probability density function can be constructed as a superposition of azimuthal and zenith distribution functions. However, such a superposition has a discontinuity in the poles of the orientation unit sphere, so it would be preferable to use Fisher or Kent distributions. The backscattering Mueller matrix has been calculated for the hexagonal ice columns and plates for the first time. Possibility of retrieving the orientation distributions of the crystals from a vertically pointing polarization lidar measuring the full Mueller matrix is considered. It is shown that for the columns the element m44 or, equivalently, the circular depolarization ratio distinguishes between the low and high zenith tilts of the crystals. Then, at their low or high zenith tilts, off-diagonal elements should be measured to retrieve the azimuth tilts.