Measurements of backscattering phase matrices of crystalline clouds with a lidar

摘要

Abstract: Backscattering phase matrices (BPM) have been measured by a polarization lidar with controllable polarization of output laser radiation for measuring all the Stokes parameters at $lambda $EQ 532 nm. The degree of orientation and the preferred orientation of particles are determined. To this end, the properties of BPM for the model of axisymmetric prolate particel (APP) ensemble are used. It has been suggested that scattering anisotropy of an aeosol layer, whose BPM is not described by the APP model, is caused by birefringence. In laser sensing of the atmosphere ensembles of aerosol particles being sounded are characterized by the backscattering coefficient. Very often, in addition to this parameter, the intensities of polarized and crosspolarized components of backscattered radiation are determined provided that lineraly polarized laser radiation is used. The ratio of these components is called depolarization and it is assumed a measure of the particle nonsphericity. Use of the above characteristics is based on the concept that atmospheric aerosols are ensembles of spherical or nonspherical randomly oriented particles. An experience of optical studies has shown that such a concept is quite justifiable for the majority of atmospheric aerosols. However, these exists quite a wide class of natural aerosols in the atmosphere, namely, the crystalline clouds, for which the lidar equation in scalar form is insufficient since such aerosol ensembles should be described with a backscattering phase matrix. Below we shall demonstrate this by an example. Of course, the necessity of using the BPM to describe such aerosols is, in certain sense, obvious because anomalous optical phenomena resulting from a pronounced anisotropy of light scattering by crystal clouds have been known long ago. Nevertheless, such phenomena are too rare and it is not a proiri clear how often essential deviations from the scalar approximation occur. Thus, the experimental material available for our analysis at present and partially described in references allow us to arrive at the conclusions that in 30-40 percent lidar observations of crystalline clouds either the backscattering coefficient depends on the direction of sounding radiation polarization or the polarization of scattered light becomes elliptical, or both these effects occur simultaneously. !8