Determination of the orientation of the ice crystals in a cloud

摘要

Abstract: Theoretical grounds are given in this paper for two methods of determining preferred orientation of crystal particles in a cloud. The methods proposed in the paper enable one to do this in a much simpler way than it could be done when measuring full backscattering phase matrix. One of the methods proposed assumes that a polarization lidar can be rotated as a whole, while the second technique uses rotation of the polariztion plane of a linearly polarized sounding beam. Feasibility of the former technique is illustrated in the paper with the results of field experiments on sounding of a snowfall. Recent experimental studies of crystal clouds conducted with a polarization lidar capable of measuring backscattering phase matrices (BPM) have revealed the fact that preferred orientation of symmetry axes of particles in crystal clouds is very often observed to be in horizontal plane. This conclusion is drawn from the fact that off-diagonal elements of BPMs measured differ from zero. Using a model ensemble of crystal particles of axially symmetric plates and columns one can determine the direction of preferred orientation and the degree of particles orientation about this direction. For many practical reasons it is quite desirable to try to construct a technique for detecting situations in clouds under study when a preferred orientation of crystal particles occurs, which is more simple than that based on measurements of BPMs of clouds. Below we describe two possible versions of lidar measurements using a polarization lidar with a linearly polarized sounding radiation. Such a lidar can record two cross polarized components of lidar returns from scattering medium, i.e. two first Stokes parameters. One of the versions assumes that a lidar facility can be turned around the sounding beam axis as a whole, white in the second version we need to use a $lambda@/2 phase plate in the lidar transmitter to enable changes of sounding beam polarization. In order to make understanding of the techniques proposed easier, let us remind basic relationships for a polarization lidar sensing scheme. !6