Laser Sensing of Cloud Composition: A Backscattered Depolarization Technique

Abstract

Theoretical analyses reveal that the backscattered radiation from spherical water droplets retains the polarization of the incident energy, whereas radiation backscattered from non-spherical ice crystals is partially depolarized. It is demonstrated that depolarization from hexagonal crystals arises from rays undergoing internal reflections within the crystals. For randomly oriented ice crystals, depolarization is found to be independent of the incident polarization with a value of about 29%. However, preferred orientation may lead to the dependence of depolarization on the polarization state of the incident energy as well as the orientation plane. Laboratory, experiments employing a 6328.k heliuin-neon laser have been carried out for the studies of the backscattered depolarization from clouds. A 2–4% depolarization ratio is observed from dense water clouds. For ice crystal clouds in the initial stage, the depolarization ratio is about 35%. Moreover, the dependence of depolarization on the incident polarization state and the size of ice crystals is also indicated in the experimental results. If the effect of multiple scattering is recognized, the experiments appear to verity fairly well the concept derived from the theoretical analyses. On the basis of these theoretical and experimental studies, we demonstrate that the backscattered depolarization technique can be utilized to differentiate between ice and water clouds and to obtain valuable information on ice cloud composition.