Polarization Anisotropy in Lidar Multiple Scattering from Atmospheric Clouds

Abstract

The use of polarization information in lidar measurements of the atmosphere, has proven to be a very useful technique, particularly for cloud diagnostics.1 The backscattering of liquid water droplets has very different polarization characteristics from the backscattering of ice crystals, and in many situations it is possible to use the polarization signatures to discriminate between ice and water in atmospheric clouds. Also of considerable importance is the utilization of lidar polarization information for separating the contributions of single and multiple scattering in water droplet clouds. When such clouds of perfectly spherical scatterers are illuminated by linearly polarized radiation, the single scattering in the backward direction, retains the linear polarization of the incident beam, while the multiple scattering introduces a cross-polarized component. (We designate the parallel and cross-polarized components as I‖ and I⊥ respectively). Because the I⊥ component can only arise from the multiple scattering process, observation of the polization components provides a direct measure of the multiple scattering taking place in the cloud. Since most lidar analyses assume that only single scattering is occurring, the ability to quantitatively detect the presence of multiple scattering is extremely valuable in most cloud lidar investigations. As a result lidar systems have been designed to permit such measurements. In the lidar systems, the polarization measurement is performed on the backscattered laser radiation which falls within the field of view of the lidar receiver. Implicit in the polarization measurement is the assumption that the polarization state is uniform within the field of view and that the measured properties reflect the polarization state of the entire scattering volume viewed by the receiver.