Depolarization of Laser Light Backscattered by Artificial Clouds

Abstract

An apparatus is described which measures the linear depolarization ratios of vertically polarized laser light (6328 Å) backscattered by mixed phase particle clouds grown in a laboratory cold chamber. Variations in linear depolarization ratios during the phase transitions of undercooled clouds seeded by dry ice and silver iodide agents have been monitored by simultaneously measuring the returned signal intensities in the parallel and orthogonal planes of polarization. The experimental results yield linear depolarization ratio values of approximately 3% for pure water clouds (3 to 10 ?m diameter droplets), to 50 ± 2.5% for pure ice crystal clouds (5 to nearly 200 ?m maximum dimensions) for particle concentrations up to ?104 particles cm?3. The values for mixed particle clouds were found to be related to the ratio of ice to water particle concentrations. Variations in linear depolarization ratios with the elevation angle of the laser receiver system have amounted to only a few percent of the average values in most cases. It is believed that these results are applicable to the lidar probing of atmospheric clouds with similar particle spectra, provided that lidar transmitter and receiver beamwidths are restricted to values which can limit depolarized energy resulting from photon multiple scattering. The linear depolarization ratios of individual signal spikes were found to vary from at most a few percent when plate crystals were present, up to 100% for hollow columnar crystals. The onset and intensity of these events in the recorded data are also related to ice crystal size, indicating that when small cloud volumes are being examined, this method of analysis is useful in sensing the size and habit of ice crystals.