Dual-polarization cloud lidar using a liquid crystal variable retarder

Abstract

Information on local cloud coverage, with high spatial and temporal resolution, is useful for studying how the radiative properties of clouds affect the climate. The resolution of a lidar allows for detection of subvisual cloud and aerosol layers, and for determining particle sizes of the scatterers. A cloud lidar sensitive to polarization can distinguish between ice and water in clouds, since ice crystals are more depolarizing than water droplets. Cloud lidars complement either ground-based or space-based cloud imagers by supplying the missing vertical dimension. This paper describes the design and characterization of a lidar system for the direct detection of clouds, using a liquid crystal to discriminate between backscattered polarization states on alternate laser pulses (at 30 Hz). The source is a Nd:YAG laser at a wavelength of 532 nm and with pulse energies of 118 mJ. The system is designed to be compact and robust enough for transport and deployment. Data presented show the lidar system is capable of detecting clouds up to 9.5 km above ground level (the normal operating range is 15 km) with a 1.5 m range resolution. The receiver field of view is conveniently variable up to 8.8 mrad. Daytime operation is possible, thanks to laser-line interference filters and a gated photomultiplier tube. Polarization discrimination is sufficient to measure depolarization ratios with an additive error of less than 0.4%.