Dual-polarization cloud lidar design and characterization

Abstract

Lidar has proven to be a very useful tool in many kinds of atmospheric research and remote sensing applications. Specialized lidar instruments have been built to detect certain atmospheric constituents, some with additional capabilities such as polarization sensitivity or multiple wavelength operation. Many lidar systems are large, expensive and designed for one application only. This thesis describes the design and characterization of a lidar system for the direct detection of clouds, but which is versatile enough to be reconfigured for other applications. The source is an Nd:YAG laser at a wavelength of 532 nm and with pulse energies of 118 mJ. A dual-polarization receiver is implemented with a liquid crystal polarization rotator. The system is designed to be compact (a 31 cm x 46 cm x 97 cm optics package plus a half-size rack of electronics) and robust enough for transport and deployment. The control software was developed in LabVIEW and runs on a Windows platform. These design criteria make the system useful both as a science tool and as an educational tool. 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. 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 3 m range resolution. Two orthogonal polarization states are measured on alternate laser pulses (at 30 pulses/second). Polarization discrimination is sufficient to measure depolarization ratios with better than 0.4% accuracy. 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.