Using cloud radar polarization measurements to evaluate stratus cloud and insect echoes

Suspended insects and other nonhydrometeor particulates are commonly detect ed in and near the planetary boundary layer by high-sensitivity millimeter- wave cloud radars, Radar reflectivity patterns of these targets are difficu lt to discern from those of stratus clouds, and therefore these particles r epresent undesirable contaminants for automated cloud-detection algorithms. Radar polarization measurements, however, offer a means of distinguishing between cloud droplets and these contaminants, based on the shape informati on contained in depolarization ratio observations. A procedure is presented which allows dual-polarization cloud radar data and simultaneous lidar and laser ceilometer data to determine the presence of cloud droplets alone, i nsects alone, and mixtures of the two at the Atmospheric Radiation Measurem ent program's Cloud and Radiation Test bed sites. Theory is also developed for estimating the contribution of cloud droplets to the observed reflectiv ity of an insect plus droplet mixture using dual-polarization cloud radar d ata. Preliminary data indicate that the method provides more accurate estim ates if the radar transmits circular rather than linear polarization becaus e the effects of target orientation are minimized.