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

Suspended insects and other nonhydrometeor particulates are commonly detected in and near the planetary boundary layer by high-sensitivity millimeter-wave cloud radars, Radar reflectivity patterns of these targets are difficult to discern from those of stratus clouds, and therefore these particles represent 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 information 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, insects alone, and mixtures of the two at the Atmospheric Radiation Measurement program's Cloud and Radiation Test bed sites. Theory is also developed for estimating the contribution of cloud droplets to the observed reflectivity of an insect plus droplet mixture using dual-polarization cloud radar data. Preliminary data indicate that the method provides more accurate estimates if the radar transmits circular rather than linear polarization because the effects of target orientation are minimized.