Demonstration of novel polarization lidar technique for identifying horizontally oriented ice crystals

Ice crystals are known to horizontally orient in the atmosphere when drag forces overcome the randomizing effects of Brownian motion. Such ice crystals have been shown to have an impact on radiative transfer, reflecting a greater portion of incident sunlight than their randomly oriented counter parts. However, regular identification of oriented ice crystals in the atmosphere has proven challenging. Existing lidar techniques rely on detection of strong specular backscatter from oriented platelets. These measurements are not common to most lidar systems, and are in fact, frequently avoided because such strong specular signals generally overwhelm lidar detector systems designed for typical cloud and aerosol studies. When lidars are tilted to avoid these specular returns, the low polarization ratio observed in some clouds consisting of oriented ice crystals will cause researchers to incorrectly conclude they are composed of liquid water, thereby skewing cloud phase statistics and providing an incorrect estimate of the cloud's impact on radiative transfer. To address these problems, we apply a novel lidar configuration, which provides a unique polarization capability that detects oriented ice crystals. By tilting the lidar off zenith and performing three polarization measurements, diattenuation, a polarization attribute only exhibited by oriented ice crystals, can be measured. This allows us to disambiguate clouds consisting of oriented ice crystals and water. We present here some of the first measurements of diattenuation for detection of oriented ice crystals as performed by the CAPABL lidar system in Summit Camp, Greenland. This polarization technique avoids detecting the strong specular reflections commonly used to identify oriented ice crystals, allowing return signals from oriented crystals to remain in the same dynamic range as other clouds and aerosols. This feature makes it possible for CAPABL to perform accurate, high performance measurements of all clouds and aerosols, even when oriented crystals are present.