A global survey of CALIPSO linear depolarization ratios in ice clouds: Initial findings

Linear depolarization ratio (δ) data from the summer/winter seasons over the first 2 years of Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite operations in the green (0.532 μm) laser channel are analyzed and interpreted in terms of ice cloud microphysical properties. That is, we use variations in δ as a proxy of cloud formation and environmental conditions that affect ice crystal shape and orientation. The cloud detection algorithm is tuned mainly to cirrus clouds, but also includes polar stratospheric clouds (PSC) and optically thin, low and midlevel ice clouds at high latitudes. As anticipated from ground‐based polarization lidar studies, δ increase with increasing height/decreasing temperature, and the effects of horizontally oriented plate crystals in lowing δ are evident by comparing data obtained close to the nadir (0.3°) and off‐nadir (3.0°) pointing directions. These differences in δ average 0.01–0.03, although this anisotropic scattering effect is particularly apparent at low altitudes in the mid and high latitudes. Unexpected findings include decreasing δ with increasing latitude, and δ in detected PSC that are usually similar to cirrus clouds. However, δ in PSC are lower in a belt in the lower stratosphere in the Southern Hemisphere and generally lower in the Northern Hemisphere, but higher in lower stratospheric nacreous clouds in both hemispheres. There are also significant differences in the ice cloud δ measured at night and day, but this is assumed to result from factors associated with day/night differences in CALIPSO data collection. Global average δ are 0.34–0.36 for day, and 0.23–0.26 for night.