Ice cloud depolarization for nadir and off‐nadir CALIPSO measurements

Currently approaching its sixth year in space, the CALIPSO satellite collects lidar linear depolarization ratios δ from 0.532 μm laser backscatter, an indicator of particle phase, shape, and orientation. We examine one‐yearδaverages for day and night periods when the lidar was pointing close to the nadir (0.3°) and off‐nadir (3.0°), in terms of geographic location and zonal height averages. For the first time, also given is the dependency ofδon temperature versus latitude for ice clouds. The analysis involves all ice clouds with a cloud top temperature of <−40°C, which include mainly cirrus and altostratus, as well as some polar stratospheric clouds identified by CALIPSO. We find significant differences from ∼−10° to −30°C between the nadir and off‐nadir data, consistent with the effects of horizontally oriented plate crystals: overall the off‐nadirδ are increased by ∼0.05 globally. Strong dependencies of δ also occur with latitude and height. Day minus night δ differ by 0.02–0.03. The global average day plus night δfor nadir and off‐nadir data are 0.318 and 0.365, respectively. As expected from ground‐based studies,δ increase steadily with decreasing temperature, which is particularly apparent in the nadir data because of oriented plate effects. These findings have implications for the modeling of radiative transfer through ice clouds.