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Ice cloud depolarization for nadir and off‐nadir CALIPSO measurements
Author(s) -
Sassen Kenneth,
Kayetha Vinay Kumar,
Zhu Jiang
Publication year - 2012
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2012gl053116
Subject(s) - nadir , cirrus , environmental science , lidar , latitude , atmospheric sciences , ice cloud , ice crystals , radiative transfer , satellite , remote sensing , climatology , geology , meteorology , geodesy , geography , physics , optics , astronomy
Currently approaching its sixth year in space, the CALIPSO satellite collects lidar linear depolarization ratios δ from 0.532 μ m laser backscatter, an indicator of particle phase, shape, and orientation. We examine one‐year δ averages 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 of δ on 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‐nadir δ 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 δ for 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.