On the spectral dependence of backscatter from cirrus clouds: Assessing CALIOP's 1064 nm calibration assumptions using cloud physics lidar measurements

Recent space‐based lidar missions rely on assumptions about the spectral dependence of the backscatter signals from cirrus clouds to calibrate measurements made at 1064 nm. In particular, the calibration procedure employed by the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission assumes that the backscatter color ratio, defined as the ratio of particulate backscatter coefficients at 1064 nm and 532 nm, has a value of 1.00, with an expected standard deviation on the order of 0.04. This work assesses the accuracy of this assumption, and its implications for the CALIPSO 1064 nm calibration scheme, using over 400 h of lidar measurements acquired in the northern hemisphere between 2002 and 2007 by the Cloud Physics Lidar (CPL). For the strongly scattering cirrus clouds typically used for CALIPSO calibrations, the uncorrected CPL‐derived backscatter color ratio is 0.83 ± 0.19. Accounting for computational biases introduced by the CPL assumption of pristine air in the calibration region yields a best estimate cirrus cloud color ratio of 1.01 ± 0.25.