CALIPSO 1064 nm Calibration Biases Inferred from Wavelength-Dependent Signal Attenuation by Stratospheric Aerosols
Abstract. Calibration of lidar signals at 1064 nm from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite depends on the prior calibration of the primary 532 nm channel. However, the 1064 nm calibration procedure also requires knowledge of the ratio of stratospheric signal attenuations at 1064 nm and 532 nm, which is not available a priori and thus is assumed to be 1. This assumption introduces a potential bias in the computed 1064 nm calibration coefficients. In this work we assess this bias by using independent multi-channel occultation retrievals of stratospheric aerosol extinction from the Stratospheric Aerosol and Gas Experiment (SAGE III) on the International Space Station (ISS) for the period 2017 onwards. We also use the GLObal Space based Stratospheric Aerosol Climatology (GloSSAC) to provide a historical background during the SAGE II era (1984 through 2005). The results show that the magnitude of the CALIOP 1064 nm calibration bias is less than 1–2 % within the tropics under stratospheric background conditions. However, recent biases can be as high as 5 % when volcanic perturbations and/or pyro-cumulonimbus (pyroCb) injections dominate the stratospheric aerosol loading. We explore the effects of this bias on CALIOP’s level 2 science retrievals by estimating the anticipated perturbations in cloud-aerosol discrimination (CAD) performance and by quantifying the non-linear propagation of errors in CALIOP’s 1064 nm extinction coefficients. This global characterization of the spectral attenuation differences should provide useful information for future spaceborne elastic lidars operating at 1064 nm.
The present manuscript deals with the calibration of the 1064 nm channel of CALIPSO. The calibration of the 1064 nm signal is done via the 532 nm signal. However, some corrections are necessary due to wavelength dependent signal attenuation in the stratosphere above the cirrus clouds. It once more shows the great care and effort by the CALIPSO team to provide the best possible data set from the CALIPSO mission. The scientific world can learn from their great expertise to solve the challenges of spaceborne lidar observations. The manuscript is clearly written and just needs minor revisions before publication.
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