Abstract
The Cloud-Aerosol Transport System (CATS) lidar on board the International Space Station (ISS) operated from 10 February 2015 to 30 October 2017 providing range-resolved vertical backscatter profiles of Earth's atmosphere at 1064 and 532 nm. The CATS instrument design and ISS orbit lead to a higher 1064 nm signal-to-noise ratio than previous space-based lidars, allowing for direct atmospheric calibration of the 1064 nm signals. Nighttime CATS Version 3-00 data were calibrated by scaling the measured data to a model of the expected atmospheric backscatter between 22 and 26 km above mean sea level (AMSL). The CATS atmospheric model is constructed using molecular backscatter profiles derived from Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) re-analysis data and aerosol scattering ratios measured by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The nighttime normalization altitude region was chosen to simultaneously minimize aerosol loading and variability within the CATS data frame, which extends from 28 km to -2 km AMSL. Daytime CATS Version 3-00 data were calibrated through comparisons with nighttime measurements of the layer integrated attenuated total backscatter (iATB) from strongly scattering, rapidly attenuating opaque cirrus clouds. The CATS nighttime 1064 nm attenuated total backscatter (ATB) uncertainties for clouds and aerosols are primarily related to the uncertainties in the CATS nighttime calibration technique, which are estimated to be ~9%. Median CATS V3-00 1064 nm ATB relative uncertainty at night within cloud and aerosol layers is 7%, slightly lower than these calibration uncertainty estimates. CATS median daytime 1064 nm ATB relative uncertainty is 21% in cloud and aerosol layers, similar to the estimated 16-18% uncertainty in the CATS daytime cirrus cloud calibration transfer technique. Coincident daytime comparisons between CATS and the Cloud Physics Lidar (CPL) during the CATS-CALIPSO Airborne Validation Experiment (CCAVE) project show good agreement in mean ATB profiles for clear-air regions. Eight nighttime comparisons between CATS and the PollyXT ground based lidars also show good agreement in clear-air regions between 3-12 km, with CATS having a mean ATB of 19.7 % lower than PollyXT. Agreement between the two instruments (~7%) is even better within an aerosol layer. Six-month comparisons of nighttime ATB values between CATS and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) also show that iATB comparisons of opaque cirrus clouds agree to within 19%. Overall, CATS has demonstrated that direct calibration of the 1064 nm channel is possible from a space based lidar using the atmospheric normalization technique.
Highlights
Lidar plays a crucial role in observing the Earth’s atmosphere as it enhances our understanding of the roles clouds and aerosols play in the climate system by providing vertical profiles of backscatter coefficient and other optical properties
Despite the complicated terrain and smoke, the mean attenuated total backscatter (ATB) profiles from Cloud-Aerosol Transport System (CATS) and Cloud Physics Lidar (CPL) still shows good agreement in the clear-sky region above the smoke, with the average CPL and CATS mean ATB between 7 and 15 km equal to 4.1927 × 10−5 and 4.0972 × 10−5 km−1 sr−1 respectively, meaning the CATS average ATB was 2.28 % below CPL. This agreement is surprising since the CATS daytime calibration uncertainty is ∼ 16 %–18 %, but this case occurred near local twilight when CATS signal-to-noise ratio (SNR) is higher and the 1.27 value of the 1064 nm particulate scattering ratio used for CPL could be too low, introducing errors in the CPL 1064 nm ATB profile
The CATS ATB was compared with the ground-based EARLINET systems and found to be within 20 % of the calibrated EARLINET data
Summary
Lidar plays a crucial role in observing the Earth’s atmosphere as it enhances our understanding of the roles clouds and aerosols play in the climate system by providing vertical profiles of backscatter coefficient and other optical properties. Ground-based lidars (e.g., Micro-Pulse Lidar Network (MPLNet); Welton et al, 2001) calibrate by normalizing their signal to the molecular profile but require knowledge of the aerosol optical depth of the atmosphere between the instrument and the calibration region (Welton et al, 2002). Using cirrus comprised of ice crystals assumed to be larger than the lidar wavelength ensures that the incloud backscatter coefficients at 1064 and 532 nm are essentially identical (Reagan et al, 2002; Vaughan et al, 2010; Haarig et al, 2016), enabling calculation of a 532-to1064 calibration scale factor for each qualifying cirrus cloud identified in the CALIPSO backscatter data These calibration scale factors are composited into a continuous time history using a two-dimensional moving-window averaging scheme that spans multiple orbits.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
