Assessing CALIPSO Version 4 Attenuated Backscatter 532 nm Calibration with NASA LaRC Airborne High Spectral Resolution Lidar Measurements

The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) platform, has provided profiles of aerosols and clouds globally since June 2006. The accuracy of the Level 2 CALIOP data products (e.g., aerosol extinction profiles) depends critically on the accuracy of the calibration of the 532 nm Level 1 total attenuated backscatter (TAB) profiles, as they are used to create the Level 2 products. Aerosol backscatter measurements from the NASA Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) have been leveraged to assess the calibration of the CALIOP TAB profiles since the satellite instrument became operational. A past study summarized the efforts of several validation flights conducted from 2006 through 2009, and found that collocated Version 3 CALIOP 532 nm TAB and HSRL TAB agree on average within 2.9% ± 3.9% (CALIOP lower) for daytime conditions and 2.7% ± 2.1% (CALIOP lower) at night. Since then, the CALIOP team has completely redesigned their calibration procedures, and will release an updated version (i.e., Version 4) of the CALIOP Level 1 dataset. This study compares HSRL measurements from the same 2006-2009 flights with the TAB reported in Version 4 of the CALIOP Level 1 dataset to examine the relative change in the agreement between the two sensors. Our results show that Version 4 CALIOP 532 nm and HSRL TAB agree on average within 0.6% ± 4.5% during daytime and 1.1% ± 2.5% during nighttime, indicating an improvement in the CALIOP Version 4 calibration. We also conduct an analysis including additional validation flights deployed from 2010 through 2014, and found that the overall agreement of TAB from both instruments for the 2006-2014 period is -0.2% ± 3.9% (daytime) and 0.4% ± 2.4% (nighttime).