CALIPSO lidar calibration at 1064 nm: version 4 algorithm

Mark Vaughan, Anne Garnier, Damien Josset, Melody Avery, Kam-Pui Lee, Zhaoyan Liu, William Hunt, Jacques Pelon, Yongxiang Hu, Sharon Burton, Johnathan Hair, Jason L. Tackett (+3 others)
2019 Atmospheric Measurement Techniques  
<p><strong>Abstract.</strong> Radiometric calibration of space-based elastic backscatter lidars is accomplished by comparing the measured backscatter signals to theoretically expected signals computed for some well-characterized calibration target. For any given system and wavelength, the choice of calibration target is dictated by several considerations, including signal-to-noise ratio (SNR) and target availability. This paper describes the newly implemented procedures used to calibrate the
more » ... to calibrate the 1064<span class="thinspace"></span>nm measurements acquired by CALIOP (i.e., the Cloud-Aerosol Lidar with Orthogonal Polarization), the two-wavelength (532 and 1064<span class="thinspace"></span>nm) elastic backscatter lidar currently flying on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission. CALIOP's 532<span class="thinspace"></span>nm channel is accurately calibrated by normalizing the molecular backscatter from the uppermost aerosol-free altitudes of the CALIOP measurement region to molecular model data obtained from NASA's Global Modeling and Assimilation Office. However, because CALIOP's SNR for molecular backscatter measurements is prohibitively lower at 1064<span class="thinspace"></span>nm than at 532<span class="thinspace"></span>nm, the direct high-altitude molecular normalization method is not a viable option at 1064<span class="thinspace"></span>nm. Instead, CALIOP's 1064<span class="thinspace"></span>nm channel is calibrated relative to the 532<span class="thinspace"></span>nm channel using the backscatter from a carefully selected subset of cirrus cloud measurements. In this paper we deliver a full account of the revised 1064<span class="thinspace"></span>nm calibration algorithms implemented for the version 4.1 (V4) release of the CALIPSO lidar data products, with particular emphases on the physical basis for the selection of "calibration quality" cirrus clouds and on the new averaging scheme required to characterize intra-orbit calibration variability. The V4 procedures introduce latitudinally varying changes in the 1064<span class="thinspace"></span>nm calibration coefficients of 25<span class="thinspace"></span>% or more, relative to previous data releases, and are shown to substantially improve the accuracy of the V4 1064<span class="thinspace"></span>nm attenuated backscatter coefficients. By evaluating calibration coefficients derived using both water clouds and ocean surfaces as alternate calibration targets, and through comparisons to independent, collocated measurements made by airborne high spectral resolution lidar, we conclude that the CALIOP V4 1064<span class="thinspace"></span>nm calibration coefficients are accurate to within 3<span class="thinspace"></span>%.</p>
doi:10.5194/amt-12-51-2019 fatcat:bjrufpch4vam7ohhgrp2lhblhu