Abstract. 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 1064 nm measurements acquired by CALIOP (i.e., the
Cloud-Aerosol Lidar with Orthogonal Polarization), the two-wavelength (532
and 1064 nm) elastic backscatter lidar currently flying on the Cloud-Aerosol
Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission.
CALIOP's 532 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 nm than at 532 nm,
the direct high-altitude molecular normalization method is not a viable
option at 1064 nm. Instead, CALIOP's 1064 nm channel is calibrated relative
to the 532 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 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 nm calibration coefficients of 25 % or more, relative to previous
data releases, and are shown to substantially improve the accuracy of the V4
1064 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 nm calibration
coefficients are accurate to within 3 %.
The characterization of deep convective cloud albedo as a calibration target using MODIS reflectances
