CALIPSO lidar calibration at 1064 nm: version 4 algorithm

Abstract

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 %.

Discriminating between clouds and aerosols in the CALIOP version 4.1 data products

“…Overall, the nighttime net cloud and aerosol fractions remain unchanged from V3 to V4, but the daytime net aerosol fraction is increased by about 2 % and the daytime net cloud fraction is decreased by about 2 %. Vernier et al, 2013;Yu et al, 2015;Ma et al, 2015;Cesana and Waliser, 2016;Jing et al, 2016;Tan et al, 2016;Behrenfeld et al, 2017). The cloud-aerosol discrimination (CAD) algorithm uses CALIPSO backscatter measurements and retrieved spatial properties to separate clouds from aerosols and must perform reliably under a wide variety of conditions to deliver the necessary information for additional level 2 lidar data processing and support the widest possible range of scientific investigations.…”

Cloud Aerosol Transport System (CATS) 1064 nm Calibration and Validation

“…This study presents a detailed discussion of the CATS 1064 nm calibration algorithm, as well as validation using three (Vaughan et al, 2019). The accuracy of the Rayleigh normalization technique, which is also used for CALIOP 532 nm data, is dependent on an accurate estimate of the aerosol loading in the calibration altitude region.…”

“…Clouds are identified for use in the calibration algorithm based on thresholds applied to the magnitude of the 532 nm layerintegrated attenuated backscatter, cloud base and top altitudes, cloud temperature, and the layer-integrated 532 nm volume 25 depolarization ratio (Vaughan et al, 2019). Using cirrus comprised of large ice crystals ensures that the in-cloud backscatter coefficients at 1064 nm and 532 nm are essentially identical (Reagan et al, 2002, Haarig et al, 2016, thus enabling calculation of a 532-to-1064 calibration scale factor for each qualifying cirrus cloud identified in the CALIPSO backscatter data.…”

“…These calibration scale factors are then composited into a continuous time history using a two-dimensional moving window averaging scheme that spans multiple orbits. For any individual profile, the CALIPSO 1064 nm calibration 30 coefficient is simply the product of the interpolated instantaneous value of the scale factor time history and the corresponding 532 nm calibration coefficient (Vaughan et al, 2019).…”

Improved Hydrometeor Detection Method: An Application to CloudSat

“…Superimposed is the mean cloud top (solid line) and base height (dash line) of single-layer cirrus identified through the criterion that cloud optical depth does not exceed 3 and cloud base temperature is lower than −40°C from CALIPSO version 4 level 2, 5 km cloud profile product (L2_05km_CPro) (Gasparini et al, 2018). Note that polar stratospheric clouds detected by CALIPSO, which are optically very thin (Kato et al, 2010;Kohma & Sato, 2011;Vaughan et al, 2019), cause the cirrus top and base lines to rise poleward of 60°S. Overall, G17 (Figure 7c) maintains a similar pattern as R04 (Figure 7a).…”