CALIPSO lidar calibration at 532 nm: version 4 nighttime algorithm

Abstract

Abstract. Data products from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) were recently updated following the implementation of new (version 4) calibration algorithms for all of the Level 1 attenuated backscatter measurements. In this work we present the motivation for and the implementation of the version 4 nighttime 532 nm parallel channel calibration. The nighttime 532 nm calibration is the most fundamental calibration of CALIOP data, since all of CALIOP's other radiometric calibration procedures – i.e., the 532 nm daytime calibration and the 1064 nm calibrations during both nighttime and daytime – depend either directly or indirectly on the 532 nm nighttime calibration. The accuracy of the 532 nm nighttime calibration has been significantly improved by raising the molecular normalization altitude from 30–34 km to the upper possible signal acquisition range of 36–39 km to substantially reduce stratospheric aerosol contamination. Due to the greatly reduced molecular number density and consequently reduced signal-to-noise ratio (SNR) at these higher altitudes, the signal is now averaged over a larger number of samples using data from multiple adjacent granules. Additionally, an enhanced strategy for filtering the radiation-induced noise from high-energy particles was adopted. Further, the meteorological model used in the earlier versions has been replaced by the improved Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), model. An aerosol scattering ratio of 1.01±0.01 is now explicitly used for the calibration altitude. These modifications lead to globally revised calibration coefficients which are, on average, 2–3 % lower than in previous data releases. Further, the new calibration procedure is shown to eliminate biases at high altitudes that were present in earlier versions and consequently leads to an improved representation of stratospheric aerosols. Validation results using airborne lidar measurements are also presented. Biases relative to collocated measurements acquired by the Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) are reduced from 3.6 %±2.2 % in the version 3 data set to 1.6 %±2.4 % in the version 4 release.

CALIPSO Lidar Calibration at 532 nm: Version 4 Daytime Algorithm

“…These include flights over the Caribbean (19 August 2010-28 September 2010, the DEVOTE field campaign (04 October 2011 -08 October 2011), and flights over the Azores (17 October 2012) and Bermuda (10 June 2014-19 June 2014. In the process of reproducing the Rogers et al (2011) V3 results, a bug was discovered in the code used to estimate the overlying two-way 5 transmittance differences between the two sets of measurements (see Appendix A in Kar et al, 2018). Accounting for this error led to a small upward revision of the daytime biases in the V3 dataset, which we now estimate at 3.3% ± 3.1%.…”

“…In doing the comparisons, the HSRL signals are corrected for known attenuations that occur between the CALIPSO satellite altitude and the HSRL aircraft altitude (e.g., molecular and ozone attenuation). However, as explained in Kar et al (2018),which focused on the nighttime comparisons between CALIOP and HSRL, the HSRL measurements cannot be corrected for any attenuation due to undetected cloud or aerosol layers in this altitude regime (e.g., the background stratospheric aerosol layer). Failure to correct for an 15 undetected optical depth of 0.005 yields an attenuation bias of ~1%, a value that is essentially identical to our current estimate of the bias between CALIOP and HSRL.…”

“…Due to differences in signal-to-noise ratios (SNR) and intra-orbit thermal stability, CALIOP uses 5 different techniques to calibrate the 532 nm daytime and nighttime measurements. The calibration procedure for the nighttime 532 nm parallel signals, which is the basis for all other calibrations, uses a high-altitude molecular normalization technique (Russell et al, 1979;Powell et al, 2009;Kar et al, 2018), in which calibration coefficients are determined by taking the ratio of the measured signal to the expected signal computed using an atmospheric model.…”

Polar stratospheric cloud climatology based on CALIPSO spaceborne lidar measurements from 2006 to 2017

“…3. Further details on the CALIPSO v4.10 level 1 data processing and calibration approach can be found in Kar et al (2018).…”

Laser pulse bidirectional reflectance from CALIPSO mission

“…Compared with version 3 (V3) data, the CALIOP V4.1 data product substantially improves both the 532 and 1064 nm calibration accuracies Kar et al, 2018;. Changes to the calibration techniques are firmly rooted in a thoroughly documented and peer-reviewed approach Kar et al, 2018;. More information about the data products, including data availability, user documentation, quality statements, sample read software, and tools for working with the data, etc.…”