Mitigating impacts of low energy laser pulses on CALIOP data products

Abstract. The spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) experienced an increasing number of intermittent low energy laser pulses in the final 7 years of the Cloud-Aerosol Lidar and Infrared Pathfinder Observations (CALIPSO) mission due to coronal arcing within the laser canister as internal pressure levels decreased. Degraded data quality was initially observed primarily over the South Atlantic Anomaly (SAA) region, with impacts eventually spreading globally at lower rates. To preserve the integrity of the CALIOP data record, a suite of low energy mitigation (LEM) procedures was developed to reject data that is substantially impacted by low energy pulses on small, targeted scales to minimize data loss. LEM corrects level 1B calibration biases of −3 to −4 % and reduces calibration uncertainties by 20–40 % at SAA-latitudes (0–50°S) for the 532 nm daytime and 1064 nm channels. LEM rejection in level 2 processing substantially reduces the occurrence of false feature detections. Horizontally averaged data segments that are LEM-affected (i.e., some low energy shots are present, but LEM determines the filtered profile is still acceptable) experience a signal-to-noise ratio reduction of 6–9 % which increases the probability of false detections, though this is mitigated somewhat by a slight increase in feature detection thresholds. Features identified in LEM-affected data are similar to unaffected features in terms of measured layer-averaged properties that are important for classification and subtyping. Overall, this evidence suggests that LEM has eliminated the major impacts of low energy pulses on data quality. LEM procedures were implemented in the version 4.51 level 1B data release and are implemented in all data levels of the final version 5 CALIOP data release.