CALIPSO Lidar Calibration at 532 nm: Version 4 Nighttime Algorithm.

Area of Science:

• Atmospheric Science
• Remote Sensing
• Lidar Technology

Background:

• The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on the CALIPSO satellite has undergone calibration updates.
• Version 4 calibration algorithms were implemented for level 1 attenuated backscatter measurements.
• Accurate calibration is crucial for reliable atmospheric data products.

Purpose of the Study:

• To present the motivation and implementation of the version 4 nighttime 532 nm parallel channel calibration for CALIOP.
• To improve the accuracy and reduce biases in CALIOP data, particularly for stratospheric aerosol characterization.
• To validate the new calibration using airborne lidar measurements.

Main Methods:

• Increased the molecular normalization altitude from 30-34 km to 36-39 km to minimize stratospheric aerosol contamination.
• Averaged signals over multiple adjacent granules to improve signal-to-noise ratio at higher altitudes.
• Implemented enhanced filtering for radiation-induced noise, adopted the MERRA-2 meteorological model, and used an aerosol scattering ratio of 1.01 ± 0.01.

Main Results:

• Globally revised calibration coefficients are 2-3% lower on average compared to previous versions.
• The new calibration eliminates high-altitude biases present in earlier versions.
• Validation shows reduced biases (1.6% ± 2.4%) compared to airborne lidar measurements, down from 3.6% ± 2.2% in version 3.

Conclusions:

• The version 4 calibration significantly improves the accuracy of CALIOP nighttime 532 nm data.
• Enhanced stratospheric aerosol representation and reduced measurement biases are key outcomes.
• The updated calibration provides a more reliable dataset for atmospheric research.