Implementation of CALIOP 1064-nm Channel in the CALIPSO Layer Detection Algorithm

Recent publications have demonstrated that the CALIPSO layer detection algorithm fails to detect the full extent of dense, highly attenuating aerosol layers, thus leading to underestimates of aerosol optical depths. This in turn can introduce large biases in radiative forcing calculations [1,2]. The detection failures occur because the layer detection scheme only relies on the 532 nm total attenuated backscatter measurements; 1064 nm attenuated backscatter and 532 nm volume depolarization are not considered when ascertaining layer boundaries. The focus of future CALIOP Level 2 data releases will be to include 1064 nm attenuated backscatter and 532 nm volume depolarization into the detection algorithm. Inclusion of the 1064 nm signal should help refine layer boundaries, particularly for dense smoke plumes, while depolarization should increase the sensitivity to faint features. Some work on including the 1064 nm channel into the layer detection algorithm has already been done. The V4 CALIOP Level 1 532 nm daytime calibration algorithm was designed to use the 1064 nm channel in the layer detection algorithm for identifying feature-free regions used in the calibration process [3]. Tuning the layer detection algorithm for the 1064 nm signal was fairly straightforward, but was done based on several assumptions. First, the horizontal resolution would only be a fixed 200 km, whereas the 532 nm approach uses an iterated, multi-resolution averaging scheme. Second, only the upper portion of the atmosphere was considered for determination of a singular 1064 nm threshold, while the 532 nm threshold changes as a function of altitude and the profiles are searched from the top of the atmosphere all of the way to the surface. Finally, the 1064 nm approach only had to identify if the column from the top of the atmosphere to the base of the calibration region had a feature within, typically clouds. The ability to interactively detect and clear the signal to account for multiple layers, as done in the 532 nm approach, was not needed. The following will highlight the work done to extend the 1064 nm channel to meet the more rigorous demands imposed by the Level 2 processing requirements.

[1] Lu et al., 2018: Biomass smoke from southern Africa can significantly enhance the brightness of stratocumulus over the southeastern Atlantic Ocean, PNAS, 115, 2924-2929, doi:10.1073/pnas.1713703115.

[2] Rajapakshe et al., 2017: Seasonally Transported Aerosol Layers over Southeast Atlantic are Closer to Underlying Clouds than Previously Reported, Geophys. Res. Lett., 44, 5818–5825, doi:10.1002/2017GL073559.

[3] Getzewich et al., 2018: CALIPSO Lidar Calibration at 532 nm: Version 4 Daytime Algorithm, in preparation, 2018.