The CALIPSO Automated Aerosol Classification and Lidar Ratio Selection Algorithm

TLDR

The CALIPSO aerosol products rely on algorithms that classify aerosol types and select extinction‑to‑backscatter ratios, with six predefined types—clean continental, clean marine, dust, polluted continental, polluted dust, and smoke—each assigned specific lidar ratios. The study aims to present global distributions of CALIPSO aerosol types, their integrated attenuated backscatter, and volume depolarization ratios. The authors analyze one year of CALIPSO level‑2 v2 data, evaluate algorithm robustness through interannual variability of polluted dust, and partition aerosol distributions by surface type and spatial resolution, using surface type to refine the typing algorithm. The analysis shows that, except for clean marine and polluted continental, all aerosol types are preferentially detected at 80‑km resolution, with smoke and polluted dust largely over water, dust and polluted continental evenly over land and water, and that adding surface type to the algorithm does not introduce abrupt changes in aerosol classification or extinction.

Abstract

Abstract Descriptions are provided of the aerosol classification algorithms and the extinction-to-backscatter ratio (lidar ratio) selection schemes for the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) aerosol products. One year of CALIPSO level 2 version 2 data are analyzed to assess the veracity of the CALIPSO aerosol-type identification algorithm and generate vertically resolved distributions of aerosol types and their respective optical characteristics. To assess the robustness of the algorithm, the interannual variability is analyzed by using a fixed season (June–August) and aerosol type (polluted dust) over two consecutive years (2006 and 2007). The CALIPSO models define six aerosol types: clean continental, clean marine, dust, polluted continental, polluted dust, and smoke, with 532-nm (1064 nm) extinction-to-backscatter ratios Sa of 35 (30), 20 (45), 40 (55), 70 (30), 65 (30), and 70 (40) sr, respectively. This paper presents the global distributions of the CALIPSO aerosol types, the complementary distributions of integrated attenuated backscatter, and the volume depolarization ratio for each type. The aerosol-type distributions are further partitioned according to surface type (land/ocean) and detection resolution (5, 20, and 80 km) for optical and spatial context, because the optically thick layers are found most often at the smallest spatial resolution. Except for clean marine and polluted continental, all the aerosol types are found preferentially at the 80-km resolution. Nearly 80% of the smoke cases and 60% of the polluted dust cases are found over water, whereas dust and polluted continental cases are found over both land and water at comparable frequencies. Because the CALIPSO observables do not sufficiently constrain the determination of the aerosol, the surface type is used to augment the selection criteria. Distributions of the total attenuated color ratios show that the use of surface type in the typing algorithm does not result in abrupt and artificial changes in aerosol type or extinction.