Global Above-cloud Aerosol Optical Depth Dataset from CALIOP Depolarization Ratio Method

The Depolarization Ratio (DR) method, developed by Hu et al. (2007), was applied to the long term (2006-2022) CALIOP global, cloud-sky observations to retrieve aerosol optical depth above clouds (ACAOD) at 532 nm. The DR method was among the first lidar-based remote sensing technique to quantify the aerosol loading over clouds without involving any specific assumptions about detailed vertical profiles of aerosols and their optical-microphysical properties, except for the lidar ratio of the cloud layer. Physically based on the two-way transmission of lidar signal, the DR method uses 1) layer-integrated attenuated backscatter at 532 nm, γ'532, and 2) layer-integrated attenuated depolarization ratio at 532 nm, d'532, along with CALIOP Level-2 aerosol and cloud layer products for identifying respective layers, to deduce the optical depth of aerosols above opaque water clouds (OWCs). The validation of DR-retrieved ACAOD against the direct airborne measurements acquired from HSRL-2 lidar and 4STAR sun photometer operated during the ORACLES field campaign in 2016-2018 over the southeastern Atlantic Ocean showed that both datasets correlate well (R2~0.95 for HSRL-2 and R2~0.75 for 4STAR) against airborne measurements. However, DR ACAOD retrievals were biased high by ~0.088. The Extinction Ångström Exponent was derived from the combination of DR method ACAOD and measured color ratio (1064-532 nm) information from CALIOP. Taking advantage of extinction optical depth and backscatter measurements, the above-cloud aerosol lidar ratio was also calculated. The multiyear (2006-2022) dataset of optical properties of above-cloud aerosol derived from CALIOP daytime and nighttime observations is presented here. This dataset is associated with the following publication: Jethva, H., Kayetha, V., Torres. O., Hu, Y.: Global Retrieval Dataset of UV-VIS Extinction and Absorption of Above-cloud Aerosols from Synergy of A-train Active-Passive measurements: Part I—Above-cloud Aerosol Optical Depth from CALIOP Depolarization Ratio Method, Frontiers Environmental Science, Enivronmental Informatics and Remote Sensing, Accepted, 2026.