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

We present the global above-cloud aerosol optical depth (ACAOD) retrieval dataset derived using the Depolarization Ratio (DR) method applied to CALIOP cloudy-sky measurements. 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, δ′ 532 , to deduce the optical depth of aerosols above opaque water clouds (OWCs) at 532 nm. First, we derived a global climatology of calibration constant, γ′ water, unobstructed , which represents the layer-integrated attenuated backscatter of the unobstructed water clouds. Second, we applied the DR method with an optimal configuration using the CALIOP Level 2 cloud and aerosol layer products to derive ACAOD dataset globally. Third, we validate the 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. DR ACAOD correlates well (R 2 ∼0.95 for HSRL-2 and R 2 ∼0.75 for 4STAR) against airborne measurements, however, with an overall positive bias of ∼0.088. An inter-satellite comparison of CALIOP-retrieved ACAOD against the research dataset derived from Aqua-MODIS over the southeastern Atlantic Ocean show a good-level of agreement, whereas the operational retrievals from OMI were found to show a negative bias of ∼0.13. The Extinction Ångström Exponent derived from the combination of DR method ACAOD and color ratio of 1064–532 nm provided the known regional distribution of fine-mode carbonaceous smoke and coarse-mode mineral dust aerosols. We also calculated the above-cloud aerosol lidar ratio from the derived extinction optical depth and backscatter measurements. The global dataset of reliable ACAOD from CALIOP paves the way for a potential synergy with NASA’s A-train sensors, such as OMI and MODIS, for constraining the UV-VIS observations to derive spectral aerosol absorption properties over clouds, which are essential for estimating direct and semi-direct radiative effects of aerosols in cloudy atmospheres.