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.退偏比（Depolarization Ratio, DR）方法由Hu等人于2007年提出，本研究将其应用于2006-2022年的长期全球CALIOP云天观测数据，以反演532 nm波段下的云上方气溶胶光学厚度（aerosol optical depth above clouds, ACAOD）。该DR方法是首批基于激光雷达（lidar）的遥感技术之一，可量化云上方的气溶胶载荷，且无需对气溶胶的详细垂直分布及其光学-微物理特性做出特定假设，仅需预设云层的激光雷达比。该方法基于激光雷达信号的双向传输物理机制，通过以下参数反演不透明水云（opaque water clouds, OWCs）上方的气溶胶光学厚度：1）532 nm波段的层积分衰减后向散射系数γ'532；2）532 nm波段的层积分衰减退偏比d'532，并结合CALIOP二级气溶胶与云层产品实现各分层的识别。 本研究将DR方法反演得到的ACAOD与2016-2018年在东南大西洋开展的ORACLES野外试验中，由HSRL-2激光雷达和4STAR太阳光度计获取的直接机载测量数据进行验证，结果显示两类数据与机载测量结果均具有良好的相关性（HSRL-2的决定系数R²≈0.95，4STAR的R²≈0.75），但DR反演的ACAOD整体偏高约0.088。 消光安斯特朗指数（Extinction Ångström Exponent）可通过结合DR方法反演的ACAOD与CALIOP获取的1064-532 nm波段颜色比实测数据得到；同时利用消光光学厚度与后向散射测量数据，本研究还计算得到了云上方气溶胶的激光雷达比。 本数据集包含2006-2022年多年期的、由CALIOP昼夜观测反演得到的云上方气溶胶光学特性数据。 本数据集关联的已录用论文如下： Jethva H, Kayetha V, Torres O, Hu Y. 基于A列车星座主动-被动观测协同反演的云上方气溶胶紫外-可见光消光与吸收全球数据集：第一部分——利用CALIOP退偏比方法反演云上方气溶胶光学厚度. 《前沿环境科学》（Frontiers Environmental Science）、《环境信息学与遥感》（Environmental Informatics and Remote Sensing），2026年已录用。