An overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) project: aerosol-cloud-radiation interactions in the Southeast Atlantic basin

Southern Africa produces almost a third of the Earth’s biomass burning (BB) aerosol particles, yet the fate of these particles and their influence on regional and global climate is poorly understood. ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) is a five-year NASA EVS-2 (Earth Venture Suborbital-2) investigation with three Intensive Observation Periods designed to study key atmospheric processes that determine the climate impacts of these aerosols. During the Southern Hemisphere winter and spring (June-October), aerosol particles reaching 3-5 km in altitude are transported westward over the South-East Atlantic, where they interact with one of the largest subtropical stratocumulus subtropical stratocumulus (Sc) cloud decks in the world. The representation of these interactions in climate models remains highly uncertain in part due to a scarcity of observational constraints on aerosol and cloud properties, and due to the parameterized treatment of physical processes. Three ORACLES deployments by the NASA P-3 aircraft in September 2016, August 2017 and October 2018 (totaling ~350 science flight hours), augmented by the deployment of the NASA ER-2 aircraft for remote sensing in September 2016 (totaling ~100 science flight hours), were intended to help fill this observational gap. ORACLES focuses on three fundamental science themes centered on the climate effects of African BB aerosols: (a) direct aerosol radiative effects; (b) effects of aerosol absorption on atmospheric circulation and clouds; (c) aerosol-cloud microphysical interactions. This paper summarizes the ORACLES science objectives, describes the project implementation, provides an overview of the flights and measurements in each deployment, and highlights the integrative modeling efforts from cloud to global scales to address science objectives. Significant new findings on the vertical structure of BB aerosol physical and chemical properties, chemical aging, cloud condensation nuclei, rain and precipitation statistics, and aerosol indirect effects are emphasized, but their detailed descriptions are the subject of separate publications. The main purpose of this paper is to familiarize the broader scientific community with the ORACLES project and the data set it produced.

南部非洲贡献了全球近三分之一的生物质燃烧（Biomass Burning, BB）气溶胶颗粒物，但学界对这类颗粒物的归宿及其对区域与全球气候的影响仍知之甚少。ORACLES项目（全称为ObseRvations of Aerosols above CLouds and their intEractionS，即云上方气溶胶及其相互作用观测计划）是一项为期五年的NASA EVS-2（地球亚轨道探索计划2，Earth Venture Suborbital-2）研究项目，包含三个密集观测期，旨在揭示决定这类气溶胶气候影响的关键大气过程。在南半球冬春季节（6月至10月），海拔3至5千米处的气溶胶颗粒物会向西输送至东南大西洋海域，并与全球规模最大的亚热带层积云（Stratocumulus, Sc）群落之一发生相互作用。气候模型中对这类相互作用的表征仍存在极大不确定性，部分原因在于气溶胶与云属性的观测约束数据匮乏，以及物理过程的参数化处理方式存在局限。NASA分别于2016年9月、2017年8月及2018年10月利用P-3型飞机开展了三次ORACLES野外部署任务，累计科学飞行时长约350小时；此外于2016年9月部署ER-2型飞机开展遥感观测，累计科学飞行时长约100小时，旨在填补这一观测空白。ORACLES围绕非洲生物质燃烧气溶胶的气候影响设定了三大核心科学主题：(a) 气溶胶直接辐射效应；(b) 气溶胶吸收对大气环流与云的影响；(c) 气溶胶-云微物理相互作用。本文概述了ORACLES的科学目标，介绍了项目实施概况，梳理了各次野外部署的飞行任务与观测内容，并重点阐述了从云尺度到全球尺度的一体化建模工作，以响应相关科学目标。本文着重介绍了生物质燃烧气溶胶物理化学属性垂直结构、化学老化、云凝结核、降雨与降水统计特征以及气溶胶间接效应等方面的重要新发现，但相关详细内容将在后续单独发表的论文中阐述。本文的核心目的是向更广泛的科学界介绍ORACLES项目及其产出的数据集。