Abstract
Abstract. 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 5-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 southeast Atlantic, where they interact with one of the largest 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, as well as 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, and (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 dataset it produced.
Highlights
The radiative and cloud-altering impacts of anthropogenic aerosol particles constitute the largest source of uncertainty in anthropogenic climate forcing (IPCC, 2013)
The ORACLES dataset permits the study of aerosol radiative and cloud-nucleating properties, their vertical distribution relative to clouds, the locations and degree of aerosol mixing into clouds, and cloud changes in response to such mixing
– biomass burning (BB) aerosol layers appear to be in much more frequent contact with the Sc cloud deck underneath than previously estimated, but the correlation of cloud droplet number concentrations with above-cloud smoke properties is weak (Diamond et al, 2018); in situ data from August 2017 suggest that cloud droplet number and above-cloud aerosol can be anticorrelated, owing to the anticorrelation of marine boundary layer (MBL) aerosol with above-cloud aerosol amount (Kacarab et al, 2020);
Summary
The radiative and cloud-altering impacts of anthropogenic aerosol particles constitute the largest source of uncertainty in anthropogenic climate forcing (IPCC, 2013). Aerosol particles interact directly with solar radiation through scattering and absorption of radiation, which leads to a direct radiative forcing whose sign depends on the ratio of the absorption to the total aerosol extinction and on the albedo of the underlying surface–atmosphere system (Coakley and Chylek, 1975). Aerosol particles serve as cloud condensation nuclei (CCN), which can enhance cloud albedo by increasing the concentration of cloud droplets and reducing their size when aerosol concentrations increase at fixed liquid water content (Twomey, 1974), and potentially drive changes in cloud condensate or cloud cover by altering cloud lifetimes (Simpson and Wiggert, 1969; Albrecht, 1989; Ackerman et al, 2004; Wood, 2007).
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