Investigations of Interactions between Biomass-Burning Aerosols and Low Clouds: First Results from Ascension Island in the Remote South Atlantic (Invited Presentation)

Almost one-third of the world’s biomass-burning aerosols originate from Africa every year, providing the global majority of the shortwave-absorbing aerosols overlying low clouds, primarily over the southeast Atlantic. These aerosols can provide a climate warming, but coincident thickening and increase in the low cloud deck can overcompensate, so that the net coupled-aerosol-cloud climate effect is a cooling, along with a large-scale vertical redistribution of the diabatic heating. Much uncertainty still exists on how to best represent this coupled system within larger-scale models. Ascension Island, a remote, near-equatorial island midway between Africa and South America, lies within the trade-wind cumulus regime, underneath the main outflow zone of biomass-burning aerosols capable of reaching south America from continental African fires. Trace gas measurements have been made previously at Ascension, but no aerosol composition or cloud measurements. The first-ever systematic surface-based measurements of the aerosol and cloud properties were taken on Ascension Island beginning in July of 2016, as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) deployment of a DOE ARM Mobile Facility. Early datasets clearly indicate the presence of smoke within the cloudy boundary layer, demonstrating the potential for aerosol-cloud microphysical interactions. The NASA Observations of Aerosols above Clouds and their Interactions (ORACLES) campaign further connects the island measurements with in-situ and remote-sensing measurements made over the remote southeast Atlantic from the NASA P-3 and ER-2 aircraft. The DOE LASIC and NASA ORACLES measurements will ultimately span the full annual cycle and sample several years, respectively. This presentation will discuss the relevant observational grand challenges, emphasizing the characterization of the shortwave-absorption of the aerosol, the relative aerosol-cloud vertical structure, and the low cloud response to the presence of the aerosol. In so doing we will aim to set the stage for how to make further progress towards improving our understanding of this climate regime.