First Results from Investigations of Interactions Between Biomass-Burning Aerosols and Low Clouds on Ascension Island in the Remote South Atlantic

Approximately one-half of the world’s biomass-burning aerosols originate from Africa every year, with the global majority of the shortwave-absorbing aerosols overlying the low clouds of 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, is an excellent location for assessing such aerosol-cloud characteristics towards improving our understanding of this system. The island 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. 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 a preliminary analysis focused on the LASIC August-September 2016 Intensive Observing Period. The emphasis is on characterizing the shortwave-absorption of the aerosol, the relative aerosol-cloud vertical structure, and the low cloud behavior including its diurnal cycle, and will aim to set the stage for how to make further progress addressing the relevant observational grand challenges.