Understanding Semidirect Effect within the Smoky Marine Boundary Layer over the Remote Southeast Atlantic

Ground-based observational data from Ascension Island during August 2016 and 2017 of the DOE LASIC campaign reveal that the presence of smoke within the marine boundary layer modifies the thermodynamical structure as well as the low-cloud characteristics through both radiative and microphysical processes. When smoke is present within the boundary layer, the boundary layer is more likely to deepen within the early sunlit morning followed by afternoon warming in the lowest 500 m. The biomass-burning aerosol within the boundary layer is often associated with smoke in the free-troposphere. The corresponding thermodynamics are entangled with meteorology, in that the above-cloud smoke resides within a mixed-layer of anomalously lower potential temperature. This weakens the cloud-top inversion, encouraging a positive feedback whereby more smoke from above is entrained into the boundary layer. The cloud cover is typically reduced when more smoke is present in the boundary layer. Radiative heating vertical profiles of a smoky marine boundary layer calculated at various solar zenith angles help interpret the observed thermodynamic features. Satellite observations relate the ground-based measurements to top-of-the-atmosphere albedo. Large-eddy-scale simulations of the diurnal cycle will be applied to better discriminate the radiative and microphysical impacts of the smoke, as a function of cloud fraction, of which first results will be presented at the conference.