Boundary Layer Clouds Characteristics above Ascension during Biomass Burning Seasons inferred from LASIC Field Campaign Observations

The areal extent of the absorbing aerosol above low clouds in the Southeast Atlantic is large enough for the coupled cloud-aerosol-ocean system to have a regional climate impact, but the cloud adjustments to the presence of the aerosol are still poorly understood. Cloud responses to the aerosol presence can be both radiative and microphysical, depending on the relative location of the aerosol to the cloud. This study focuses on observations collected by DOE ARM Mobile Facility during the Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign from June 2016 to October 2017, at Ascension Island (8°S, 14°W), located ~2,000 km offshore of continental Africa in the trade-wind regime. Micropulse-lidar-derived extinction profiles suggest that aerosol is almost always present in the atmospheric column during July-October, if in varying amounts and within multiple layers. The subsiding aerosol layer and deepening boundary layer over Ascension favor the mixing of smoke into the cloud layer, although this can only be indirectly determined from the lidar measurements. A two-layer cloud structure is representative of a boundary layer that is usually decoupled. The July-August observations reveal that, when absorbing aerosol is present in the cloudy boundary layer, the diurnal cycle of the potential temperature vertical profile is more prominent, the boundary layer is deeper and more well-mixed, the cloud top inversion is weaker, and the atmosphere is less stable, despite the presence of absorbing aerosol aloft. The weaker inversion under smoky conditions co-occurs with higher cloud bases at both levels. The near-surface cloud condensation nuclei concentration increase, correlating well with black carbon mass concentration. Drizzle suppression and reduced drop sizes near the cloud base are also apparent in the radar reflectivity composites under smoky conditions. The net impact of these effects upon the overall cloud fraction will be determined, and aerosol effects will be distinguished from meteorology using a sensitivity analysis based on large-eddy-scale simulations.