Synthesis of Aerosol-Cloud Interactions Over the Southeast Atlantic Throughout the Seasonal Cycle

Abstract

Climate impacts from aerosols remain the largest uncertainty in climate change projections (global mean effective forcing at -0.5 +/- 0.4 W m-2, with the forcing from aerosol-cloud interactions estimated at 0 to -0.9 W/m-2). Biomass burning aerosols can exert either a positive or a negative radiative forcing, and the uncertainty in both their direct radiative forcing and aerosol-cloud interactions is most apparent over the southeast Atlantic. The global majority of the shortwave-absorbing aerosols located above low clouds occur over the southeast Atlantic, with regional model and satellite estimates of the direct, indirect, and semi-direct aerosol-cloud-radiation effects uncertain. The Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign gathered ARM Mobile Facility 1 data sets from July 1, 2016 through October 31, 2017 over Ascension Island in the remote southeast Atlantic. Ascension Island (14W, 8S) lies within the trade-wind cumulus regime, underneath the main outflow zone of biomass-burning aerosols from continental African fires. LASIC was complemented by the UK  CLoud-Aerosol-Radiation Interaction and Forcing: Year-2017 (CLARIFY) aircraft campaign, also based on Ascension, and the NASA multi-year ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) aircraft campaign, both with similar goals as LASIC.

This proposal builds on a currently-ending grant that focused on understanding the low cloud response to the presence of the shortwave-absorbing aerosol. The current grant has highlighted the important new finding that biomass-burning aerosol can be common in the southeast Atlantic boundary layer, documented cloud-relevant aerosol properties at Ascension (e.g., cloud condensation nuclei and single-scattering-albedo), and is assessing how low cloud behavior is altered by the presence of the biomass burning aerosol. Relevant new findings include discovering that shortwave absorption within the boundary layer in August can significantly reduce low cloud fraction, but also alters its diurnal cycle, so that the low cloud fraction is a maximum in the morning after sunrise, when a radiatively-driven ascent helps support cloud development and stratiform outflow near the inversion. Ongoing work is assessing how the dominant aerosol-cloud interactions change during the biomass-burning season, with one focus being the diurnal cycle.
This work will deepen that analysis, and in particular clarify how the large-scale meteorology establishes the context for the seasonal evolution in the aerosol-cloud interactions.
Several aspects of this relationship will be probed in depth. The common occurrence of thin clear-air gaps between the aerosol and cloud layers is held to be important, in that the gaps prevent microphysical aerosol-cloud mixing, and, discourage a free-tropospheric stabilization of the cloud top inversion. One hypothesis we will investigate is that advection of a boundary layer from the southern oceans can explain the existence of clear-air gaps, as opposed to free-tropospheric subsidence. A switch in the large-scale advection near cloud top, at approximately 800 hPa, from aerosol-laden continental air to aerosol-free air from the south-southeast, is hypothesized to represent the primary synoptic influence on subsequent aerosol-cloud interactions during the biomass-burning months. This proposal will also support analysis of aerosol-cloud interaction behavior in the Southern Hemisphere fall (Jan-March), when dust from North Africa, mixed with smoke, interacts with cumuli of shallow and intermediate depth less likely to possess a stratiform outflow layer.
This proposal continues support for Zuidema in the role of LASIC lead scientist and as such helps support further synthesizing activities with other southeast Atlantic fieldwork activities.