Entrainment and Mixing of Biomass Burning Aerosol into the Namibian Stratocumulus Cloud Deck

Interactions between the seasonal biomass burning (BB) smoke plume and the semi-permanent stratocumulus cloud deck in the southeast Atlantic Ocean (SEA) remain poorly understood due to lack of direct observations, despite the importance of this region for the global energy budget. In particular, the extent to which BB aerosol is entrained and mixed into the cloud deck is poorly constrained. Although CALIOP lidar data shows that the smoke layer is clearly separated from the cloud deck near the Namibian/Angolan coast and subsides as it moves westward, the lidar may underestimate the geometric extent of the smoke layer in cases of strong attenuation. As a result, the longitude at which mixing first occurs is uncertain. Large Eddy Simulation (LES) studies suggest that the net radiative forcing of BB aerosol above cloud is negative, in large part due to the Twomey indirect effect. Thus, the extent of BB aerosol mixing is of prime climatic importance in this region.

NASA’s ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) field campaign will investigate multiple facets of the aerosol-cloud-marine boundary layer system in the SEA between August 2016 and October 2018. We present evidence from the campaign and from A-train satellite sensors to constrain the longitude of first mixing and identify a seasonal cycle in the extent of aerosol mixing. In addition, we assess the sensitivity of the CALIOP lidar to smoke in this region through radiative transfer calculations and comparison to field observations.