Interactions: Smoke and Clouds above the Southeast Atlantic Upcoming Field Campaigns Probe Absorbing Aerosol’s Impact on Climate

From July through October, smoke from biomassburning (BB) fires on the southern African subcontinent is transported westward through the free troposphere over one of the largest stratocumulus cloud decks on our planet (Fig. 1). BB aerosol (smoke) absorbs shortwave radiation efficiently. This fundamental property implicates smoke within myriad small-scale processes with potential large-scale impacts on climate that are not yet well understood. A coordinated, international team of scientists from the United States, United Kingdom, France, South Africa, and Namibia will provide an unprecedented interrogation of this smoke-and-cloud regime from 2016 to 2018, using multiple aircraft and surface-based instrumentation suites to span much of the breadth of the southeast Atlantic. The scientific motivations are many. Smoke warms the atmosphere, in contrast to the climate cooling provided by the reflected sunlight from the extensive low clouds residing mostly below the smoke layer. Yet the low clouds also respond to the presence of the smoke in counterintuitive ways that can either strengthen or weaken the low cloud deck. Smoke can stabilize the atmospheric temperature profile by warming the free troposphere and cooling the surface below. The stabilization strengthens the low cloud deck, so that the net smoke-plus-cloud effect is an enhanced cooling. This effect is thought to dominate the low-cloud response, because space-based lidar informs us that much of the BB aerosol resides above the cloud deck (Fig. 1). In contrast, if the smoke mixes directly into the cloud layer, warming provided by the smoke could reduce the relative humidity and help dissipate the cloud. Changes in the amount of aerosol nucleating the clouds also alter the cloud microphysics and the clouds' likelihood of producing rain. Other effects exist, for example, from the moisture associated with the aerosol layer, while further effects may still remain to be discovered. At a larger scale, the change in atmospheric warming from the smoke affects the neighboring precipitation distribution. The smoke’s influence on the surface energy budget ultimately affects the equatorial climate and its variability through the trade winds, and changes the energy distribution between the Northern and Southern Hemisphere.