Heating rates due to biomass burning aerosol in the southeast Atlantic
Miller, Mark A. — Rutgers University
Collow, Allison — Universities Space Research Association
Area of research:
Broadband Heating Rate Profile (BBHRP)
The remains of old crop fields are burned to ensure a good crop during the next planting season and these seasonal fires produce smoke, which is known as biomass burning aerosol. Seasonal burns in central and south-central Africa produce an extensive plume of biomass burning aerosol that drifts over the southeastern Atlantic, which is home to an extensive cloud system in the lower marine atmosphere known as the boundary layer. Marine boundary-layer clouds and biomass burning aerosols over the southeast Atlantic region play a large role in the global energy budget, yet their properties are poorly represented in atmospheric models. Biomass burning aerosols absorb sunlight, causing a local heating within the atmosphere, and impacts on the temperature elsewhere in the atmospheric column. Using a combination of idealized radiative transfer modeling and observations collected during the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) user facility's Layered Atlantic Smoke Interactions with Clouds (LASIC) field campaign, uncertainties in the aerosol optical properties and aerosol transport are leveraged to give a range of possible heating rates due to biomass burning aerosols and postulate how this may impact marine stratocumulus clouds.
Simulating biomass burning aerosol and the interaction of this aerosol with the underlying marine clouds requires that the amount of sunlight that is scattered or transmitted through the aerosol layer relative to the amount that is being absorbed be accurately represented. Absorption by these overlying aerosols acts to heat the layer above the underlying clouds, which can impact their structure and life cycle. A key variable that must be accurately represented in models is the single scattering albedo because it determines the impact of biomass burning aerosols on the vertical profile of temperature over the southeast Atlantic. Our study demonstrates the sensitivity of the specification of the single scattering albedo over the southeastern Atlantic in models and emphasizes the need for accurate measurements of this aerosol property. The study also demonstrates that the impact of the biomass burning plume on the structure of the lower atmosphere over the southeastern Atlantic depends on the details of the trajectory that it follows as it moves from Africa over the adjacent ocean. Long and circuitous plume trajectories of biomass burning aerosol as it is transported from Africa and across the southeastern Atlantic Ocean enable more time for absorption and heating and, thus, potentially more profound changes in the cloud layer below.
The Rapid Radiative Transfer Model was forced with vertical profiles of temperature, humidity, and cloud observations collected by the ARM Mobile Facility as it was stationed on Ascension Island for the LASIC field campaign over the course of the 2016 and 2017 biomass burning seasons, with analyzed aerosol fields from the MERRA-2 reanalysis. Experiments with and without aerosols, black carbon, and clouds were used to quantify the individual heating from each of the three constituents. Deficiencies in the single scattering albedo in MERRA-2 motivated additional experimentation to determine how the heating rates would change due to the relative humidity and aerosol speciation in MERRA-2. This resulted in a range of heating rates within the aerosol plume of 2-8 K, with roughly 80% of the heating within the plume directly due to black carbon. When clouds are located beneath the aerosol plume, which is common, heating is enhanced due to aerosols. As the biomass aerosol is transported, heating due to the absorption of sunlight compounds, without relief from cooling at night. This indicates that aerosols may play an even larger role in the energy budget by impacting other processes such as the transition of marine stratocumulus clouds to trade cumulus.