Atmospheric Sciences & Global Change
Dust Increases Cloud Cover
Scientists find an unexpected culprit encouraging cloud formation
The satellite image shows western Africa, where dust storms carry particles across the Atlantic Ocean and into the atmosphere. The inset graph (enlarge image) shows the inter-annual correlation between the June-July-August mean dust burden and lower tropospheric cloud fraction anomalies simulated by the model over the tropical/subtropical Atlantic Ocean. Satellite image courtesy of NASA.Enlarge Image.
Results: Surprisingly, cloud cover increases when more dust blows off the west coast of Africa, according to a long global climate simulation run by researchers from the University of California San Diego and Pacific Northwest National Laboratory. They expected that heat radiating off of dust, which absorbs solar energy, would "burn off" the clouds. Instead, the team found more clouds as more dust flows from Africa over the Atlantic Ocean. The Community Earth System Model (CESM) produced the simulations in this research.
"In our simulations, we see that above the clouds, the heat given off by dust makes the atmosphere more stable which, in turn, reduces air mixing between the clouds below and the much drier air above the clouds," said the paper's co-author Dr. Steven J. Ghan, Laboratory Fellow and atmospheric scientist at PNNL. "By reducing that mixing, more of the water evaporating from the ocean can produce clouds."
Why It Matters: Global climate models are necessary to provide a view of the future effects of increased atmospheric carbon dioxide. Distinguishing the climate impact of carbon dioxide from natural variations in the climate becomes important to understand what triggers these natural variations. Shifting wind and precipitation patterns are natural influences that provoke dust lofted in the air from Africa. The impacts of dust on clouds were previously unknown, yet appear to be an important factor in altering the Earth's energy balance for years. This research is part of the effort to describe and understand naturally occurring background climate changes. By simulating the effects of dust over North Africa without the effects of emissions from human activity, such as coal-burning power plants, they can isolate a view of this natural event to understand the full impact of dust as it influences cloud formation.
Methods: The researchers used various observation data such as dust and dust sources, cloud cover and composition, and aerosol optical depth from ground and satellite information for their study. They ran a CESM simulation for 150 years under pre-industrial conditions, i.e., without emissions caused by human activity to view the climate interactions isolated from human influence.
They then examined the cloud-aerosol relationships on a year-to-year timescale. The research found a strong cloud cover enhancement below plumes of dust transported from Africa. They identified the driving mechanism of this increase as a suppression of vertical mixing due to dust absorbing sunlight.
What's Next? The scientists are investigating the climatic signature of natural variations in other aerosol particle types, such as sea salt and wildfire smoke.
Sponsor: Development of the CESM and the research for this paper was partially funded by the U.S. Department of Energy Office of Science Biological and Environmental Research program for the Earth System Modeling Program.
Research Team: Michael J. DeFlorio, Arthur J. Miller, Daniel R. Cayan, Lynn M. Russell, and Richard C. J. Somerville of University of California San Diego; and Steven J. Ghan and Balwinder Singh of PNNL.
Research Area: Climate & Earth Systems Science
Reference: DeFlorio MJ, SJ Ghan, B Singh, AJ Miller, DR Cayan, LM Russell, and RCJ Somerville. 2014. "Semi-Direct Dynamical and Radiative Impact of North African Dust Transport on Lower Tropospheric Clouds over the Subtropical North Atlantic in CESM 1.0." Journal of Geophysical Research 119(13): 8284-8303. DOI: 10.1002/2013JD020997.