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Atmospher Sci & Global Chg
Research Highlights

October 2011

The Birth of a Cloud Droplet

Comparing simulations of how atmospheric particles become clouds

satellite image
A satellite image of the North Pacific Ocean shows the horizontal tracks of ships crossing the ocean, emitting exhaust particles that increase cloud brightness. These bright clouds have a cooling effect by reflecting the sun’s energy away from Earth. New research from PNNL helps understand methods to capture information about increased cloud brightness in climate models. Image courtesy of the National Aeronautics and Space Administration Earth Observatory and obtained from the NASA Terra satellite. Enlarge Image

Results: Wrapped in mystery, the formation of a cloud droplet comes down to physics. Pacific Northwest National Laboratory led a research team that has helped peel away another layer of the cloud droplet conundrum. The team, led by Dr. Steven Ghan, evaluated several popular computational methods that describe how tiny particles in the atmosphere serve as seeds for cloud droplets. Their state-of-the-science study found all methods performed well under most conditions, but the more complex treatments performed well under a wider range of conditions.

Why it matters: Emissions of aerosols, those tiny bits of smoke, pollution, and chemicals in the atmosphere, have changed substantially over the last 200 years as a result of burning forests, coal, and other organic materials. The changes in aerosol concentration in the atmosphere affect clouds and the energy balance of the Earth, but the complexity of this influence is a large area of uncertainty for climate change prediction. At the root of this aerosol effect is how aerosols turn into cloud droplets, a process called droplet nucleation, and how they brighten clouds. Brighter clouds scatter sunlight and have a cooling effect on the Earth. Cloud droplet size and number also affects rainfall; the more droplets, the smaller they become and the less likely they are to fall out of clouds as rain. Understanding the cooling effect of brighter clouds, and the effect on rainfall, are two important parts of the climate picture.

Methods: Led by Dr. Steven Ghan, a team compared and contrasted the assumptions used in developing the most popular cloud drop nucleation parameterizations, and compared them under a variety of conditions. All imaginable conditions, such as different updrafts, amounts and sizes of aerosol and cloud conditions were used. They compared these methods against a detailed model estimate to see how they measured up against the benchmark simulations. They also compared global estimates of aerosol effects on the Earth's climate using two of the parameterizations.

Their conclusions and recommendations were published in the Journal of Advances in Modeling of Earth Systems.

What's next: The methods reviewed in this study focused on droplet formation at cloud base, the lowest altitude of a cloud. The team is now exploring methods to represent droplet formation inside clouds, where updrafts are stronger but droplet surface area, which affects the transfer of water vapor to the particles, is larger than at cloud base.

Acknowledgments: The research was funded by the U.S. Department of Energy Atmospheric Systems Research program and performed by Drs. Steven J. Ghan, Xiaohong Liu, and Mikhail Ovchinnikov of PNNL; Dr. Hayder Abdul-Razzak of Texas A&M University-Kingsville; Dr. Athanasios Nenes of Georgia Institute of Technology; Dr. Yi Ming of the Geophysical Fluid Dynamics Laboratory; Dr. Ben Shipway of United Kingdom Meteorology Office; Dr. Nicholas Meskhidze of North Carolina State University; Dr. Jun Xu of the Chinese Research Academy of Environmental Sciences; and Xiangjun Shi of the Institute of Atmospheric Physics Chinese Academy of Sciences.

Additional support was supplied by the National Aeronautics and Space Administration Atmospheric Composition Modeling and Analysis Program (ACMAP) and the National Science Foundation Faculty Early Career Development (CAREER).

Reference: Ghan SJ, H Abdul-Razzak, A Nenes, Y Ming, X Liu, M Ovchinnikov, B Shipway, N Meskhidze, J Xu, and X Shi. 2011. "Droplet Nucleation: Physically-Based Parameterizations and Comparative Evaluation." Journal of Advances in Modeling of Earth Systems, 3:M10001. DOI: 10.1029/2011MS000074.

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Parameterizations in climate models?

Why do scientists use parameterizations in climate models? The genesis of a cloud droplet is a complex process and very difficult to portray over the whole Earth for the multi-century simulations scientists use in climate models. Aerosols and cloud processes vary on much smaller time and space scales than climate models can simulate. Thus, parameterizations, efficient ways to represent complex processes in climate models, have been constructed to capture the essence of cloud physics at a much lower computational cost. By quickly estimating the birth of a cloud droplet, scientists can use climate models to quantify how aerosol emissions affect clouds and climate on regional and global scales.