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Atmospher Sci & Global Chg
Research Highlights

August 2010

Clouds + Mineral Dust = Rain

Researchers size up ice nuclei to more accurately model climate change

Results: A team of atmospheric scientists, including Dr. Xiaohong Liu of Pacific Northwest National Laboratory (PNNL), found a critical link between the size of dust particles in clouds and their likelihood to produce rain. The number of dust particles larger than 0.5 µm in diameter had a close correlation with the number of ice nuclei, which are known to form seeds for rain droplets in clouds. This relationship allowed the team to develop a much more accurate parameterization to predict ice nuclei numbers in clouds, reducing uncertainty by a factor of one hundred.

Why it matters:  Results from this study provide atmospheric researchers a more accurate picture of how ice nuclei form and at what temperature, so they can increase accuracy of climate models.  Past predications were based on an assumption of ten times more ice nuclei present in clouds. This breakthrough follows a 14-year study from field studies at locations around the globe. This research was published in the Proceedings of the National Academy of Sciences.

Methods: The team used ice nuclei and aerosol datasets collected in nine separate studies, including field and aircraft samples. Data was gathered from areas around the globe, from the Amazon Rainforest in Brazil to the Arctic Circle.  Ice nuclei number concentration data came from the Colorado State University Continuous Flow Diffusion Chamber (CFDC).  During aircraft measurements, the CFDC either sampled from an ambient inlet or from the outlet of a counterflow virtual impactor that passed only cloud particle residual aerosols to the CFDC.

ice nuclei
Schematic diagram of the effect of ice nuclei (IN) from various possible particle emissions on midlevel precipitating clouds and cirrus ice clouds.

What's next:  This new relationship is simple to apply and allows researchers to update climate models for more accuracy. Much work needs to be done in tandem with biologists to determine number and sources of these dust particles as a function of season and soil type, and on airborne particles acting as ice nuclei.  The newly developed Atmospheric Measurement Laboratory, housed at PNNL, will provide an ice nucleation chamber to assist in making artificial clouds that activate particles under precisely controlled temperature and supersaturation conditions and separating these particles from the rest of the aerosol to study ice nucleation.

Acknowledgments: This work was funded by DOE's Office of Biological & Environmental Research; Atmospheric System Research Grant; NASA's Modeling and Analysis Program; and National Science Foundation. The research team was made up of scientists from Colorado State University, Pacific Northwest National Laboratory, North Carolina State University, Oregon State University, University of Colorado and National Center for Atmospheric Research.

Reference: DeMott PJ, AJ Prenni, X Liu, SM Kreidenweis, MD Petters, CH Twohy, MS Richardson, T Eidhammer,and DC Rogers. 2010. "Predicting Global Atmospheric Ice Nuclei Distributions and Their Impacts on Climate," Proceedings of the National Academy of Sciences of the United States of America 107 (25): 11217-11222. DOI: 10.1073/pnas.0910818107

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