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Atmospheric Sciences & Global Change
Research Highlights

July 2011

Connecting the Dots on Aerosol Details

Multi-scale model calculates human-caused aerosol indirect effects

Energy Budget
Pollution aerosols and their effect on the Earth’s energy budget are part of the picture that climate scientists are focusing on in multi-scale climate modeling. Enlarge Image
The cloud response to aerosols shows only one-third as strong as it did in previous modeling in the study using the PNNL-MMF. Scatter plots show changes in liquid water path response (LWP) versus cloud condensation nuclei (CCN) perturbation from anthropogenic aerosols in a) MMF (new model), and b) CAM5 (previous model). Anthropogenic, or human-caused aerosols are primarily the result of fossil fuel burning. Enlarge Image

Results: Predicting future climate change hangs on understanding aerosols, considered the fine details in the atmosphere. Researchers at Pacific Northwest National Laboratory and the National Center for Atmospheric Research used a new modeling tool to bring the picture of aerosols and their actions on clouds into sharper focus. The multi-scale aerosol-climate model, an extension of a multi-scale modeling framework, examined specific aerosol-cloud interactions and their effects on the Earth's energy budget, one of the toughest climate forecasting problems. Their results show that the cooling effect of human-caused atmospheric aerosols is smaller than previously thought.

Why it matters: Current global climate models used to predict climate change account for large-scale climate processes, typically at scales greater than 100 kilometers, or about 62 miles. This means small-scale and regional features in the climate tend to be averaged out, or estimated through parameterization, a technique used to represent complex small-scale systems. Because small-scale climate features, such as clouds and atmospheric aerosol particles, have a large impact on global climate, it's important to improve the methods used to represent those climate features in the models. This study has advanced scientists' capabilities to model and predict those complex aerosol-cloud interactions on the Earth's energy budget, for a balanced and energy-sustainable future.

Methods: Scientists at PNNL developed a new aerosol-climate model as an extension of a multi-scale modeling framework model that embeds a cloud-resolving model (CRM) within each grid column of a global climate model. This model, called the PNNL-Multi-scale Modeling Framework, depicts aerosol-cloud interactions in both stratiform and convective clouds in a more realistic way than conventional global models. In addition, the PNNL-MMF is much more computationally feasible for running multi-year climate simulations than a global CRM.

The team evaluated simulated cloud fields from the multi-scale aerosol-climate model and examined how specific human-caused aerosols, such as sulfate, black carbon (soot), and organic carbon affect those clouds and, in turn, the climate. The PNNL-MMF is a more physically based way to represent the indirect effects of aerosols compared to parameterization, typically used to represent small-scale climate details in global models. The significant computational resources available to the team at the National Center for Computational Sciences at Oak Ridge National Laboratory enabled this improvement in climate modeling.

The study compared pre-industrial and present-day results in current global models to the newer high-resolution model with the PNNL-MMF extension. Comparisons show a lesser effect on the Earth's energy budget, considering the additional burden of human-caused aerosols. These results confirm the need to use global high resolution models to study the aerosol indirect effects.

What's next: The researchers are working to understand the differences found between global models and the more detailed PNNL-MMF results. In future work, the team will tackle the aerosol effects on precipitation.

Acknowledgments: This research was funded by the National Aeronautics and Space Administration Interdisciplinary Science Program and the U.S. Department of Energy Office of Science, Atmospheric System Research program and Scientific Discovery through Advance Computing program. Additional support to the team was through the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a DOE scientific user facility, the National Center for Computational Sciences at ORNL, and the National Science Foundation, Science and Technology Center for Multiscale Modeling of Atmospheric Processes, managed by Colorado State University. 

Reference: Wang M, S Ghan, M Ovchinnikov, X Liu, R Easter, E Kassianov, Y Qian, and H Morrison. "Aerosol Indirect Effects in a Multi-scale Aerosol-climate Model PNNL-MMF." 2011. Atmospheric Chemistry and Physics, 11, 5431-5455. DOI: 10.5194/acp-11-5431-2011.

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What is the Earth's energy budget?

Looking at the Earth's energy system like a bank account, gains and losses of energy should generally maintain a balance. Incoming energy, which comes primarily from the sun, is turned into various forms of absorbed energy, depending on terrain and atmospheric conditions such as clouds and aerosol particles. Losses leave primarily through reflected energy, which is sent back into space, and evaporation of water. When aerosols from human activities such as industrial plant and vehicle emissions are added to the system, the energy budget has to deal with the increase. Scientists are working to understand how these aerosols affect the Earth's energy budget.