06 December 2012

A Shortcut to Aerosol Signatures: Scientists find a way to separate the aerosol signal from the weather noise

Science

Scientists developed a modeling shortcut to dial in a clearer atmospheric particle signal. A research team from the Scripps Institute of Oceanography, the University of Washington, and Pacific Northwest National Laboratory (PNNL) fine-tuned the winds simulated in a global climate model to better represent the winds measured in the atmosphere. Their technique increased the signal’s clarity by greatly reducing the signal noise. Their work produced shorter, more efficient simulations of the global aerosol particle effects on clouds and a better reception of the atmospheric particle signal.

Approach

Aerosol particles produced by human activity are influencing climate change by making more droplets so that clouds can hold more water without raining. Isolating such aerosol effects on clouds from the influence of inherently variable effects of weather systems is fraught with difficulty. The aerosols’ signature is small but persistent, yet it involves the same highly variable quantity. The amount of liquid water in the atmospheric column is strongly influenced by weather systems.

For this study, researchers simulated aerosol effects on clouds using the Community Atmosphere Model, CAM5. They nudged simulations of pre-industrial and present-day aerosols toward the same winds so that the weather systems were similar in both simulations. They also simulated pre-industrial and present-day aerosols without nudging so that the weather systems could evolve freely. In both pairs of simulations, the aerosols’ impact on Earth’s energy balance was estimated by comparing Earth’s energy balance to each pair of pre-industrial and present day aerosol simulations.

Nudging greatly reduced the variations in the column water without significantly changing the sensitivity of the column liquid water to the aerosol, thus permitting global estimates of aerosol effects on clouds in much shorter simulations.

Impact

Although scientists agree that the Earth is warming because of increasing concentrations of greenhouse gases, there are other considerations. Increased levels of atmospheric particles, or aerosols, inside clouds can have a cooling influence. Pollution particles make the clouds brighter and more reflective by both increasing the number of cloud droplets and reducing each droplet’s size. Scientists are working to find other factors that allow polluted clouds to hold more water, which would make them even brighter. Accounting for detailed features in the climate, such as the cooling effect of pollution particles on clouds, will help scientists understand past climate change and improve confidence in predictions of future climate change. This study gives climate researchers a valuable tool for those important predictions.

Summary

A Shortcut to Aerosol Signature: Scientists find a way to separate the aerosol signal from the weather noise

Climate scientists suspect that aerosol particles produced by human activity cool the earth by making clouds brighter. Confirming that theory has been difficult because cloud brightness depends on factors other than aerosols, such as winds. Now, researchers from the Scripps Institute of Oceanography, University of Washington, and DOE scientists from Pacific Northwest National Laboratory showed that by nudging the winds simulated in the Community Atmosphere Model (CAM5) toward the winds measured in the atmosphere, the influence of those other factors is constrained so that aerosol effects on cloud brightness can be identified. This greatly reduced the variations in the column liquid water in clouds without significantly changing the sensitivity of the column liquid water to the aerosol, thus permitting global estimates of aerosol effects on clouds in much shorter simulations. Simulations with pre-industrial and with present day emissions of aerosol and aerosol precursor gases were both nudged toward the same winds so that the weather systems were similar in both simulations. They also performed simulations with pre-industrial and with present day emissions without nudging so that the weather systems could evolve freely. In both pairs of simulations the aerosol impact on Earth’s energy balance was estimated by comparing Earth’s energy balance with each pair of pre-industrial and present day simulations. They found that much shorter simulations were needed when nudging was used. This study gives climate researchers a valuable tool for important climate change predictions to plan for a sustainable future.

Contact
Hsi-Yen Ma
Acknowledgments

Sponsors: This research was funded by the Center for Multiscale Modeling of Atmospheric Processes (a National Science Foundation (NSF) Science and Technology Center), the U.S. Department of Energy’s (DOE) Office of Science, Office of Biological and Environmental Research Earth System Modeling Program, the NOAA Climate and Global Change Postdoctoral Fellowship Program and by the DOE Office of Science and NSF Decadal and Regional Climate Prediction using Earth System Models (EaSM) program.

Research Team: Gabriel Kooperman, Richard Somerville, and Lynn Russell of Scripps Institution of Oceanography; Michael Pritchard of the University of Washington; and Steven Ghan and Minghuai Wang at PNNL.

Reference: Kooperman G J, MS Pritchard, SJ Ghan, RCJ Sommerville, and LM Russell. 2012. “Constraining the Influence of Natural Variability to Improve Estimates of Global Aerosol Indirect Effects in a Nudged Version of the Community Atmosphere Model 5.” Journal of Geophysical Research-Atmospheres. DOI: 10.1029/2012JD018588, in press.