01 June 2012

Pollution + Storm Clouds = Warmer Atmosphere

Science

For the first time, U.S. Department of Energy (DOE) researchers at Pacific Northwest National Laboratory (PNNL) have shown that pollution increases warming in the atmosphere through enlarging thunderstorm clouds. Pollution strengthens thunderstorm clouds, causing their anvil-shaped tops to spread out high in the atmosphere and capture heat—especially at night.

Approach

The scientists conducted a computational study with resolutions high enough to allow the team to see the clouds develop. They found that for warm summer thunderstorms, pollution particles lead to stronger storms with larger, anvil-shaped clouds that also last longer. The warming effect dominated by trapping more heat, especially at night, even though the larger clouds also reflected more sunlight warmth back into space during the daytime.

Previous work showed that when it's not too windy, pollution leads to bigger clouds. This occurs because more pollution particles divide up the available water for droplets, leading to a higher number of smaller droplets that are too small to rain. Instead of raining, the small droplets ride the updrafts higher, where they freeze and absorb more water vapor. Collectively, these events lead to bigger, more vigorous convective clouds that live longer. DOE researchers from PNNL, and scientists from Hebrew University in Jerusalem and the University of Maryland took to high-performance computing to study the invigoration effect on a regional scale. The simulations had a high enough resolution to allow the team to see the clouds develop. The researchers then varied conditions such as wind speed and air pollution. This is the first time researchers showed pollution increased warming by enlarging thunderstorm clouds. The warming was surprisingly strong at the top of the atmosphere during the day, when the storms occurred. The pollution-enhanced anvils also trapped more heat at night, leading to warmer nights.

Impact

Clouds are one of the most poorly understood components of the Earth’s climate system. The large amount of heat trapped by the pollution-enhanced clouds has the potential to impact regional circulation and modify weather systems. Current global climate models don’t represent these aerosol effects. Getting a better understanding of clouds and how pollution and other particles affect their size and influence on the climate is important to better predict the future of climate change. The research was supported by the U.S. Department of Energy Office of Science. The data from China were gathered under a bilateral agreement with the China Ministry of Sciences and Technology.

Summary

For the first time, DOE researchers at PNNL have shown pollution increases warming in the atmosphere through enlarging thunderstorm clouds. The scientists conducted a computational study with resolutions high enough to allow the team to see the clouds develop. For warm summer thunderstorms, they found pollution particles lead to stronger storms with larger, anvil-shaped clouds that also last longer. The warming effect dominated by trapping more heat, especially at night, even though the larger clouds also reflected more sunlight warmth back into space during the daytime. Clouds are one of the most poorly understood components of the Earth’s climate system. The large amount of heat trapped by the pollution-enhanced clouds has the potential to impact regional circulation and modify weather systems, and current global climate models don’t represent these aerosol effects. Getting a better understanding of clouds, and how pollution and other particles affect their size and influence on the climate, is important to better predict the future of climate change.
For more, see PNNL press release “Pollution teams with thunderclouds to warm atmosphere,” http://www.pnnl.gov/news/release.aspx?id=925.

Contact
Dilip Ganguly
Acknowledgments

This study was supported by the DOE Office of Science and Biological and Environmental Research (BER) Regional and Global Climate Modeling Program as part of a bilateral agreement with the China Ministry of Sciences and Technology on regional climate research and the U.S. DOE BER’s Atmospheric System Research Program (ASR). The work was performed by Drs. Jiwen Fan and L. Ruby Leung of PNNL; Dr. Daniel Rosenfeld of The Hebrew University of Jerusalem; Dr. Zhanqing Li and Yanni Ding of the University of Maryland.