Atmospheric Warming Impacted By Pollution, Thunderclouds
May 21, 2012

Atmospheric Warming Impacted By Pollution, Thunderclouds

Pollution and summer thunderstorm clouds are an adverse combination when it comes to climate change, as a new computer simulation has shown that the two seemingly unrelated entities are combining to warm the atmosphere.

The findings, which were announced by the Department of Energy's Pacific Northwest National Laboratory (PNNL) Friday and published in the May 10 edition of the journal Geophysical Research Letters, were the result of simulations conducted by PNNL climate researcher Jiwen Fan and colleagues from the Hebrew University of Jerusalem and the University of Maryland.

Fan, the lead author of the study, and associates discovered that pollution strengthens thunderstorm clouds, and as a result, their anvil-shaped tops spread out high in the atmosphere and capture heat, especially during the nighttime hours. How much this effect counters the cooling effects provided by other clouds is not yet known, and the researchers will have to incorporate this newly-discovered phenomenon into global climate models in order to find out, the PNNL said.

"Global climate models don't see this effect because thunderstorm clouds simulated in those models do not include enough detail," Fan said in a statement. "The large amount of heat trapped by the pollution-enhanced clouds could potentially impact regional circulation and modify weather systems."

Experts had previously established that pollution can lead to larger clouds under non-windy conditions, because a greater number of pollution particles divide up the water available for droplets. This leads to a greater number of smaller droplets which are too small to rain, and instead are caught up in updrafts, and rise until they freeze and absorb more water vapor, thus leading to "bigger, more vigorous convective clouds that live longer," they said.

To determine which factors contribute the most to this process, the research team created computerized simulations for two different types of storm systems -- warm summer thunderstorms in southeastern China and cool, windy frontal systems on the Great Plains of Oklahoma. They used high resolution imaging in order to watch the development of the clouds, then varied conditions such as wind speed and amount of air pollution.

"Fan and colleagues found that for the warm summer thunderstorms, pollution led to stronger storms with larger anvils," the press release said. "Compared to the cloud anvils that developed in clean air, the larger anvils both warmed more -- by trapping more heat -- and cooled more -- by reflecting additional sunlight back to space. On average, however, the warming effect dominated."

"This is the first time researchers showed that pollution increased warming by enlarging thunderstorm clouds," it added. "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."