Study Clouds Link Between Pollution and Rain

A team of scientists has looked at clouds from both sides now and found more bad news about air pollution and global warming.

For the past decade, some scientists have thought that small air-polluting particles produced by the burning of fossil fuels and then sucked into clouds acted as seeds for new cloud particles, plumping up polluted clouds with numerous and smaller cloud particles. Smaller cloud droplets are less efficient at producing rain, and the thinking was that less precipitation would yield thick balls of clouds that reflect more sunlight away from Earth.

The cooling effect was thought to be strong enough to cancel the contribution of atmospheric warming from increased carbon dioxide in the atmosphere. Carbon dioxide is the major greenhouse gas produced by human consumption of fossil fuels, along with methane, nitrous oxide, and other carbon gases.

Ship tracks, the long lines of clouds downwind of ships, provided a nice opportunity to study the effect of exhaust particles, called aerosols, on clouds. However, pesky data turned up.

Sometimes scientists measured more water and thicker clouds in the ship tracks. More often, less water was measured in the tracks, just the opposite of what was expected.

The new study involved creating a detailed three-dimensional model for the interaction of air pollution with clouds and found that the mitigating effect of pollution only works when the air above clouds is humid. The model was tested against real data from the field and found to be very realistic.

"Our findings indeed indicate that aerosol pollution will not save us from greenhouse warming to the extent that has been widely thought by the general climate community," said Andrew Ackerman of NASA's Ames Research Center. A research paper on the results by Ackerman and colleagues at the University of Tasmania, Lawrence Livermore National Laboratory, and the University of Colorado, Boulder, was published in the Dec. 23 issue of the journal Nature.

The team's approach involved a model with 45 ingredients or variables at each of a half million grid points to represent a cloud space spanning four square miles and one mile thick, solved to predict the interactions of water droplets in clouds. The approach was so complex that mathematical solutions required three days of running simulations divided into smaller pieces simultaneously on 32 computers at once, a process known as cluster computing or parallel processing.

A big surprise came when the model, in one case, cranked out a relatively dry cloud despite an increase in aerosols, or air pollution. No precipitation was falling from the cloud, as well. That's when the team noticed that the air above the clouds in this strange case was much drier than in the other meteorological cases.

"We hypothesized that the dry air above the boundary layer was reducing the precipitation, and thus leading to the unexpected behavior," Ackerman told LiveScience.

Sure enough, by drying out air above the simulated cloud layer, the team was able to reverse the response of cloud water to pollution in their models. The dry air resulted in less drizzle from the cloud and more rapid drying as it sucked in more air from above.

Ackerman and his team also learned something about "non-precipitating clouds." It can be misleading when studying the effects of pollution on clouds to ignore small amounts of precipitation locked in clouds that do not rain. Actually, the movement of cloud droplets slowly falling within clouds can be the subtle driver of the process that results in relatively dried out, polluted clouds that are less effective at offsetting global warming.