Dirty Air Fosters Precipitation Extremes

Changes to clouds encourage drought in dry areas and torrential downpours in moist ones

By Janet Raloff Web edition : Sunday, November 13th, 2011

Even clouds can suffer from inhaling air pollution, a new study finds, resulting in extreme rainfall patterns that appear to be altering climate across the globe.

Farmers, municipal water authorities and others who depend on rainfall prefer moderate, dependable precipitation. But as soot and other minute airborne particles — a class of pollutants known as aerosols — get sucked into clouds, the pollution can dramatically alter when clouds deposit rain. The discovery emerged from analyzing every one of thousands of clouds passing over federal monitoring instruments at a site in the western United States over a 10-year period, explains Zhanqing Li of the University of Maryland in College Park.

"Haze, storms, drought and flood: We found very strong evidence that they are well connected," he said in Washington, D.C., on November 10 at the Symposium on Stratospheric Ozone and Climate Change. He and colleagues published the findings online November 13 in Nature Geoscience.

"This is the first study to clearly establish the link between aerosols, precipitation and climate," says Renyi Zhang of Texas A&M University in College Station.

The effects Li and his colleagues saw depended not only on pollution concentrations but also on moisture levels and cloud types. In relatively low-lying clouds where the moisture is liquid in the form of tiny droplets, increasing aerosol concentration tended to suppress precipitation, especially when relative humidity was low.

In clouds, water molecules latch onto aerosols and continue to grow until they collect enough moisture and form rain. But Li and his colleagues found that if there were too many aerosols in a cloud, not much water attached to any individual aerosol. That left water droplets that were too small to fall as rain.

In taller clouds that contained a mix of moisture types — water in the lower regions but ice crystals up high, for example — adding aerosols had the opposite effect. Droplets too tiny to rain out would get carried by strong vertical winds to the top of the cloud. There the droplets froze, releasing the energy — known as latent heat — stored in liquid water.

"That extra energy acts like a stove in the upper atmosphere to fuel the cloud's further vertical growth," Li says. Since water prefers to stick to ice, the frozen cloud particles quickly grew to sizes too heavy to remain aloft. Those particles came down as rain in storms too heavy for the ground to efficiently absorb.

The study also "reveals unprecedented magnitudes of impacts," says coauthor Daniel Rosenfeld of the Hebrew University of Jerusalem. For instance, very dirty conditions can double the heights of clouds compared with those that develop under pristine conditions. "The probability of heavy rain is increased by 50 percent from clean to dirty conditions, whereas the chance of light rain is reduced by 50 percent," Rosenfeld says.

Two years ago, Yun Qian of the Pacific Northwest National Laboratory in Richland, Wash., and his colleagues published data from eastern China indicating that the heavy pollution there was suppressing light rains. As a result of the new study, he says his team now plans to investigate whether China's heavy pollution will similarly affect mixed phase — water and ice — clouds in ways that might foster monster rains.

For years, studies have been showing global warming can foster more extreme weather, notes Veerabhadran Ramanathan of the Scripps Institution of Oceanography at the University of California, San Diego. "What this study is showing is that there may be one more factor causing the same thing — further pushing events in a direction we do not want."

Source: Science News