Send in the clouds

Clouds remain a weak point in scientists' ability to forecast climate

During the summer of 1976, a dozen years before climate change burst on the scene as a hot-button issue, climate scientist Bruce Wielicki saw the future, and it was in clouds.

A grad student at the time, Dr. Wielicki was summering at the Woods Hole Oceanographic Institution on Cape Cod, taking part in a 12-week brainstorming session on climate.

At one point, he recalls, he teamed up with a climate modeler studying how the Earth and atmosphere balance the energy they receive from the sun. Wielicki had been working on a simple computer model to simulate the effect changing sea-surface temperatures have on climate, including cloudiness. The two decided to see what would happen if they linked their models.

"I added clouds to his model, and of course, they blew his results right up," Wielicki says, laughing. "He was sure I'd messed up something in my calculations. So we spent the summer figuring out that, no, my calculations were right and clouds were that powerful."

"That was 25 years ago," he says, "and we still haven't figured clouds out."

Indeed, Joni Mitchell's '60s-era lyrics still resonate with climate scientists today as they probe the mysteries of clouds and their impact on Earth's climate system.

"We've made a lot of progress" in understanding and modeling clouds and their impact on climate, acknowledges Jeffrey Kiehl, who heads the climate modeling section at the National Center for Atmospheric Research in Boulder. Colo. "But in the last five years, progress has flattened out in trying to sort out cloud processes." Clouds are still a weak point, he says, in reducing the uncertainties in climate forecasts.

Small clouds hard to gauge

Some of the challenges, he notes, lie in the nature of modeling itself. Many of the key processes that take place in clouds occur on scales so small that large climate models can't see them. As knowledge about these small-scale processes grows, the potential to include them in models exists, researchers say, but to do so would require an enormous increase in computing power. As an alternative, researchers are using smaller-scale models that can accurately capture features, important to events such as thunderhead formation, and then using those results as reality checks for how larger-scale models treat such phenomena.

Challenges also lie in understanding basic cloud processes, such as factors that influence where a cloud's base forms or what determines how high clouds get, Dr. Kiehl says. "These are critical to understanding whether clouds present a positive or negative feedback to the system" - essentially whether they act to heat or cool the planet.

A thick layer of stratus clouds, for example, which can range from fog at ground level to cloud masses a few hundred feet above the surface, typically moderates daytime temperatures by reflecting most of the sunlight striking them back into space.

High-flying cirrus clouds, on the other hand, can be thin enough to let sunlight through while trapping heat that rises from Earth's surface. Researchers note that when the sun is low on the horizon, sunlight can strike the underside of high-latitude cirrus layers, which reflect that radiation back toward Earth.

By some estimates, a 50 percent increase in cirrus-cloud cover could warm the climate much more than a 50 percent increase in atmospheric carbon dioxide.

In other cases, the challenge lies in more subtle interactions between clouds and other constituents of the atmosphere.

Enough progress has been made on basic cloud processes, that to continue to focus on them "may be looking under the streetlight for the lost keys," says James Hansen, a climate researcher at the National Aeronautics and Space Administration's Goddard Institute for Space Science in New York. He holds that the "real uncertainty lies in the effect aerosols have on clouds."

Aerosols are the wild card

Aerosols are tiny particles of material such as sulfates, dust, sea salt, and soot, and often they are tied to industrial pollution. During the late 1980s and early '90s, an appreciation for their direct effect on climate grew as scientists came to understand that aerosols can reflect incoming solar radiation back into space. Moreover, scientists had long known that aerosols act as seeds around which cloud droplets can form. By increasing droplet size and reducing precipitation, aerosols can help clouds retain their moisture, boosting a cloud's ability to reflect sunlight.

But recent studies have shown that aerosols and soot can have the opposite effect as well. An ambitious international experiment in 1998 and 1999 combined data from aircraft, satellites, ships, and ground stations to track the effect of aerosols and soot on cloud formation over the Indian Ocean.

Dubbed INDOEX, the experiment showed how soot particles can absorb sunlight and re-radiate it as heat. That added heat can raise air temperatures sufficiently to burn off nascent clouds before they fully form.

In addition, says Wielicki, an atmospheric scientist at NASA's Langley Research Center in Hampton, Va., the study also showed that when a cloud moves into a thick layer of soot, the soot within the cloud joins forces with the cloud's water droplets to trap heat, changing the distribution of heat in the atmosphere's boundary layer, which in turn affects cloud formation.

Beyond the impact of aerosols lies a need to get a better handle on tropical clouds, particularly tropical cirrus clouds, researchers say. Indeed, this class of cloud represents a priority for cloud research, notes Bruce Albrecht, a professor of meteorology and physical oceanography at the University of Miami's Rosenstiel School of Marine and Atmospheric Science in Coral Gables, Fla.

Tropical cirrus clouds typically form as thunderheads, transferring heat from the ocean to the atmosphere, and grow to heights of 12 kilometers (7 miles) or more. At those heights, high-altitude winds shear their tops into an anvil shape, then extend them until they become cirrus clouds. Their water droplets will have turned to ice crystals.

"These layers are relatively thin, sometimes so thin you can't see them from the ground. But they are still important" to the Earth's radiation budget, Dr. Albrecht says.

Natural thermostat?

How they respond to changing climate, however, has been the subject of debate for years. Some researchers have proposed that processes in the tropics may regulate cirrus formation in a way that serves as a natural thermostat.

A paper published earlier this year in the Bulletin of the American Meteorological Society, for example, proposes that when temperatures rise in cloudy regions of the tropical Pacific, cirrus formation drops off, allowing more heat to escape into space. The paper's lead author, Massachusetts Institute of Technology atmospheric scientist Richard Lindzen, and his colleagues suggest this as a plausible explanation for trends in cloud formation and temperatures they saw in data from satellites and buoys. They have dubbed it "the iris effect."

Others, however, suggest that newer data may indicate that the region does not exhibit a self-regulating mechanism.

Researchers hope to get a better handle on this and other issues through NASA-funded experiments scheduled for the next few years. In 2002, researchers are slated to study tropical cirrus formation from thunderheads over southern Florida, in a project dubbed CRYSTAL-FACE. That effort also is designed to serve as a warm-up to another study that will focus on the Western Pacific.

I've looked at clouds from both sides now,

From up and down and still somehow

It's cloud's illusions I recall,

I really don't know clouds at all.

- Joni Mitchell, singer-songwriter

(c) Copyright 2001. The Christian Science Monitor

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