Send in the 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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