Cloud scientists plumb some cirrus mysteries

Photos of Earth's atmosphere, taken from space, often serve as icons for the fragile balance of conditions that support life on the planet.

Graeme Stephens points to a slimmer icon - the amount of water in clouds. Stretch that water evenly around the planet, he explains, and it would form a wispy layer less than a tenth of a millimeter thick.

Yet that sliver "is absolutely crucial for life on the planet," says the Colorado State University atmospheric scientist. By helping to generate rain and snow, it "represents the renewable part of the fresh water cycle," and the clouds it forms help regulate how much heat the atmosphere keeps.

Little wonder, then, that for atmospheric science, 2006 could be dubbed the year of the clouds. On April 28, NASA launched two satellites designed to study the structure and processes that govern the rise and fall of various cloud types.

The launch comes on the heels of an intensive, three-week effort earlier this year to measure monsoon thunderheads in the tropics - and particularly the icy cirrus clouds that they form.

Thunderhead-spawned cirrus are ubiquitous over the tropical oceans. They tend to trap heat and outlast their terrestrial counterparts. As a result, they have an enormous effect on weather and climate patterns beyond their immediate neighborhood, researchers say.

But knowledge of the inner workings of clouds and the factors that influence their life cycles is about as skimpy as the tenuous layers they can form.

"For example, we can't tell you how much water in the atmosphere is in the form of ice," laments Dr. Stephens, the lead scientist on CloudSat, one of the two international satellites NASA launched late last month. "We can't even tell you what fraction of clouds that cover our skies produce rain or snow."

Such issues "go right to the heart of major uncertainties left" in estimating temperature increases from human-induced global warming, says Andrew Gettelman, a researcher at the National Center for Atmospheric Research in Boulder, Colo.

CloudSat, and its US-French companion, CALIPSO, are joining three other orbiters in a constellation of atmospheric satellites dubbed "the A-train." Unlike an earthbound train, they don't orbit single file. But they do cross the equator within minutes of each other to develop a comprehensive look at factors that affect weather and climate.

CloudSat and CALIPSO can be steered to cross the same spot within 15 seconds of each other to focus on the same sets of features. CALIPSO carries a laser-based radar to gather information on high-altitude cirrus and on particles in the atmosphere known as aerosols. These can enhance or inhibit cloud formation and precipitation, as well as reduce incoming sunlight on their own. CloudSat's radar can tease out information about a cloud's liquid and ice content, as well as precipitation. It can also pierce high-level cloud layers that have stymied other satellites from seeing what is happening underneath.

One goal is to get a better handle on where inside a cloud water vapor releases its heat as it cools, condenses, and reverts to liquid drops or ice in tropical thunderstorms.

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