Cloud scientists plumb some cirrus mysteries

New data may lead to more accurate and long-term weather forecasts.

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 millimetre thick.

Yet that sliver "is 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."

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 neighbourhood, researchers say. But knowledge of the inner workings of clouds 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 in April.

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 Centre for Atmospheric Research in Boulder, Colorado.

CloudSat, and its US-French companion, Calipso, are joining three other orbiters in a constellation of atmospheric satellites.

Why latent heat matters

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.

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.

One goal is to get a better handle on where inside a cloud water vapour releases its heat as it cools, condenses, and reverts to liquid drops or ice in tropical thunderstorms. This "latent" heat supplies the energy the storm needs to survive, and "the distribution of that heating is pretty important for the way storms develop," Stephens says. Indeed, the knowledge could lead to more-accurate long-term weather forecasts far from the thundering heads, he suggests.

In the tropics, individual thunderstorms can organise into clusters. These clusters become systems that extend up to 2,000 miles across. These systems tend to appear in the Indian Ocean and move east across the Pacific in cycles ranging from 30 to 60 days. They can influence storm formation far to the north and south of their locations.

How fast they propagate is thought to be determined in part by the altitude at which the latent heat is released. Systems that release heat at high altitudes are believed to move faster than those that release heat at low altitudes. The two satellites are expected to let scientists see where this heat is released, which could help improve forecasts of these systems and the weather patterns they affect.

Not surprisingly, these systems generate large shields of high-altitude cirrus that have their own effect on weather and climate. These clouds were the subject of a three-week field campaign this winter in Darwin, Australia. Researchers taking part in the Tropical Warm Pool International Cloud Experiment (TWP-ICE) initially hoped to have CloudSat and Calipso in the sky to augment their aircraft and ground measurements. Launch delays closed the door on that.

Still, the results stand to be the most comprehensive set of information ever collected on these clouds, says Charles Long, who oversees the US Department of Energy's atmospheric monitoring sites in the western Pacific. "This is the first field programme to look at tropical convection through its whole life cycle," adds Greg McFarquhar, a researcher at the University of Illinois at Urbana-Champaign.

Even so, because cirrus trap heat, the Australian project focused much of its effort on gathering images and other data on these tenuous, high-altitude blankets in a region that is the globe's heat engine, says James Mather, another DOE scientist who played a key role in the project.

For instance, researchers have noted that some cirrus clouds vanish within an hour or two, while others last for 12 hours or more. Some researchers looked at atmospheric chemistry and the role of aerosols. Others found a "chemical equator" - a shifting region that separated the relatively pristine air of the Southern Hemisphere from more-polluted northern air. Calling the data TWP-ICE gathered "a goldmine", Dr Long anticipates it will take at least a decade for researchers to plumb its depths.