Researchers Spying On Invisible Clouds

Skies may offer answers to pace of global warming

Toward the end of one in a rare string of clear days, the sun sits low in the Arctic sky, painting the drifts and ice ridges pale orange against the steel blue of shaded snow.

From the deck of the Canadian ice breaker Des Groseilliers, the Arctic sky overhead appears empty, cloudless. But looks can be deceiving.

A few nights ago, a unique radar on board detected the barest wisp of a cloud layer - one that could not have been seen with the naked eye or with cameras on orbiting satellites.

Such "invisible" clouds may be causing a problem as scientists attempt to better understand the myriad factors that contribute to climate change here and around the world.

To accurately forecast what effect global warming will have on the Arctic, scientists must understand what effect all clouds - even the "stealth" ones - have on radiation and surface temperature. And when scientists can't see them, it's that much harder to take them into account.

Filling in atmospheric blanks

That's where Taneil Uttal comes in. She and a number of other colleagues here are focusing their attention on filling in the atmospheric blanks.

These clouds' effect on the amount of solar radiation reaching the surface "is very significant," notes Ms. Uttal, a researcher with the National Oceanic and Atmospheric Administration's Environmental Technology Laboratory (ETL) in Boulder, Colo.

Until now, scientists trying to forecast global warming's effect on the planet have relied heavily on satellite and traditional ground-based observations. Those observations, however, are limited in what they say about clouds, because if satellites see clouds at all, they see only the tops. Ceilometer readings or human observations give information only about the bottoms of clouds. But a lot goes on in between, Uttal says.

"Two clouds can look the same, but their particles may be different sizes or their phases can be different," she says, referring to whether the particles are water droplets or ice crystals. Such differences "affect how the cloud reflects visible or infrared radiation."

Today, Uttal is relying on some of the most sophisticated equipment ever assembled for polar research.

Sitting atop the "pumpkin" - an orange prefab office trailer on the ship's helicopter deck - a unique radar trains its mechanical eye on the endless azure sky, hoping to see clouds the human eye can't. The millimeter cloud radar, the device that found the wispy clouds a few nights ago, measures the reflectivity of what its beams encounter, creating images of cloud layers from the information it receives.

Working in tandem with a radar-like device called the lidar, which uses laser light, instead of radio signals, to detect cloud features.

"The radar can see through thick layers of clouds; the lidar is looking for thin cirrus clouds or haze," says Raul Alvarez, the lidar's designer. Because the wavelength of light is much shorter than that of radio waves, the lidar can detect smaller particles, distinguish between their phases, and even can spot aerosols - extremely small particles that serve as the foundation around which water droplets and ice crystals form.

On its own, lidar signals can't penetrate a thick cloud layer - but the cloud radar can. Taken together, the lidar and millimeter cloud radar "give us a powerful set of tools" for closing the knowledge gap on Arctic clouds and their role in the exchange of heat between the atmosphere and the surface, Uttal says.

Cirrus surprise

The radar already is turning up surprises, revealing cirrus clouds in the Arctic sky. "People said we'd never see them up here," Uttal says. Moreover, the radar's readouts show layering similar to the kind occurring with clouds at lower latitudes.

Such layering can play an important role in slowing the dissipation of surface heat or slowing the influx of solar radiation - much as layers of glass in a storm window keep heat indoors and cold air outdoors.

In the end, SHEBA researchers hope to find a correlation between radiation and changes in cloud type, altitude, composition, and thickness over a full year - giving modelers a more accurate picture of the heat budget here, one that isn't just cloud-top deep.

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