Science News - February 26, 2011

research, scientists weren’t sure that the bright bands of water vapor in microwave satellite images really translated to super-soggy conditions. So teams flew research airplanes into storm systems, some of which spawned atmospheric rivers, to measure how wet things got. “You could really sense the juiciness,” Ralph says. “You could smell it in the cockpit.” Atmospheric rivers are born because of temperature differences between Earth’s tropics and its poles. During winter, a pole cools compared with the equator, creating a strong temperature gradient across the hemisphere, a difference that causes low-pressure storms to spin off in the midlatitudes. Winds within the storm can funnel moisture into a narrow band at its leading edge — the atmospheric river. At the San Francisco meeting, George Kiladis of the Boulder lab described a March 2005 river that apparently sucked moisture into the Pacific Northwest all the way from the tropics, in the “ intertropical convergence zone” where winds from both hemispheres meet. Kiladis and colleagues describe the river in a paper to appear in Monthly Weather Review.

People living on the West Coast are familiar with atmospheric rivers such as the famous “Pineapple Express,” which occasionally ferries moisture directly from Hawaii. But the rivers can also come up through the Gulf of Mexico or along the eastern seaboard.

One wet day an atmospheric river that hit california’s coastal mountains in 2009 delivered 16 inches of rain, increasing the year’s accumulation by almost 50 percent in one day.

Rainfall at Three Peaks Summit in California

52

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16

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0

10/12/09 10/13/09 10/14/09

Day and time

source: m. ralph, monterey county water resources aGency www.sciencenews.org

Scientists are now moving from spotting atmospheric rivers to understanding them and trying to predict their impact. Leading the way is California, which is setting up four atmospheric river observatories along its coast to track the rivers as they arrive. Each river can have strikingly different effects depending on the angle and speed at which it approaches mountain ranges and watersheds.

“To be able to nail down specific water basins that are most prone to flooding, you really need to know precisely where that atmospheric river will make landfall,” Neiman says. “That’s the tough part.” During a November 1994 storm, now known to be an atmospheric river, for instance, forecasters predicted that less than one-tenth of an inch of rain would hit some parts of the San Francisco Bay area. In places, more than 11 inches fell, David Reynolds, a meteorologist with the National Weather Service’s office in Monterey, Calif., said at the geophysics meeting. How that moisture is distributed within the river and how long it sits in one location determine what areas will see the most flooding.

Not all atmospheric rivers are devastating — in fact, most of them are weak — but they cause many of the most extreme West Coast floods. In one study, Ralph and colleagues looked at all seven floods that occurred on California’s Russian River between 1997 and 2006. All were due to atmospheric rivers, the researchers found. The amount of intense rainfall the West Coast gets from atmospheric rivers over time, says Ralph, is comparable to the soakings the Gulf Coast and southeastern United States receive from major landfalling hurricanes.

In January, California emergency planners met in Sacramento to run through a doomsday scenario dubbed ARkStorm. Officials tested how they would respond if a series of atmospheric rivers hit the coast one after the other. That scenario was modeled on the rivers that hit in the winter of 1861–62 and flooded the state’s central valley. The capital had to be temporarily moved from Sacramento to San Francisco, and the governor took a rowboat to his inauguration.

In December, an atmospheric river led to
flooding in parts of Orange County, Calif.

To better predict such disasters, researchers at NOAA and the Scripps Institution of Oceanography in La Jolla, Calif., are working with state officials to pinpoint the most vulnerable areas. Many times, the river of humidity runs into a mountain, is forced upward, and rains out its water. Other times the river hits the base of a range and shifts to flow around it. Figuring out which process dominates in which locations will help officials better prepare, Ralph says.

For instance, in 2009 planners in Washington faced a crisis when the Howard Hanson Dam, on the Green River above Seattle’s southern suburbs, began leaking just as an atmospheric river was aiming right at it. The scientists analyzed how the river would run into the watershed and dump its water, and predicted it would not cause lots of rainfall above the dam. The U.S. Army Corps of Engineers decided not to assume emergency control, and the rain did taper off quickly.

Atmospheric rivers may become even more relevant as global temperatures rise. Researchers aren’t sure exactly how climate change will affect the rivers, but warmer air generally means that the atmosphere can hold more water vapor, says Neiman. On the other hand, winds may weaken in a globally warmed world, meaning the rivers might carry more water but be less effective at delivering it.

More answers may come within the next few months, as NOAA scientists plan to fly unmanned aircraft into several storms to try to learn even more about atmospheric rivers. s