Rivers in the Sky: Atmospheric Bands of Water Vapor Can Cause Flooding and Extreme Weather

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Scientific conferences usually don't physically experience their subjects. But during a session on "atmospheric rivers" last December at a geophysics meeting in San Francisco, one of those very rivers was barreling down on meeting attendees.

Like freight trains loaded with water vapor, atmospheric rivers are long, narrow bands whose winds funnel huge amounts of moisture through the sky. When they hit coasts, these rivers can drop their moisture as rain and cause destructive flooding, as in January 2005 when more than 20 inches of rain soaked southern California, killing 14 people and causing hundreds of millions of dollars in damage.

Scientists (and San Francisco) managed to escape December's atmospheric river without such harm, but the storm dumped more than 10 feet of snow in parts of the Sierra Nevada, putting the mountains on track for their wettest recorded season. That sort of impact underscores why researchers have recently become fascinated with atmospheric rivers. Completely unknown just over a decade ago, these rivers turn out to be not only a key factor in Western flooding and water supply, but also a major player in the planet's water cycle.

"Water is life, and atmospheric rivers provide water," says Paul Neiman, a meteorologist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo. New re search is revealing how these rivers work, as well as helping forecasters better predict their consequences.

At any given time, somewhere between three and five atmospheric rivers are typically ferrying water in each hemisphere. More than 1,000 kilometers long, they are often no wider than 400 kilometers and carry the equivalent, in water vapor, of the flow at the Mississippi River's mouth. "That has really captured the imagination of scientists," says Marty Ralph, also a meteorologist at the Boulder lab. "There are only a handful of these events, and yet they do the work of transporting 90-plus percent of water vapor on the planet."

Ordinary clouds don't carry lots of water vapor long distances; they rain out as soon as water droplets coalesce and get heavy enough to fall as precipitation. In the 1990s, MIT researchers calculated from wind and moisture data that jets in the atmosphere, which the scientists termed atmospheric rivers, must exist to help ferry water around the planet.

Since then researchers have gotten a better look at the rivers, using microwave-sensing instruments carried on polar-orbiting satellites. Solar radiation bouncing off Earth's surface in microwave wavelengths is affected by the amount of water vapor between the ground and the satellite, but microwaves aren't affected by clouds the way visible and infrared radiation are. So microwave instruments are able to photograph ribbons of water vapor coursing through the atmosphere.

In the early days of atmospheric river 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 "inter-tropical convergence zone" where winds from both hemispheres meet. …