Science News - February 26, 2011

Atmospheric bands
of water vapor can
cause flooding and
extreme weather

By Alexandra Witze Scientific conferences usually don’t physically experience their subjects. But during a ses- sion on “atmospheric rivers” last December at a geophysics meeting in San Francisco, one of those very rivers was barreling down on meeting attendees.

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 research 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

An atmospheric river that hit California
in late December (warmer colors sig-
nify more water vapor) ferried moisture
from southwest of Hawaii.

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 micro-wave-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.