Atmospheric river storms are hailed as drought-busters when they bring needed rain and snow, but they have a well-known dark side: damaging floods. A new NASA study documents a second destructive force in these storms: high winds.

The study shows that atmospheric rivers were associated with almost half of the most extreme mid-latitude windstorms globally for the past 20 years, doing billions of dollars in damage.

An atmospheric river is a long, narrow stream of water vapor carried by wind that can cause storms. Satellite observations show that this weather pattern occurs all over the world, even around Antarctica. Atmospheric river storms are common in wintertime along the U.S. West Coast, where rain and snow are often needed. For that reason, Americans tend to think of them mainly in terms of precipitation.

"Our study highlights the risks of extreme and hazardous winds that can occur with atmospheric river storms, in addition to the more well-known risks from heavy precipitation," said Duane Waliser of NASA's Jet Propulsion Laboratory, Pasadena, California. Waliser is lead author of the new study published Feb. 20 in Nature Geoscience.

Waliser and Bin Guan, of JPL's and UCLA's Joint Institute for Regional Earth System Science and Engineering, used global weather analysis records and NASA satellite precipitation data for their study. They examined what fraction of global storms that feature extreme wind and precipitation - over both land and sea - occurred in conjunction with atmospheric rivers.

Over mid-latitude oceans, where high winds can be a threat to shipping, this weather pattern is associated with up to half of the most extreme wind and precipitation occurrences.

Similarly, the researchers studied the correlation between atmospheric rivers and extreme precipitation, also looking at the top 2 percent of precipitation-producing storms. They found that, as with windstorms, atmospheric rivers are associated with up to half of these extreme precipitation events across the same mid-latitude regions.

Atmospheric rivers that make landfall have a greater potential for destruction. The researchers examined the most destructive windstorms of the last 20 years - the top 2 percent in terms of wind speeds near Earth's surface. They found that atmospheric rivers were associated with up to half of these storms along the world's mid-latitude coastlines. Often, the highest wind speed ever recorded on a coastline was associated with an atmospheric river storm.

To get an idea of the potential economic consequences of these storms, Waliser and Guan consulted a database of the 19 most expensive European windstorms, in terms of insurance losses, between 1997 and 2013. They found that atmospheric rivers were associated with 14 (about 75 percent) of these events. Together, these 14 storms accounted for more than $25 billion in insured losses.

The extent of these correlations came as something of surprise to the researchers. Since atmospheric rivers are, by definition, extreme cases of winds transporting moisture, "We expected that there would be an association," Guan said, "but the degree of the connection exceeded our expectation."

NASA collects data from space, air, land and sea to increase our understanding of our home planet, improve lives and safeguard our future. NASA develops new ways to observe and study Earth's interconnected natural systems with long-term data records. The agency freely shares this unique knowledge and works with institutions around the world to gain new insights into how our planet is changing.

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