How Warming May Alter Critical ‘Atmospheric Rivers’

The influx of much-needed rain over Northern California comes courtesy of a feature called an atmospheric river that is a key source of much of the state’s precipitation and water supply.  A relatively recent meteorological discovery, these ribbons of water vapor in the sky are something scientists are trying to better understand.  They are flying research planes into the heart of the current storm to study what fuels it, which could help improve forecasts of the events.

Because the atmospheric rivers are critical for California’s water supply, scientists also want to pin down how they are being affected by climate change.

“We need to understand more about how climate change will change the amounts of precipitation that they will drop in the future,” Michael Dettinger, a research hydrologist with the U.S. Geological Survey who is actively investigating that question, said.

Wet, Dry, Wet

The relentless drought in California, now in its fourth year, has left the state with a dismal snowpack and below-normal stores in many reservoirs, which has hobbled the state’s agriculture and sent some communities to the brink of waterlessness, among numerous other impacts.

The drought has reached those depths because of record warmth that has increased water use and evaporation, and an atmospheric setup that has kept most storms over the Pacific from reaching the state during the past few winters, typically California’s wet season.  In particular, a tenacious region of high pressure has diverted those storms — and their rains — further north.
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Now, the high pressure system has shifted again and the flow of moisture has turned back on.  This atmospheric river is expected to bring significant rainfalls of 5 to 10 inches in mountainous areas and 1 to 5 inches in valleys as the storm hits the mountains of Northern California.  (The mountains force the air upward, causing it to cool and the water vapor in it to condense out and fall as rain.)
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What Will Warming Do?

Another issue researchers are trying to better understand is how a warmer atmosphere will impact atmospheric rivers — a key consideration for areas that so depend on these features for water.

Climate models and basic physics suggest that atmospheric rivers will become moister and more intense in the future, as a warmer atmosphere can hold more water vapor (about 4 percent more for every degree 1°F of warming).

But, as with this storm, that may only mean more rain, not snow.  Winters in California are expected to continue warming, so “it's pretty likely that the average elevation at which snow falls during [atmospheric rivers] (and at other times) will increase,” Swain said.

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