By flying over atmospheric rivers, scientists aim to improve forecasts

The term “atmospheric river” may sound airy and ethereal, but these massive, fast-moving, soggy storms can hit as hard as a freight train. Since December, the western US has been battered by back-to-back atmospheric rivers, the most recent flooding the state on March 15 and another forecast to hit the state next week. These powerful currents of water vapor come with strong winds, heavy rain, and thick snow, generating floods, landslides, and avalanches.

As big as they are, these storms are surprisingly hard to see coming. The one-week warning is the best forecasters can do right now.

A team of scientists is trying to change that. In the past few months alone, more than three dozen reconnaissance missions have flown into the storms. They have launched dozens of weather balloons high into the stratosphere, each with instruments to measure temperature, humidity, air pressure, and wind. And scientists have crunched through reams of data and run hundreds of computer simulations, all to forecast when the next atmospheric river will arrive and how intense it is likely to be.

The goal of this effort, the team says, is to improve predictions, give people in the path of storms more time to prepare for flooding, and ultimately find ways to manage water for the drier months of the region.

It’s a tall order, particularly during this year’s seemingly relentless barrage of storms. “We’ve been hit here: December, January, February, March,” says meteorologist Marty Ralph. “It’s been a long and active season.”

In December and January alone, nine atmospheric rivers relentlessly battered the western United States and Canada, dumping record-breaking rain and snow across the region. More than 121 billion metric tons of water fell in California alone, according to the US National Environmental Satellite Data and Information Service.

And this task is likely to become even more challenging, given the lingering uncertainty about how atmospheric rivers will change in intensity and frequency as the planet continues to warm.

rivers in the sky

atmospheric rivers are long narrow bands of condensed water vaportypically about 1,500 kilometers long and 500 kilometers wide (Serial number: 02/11/11). Streams form over warm ocean waters, often in the tropics, and meander through the sky, carrying vast amounts of water. An atmospheric river, on average, can carry up to 15 times the volume of water at the mouth of the Mississippi River. When these storms come ashore, they can release that water as rain or snow.

While atmospheric rivers can bring welcome water to a dry region, they are also “the leading, almost exclusive cause” of flooding on the US West Coast, Ralph says.

In 2013, he and his colleagues created the Center for Western Climate and Water Extremes, or CW3E, at the Scripps Institution of Oceanography in La Jolla, California. The group then created the first weather model designed to predict atmospheric rivers on the West Coast of the US. This year, the team also created a river atmospheric intensity scaleclassifying the events according to the size and the amount of water they carry.

To improve their landfall and intensity forecasts, the team collects data from drifting ocean buoys, weather balloons and aircraft. The group even enlisted the help of the US Air Force. hurricane hunters — most famous for flying into the eyes of tropical cyclones from June to November — for aerial reconnaissance (Serial number: 05/18/12).

The data collected by the planes fills an important information gap, says Anna Wilson. She is an atmospheric scientist at Scripps who also manages field research for CW3E. Weather balloons are the workhorses of weather forecasting, but they’re launched over land and “it’s important to see what happens before (an atmospheric river) makes landfall,” says Wilson.

Satellites can provide valuable atmospheric data over the ocean, but they generally cannot see through clouds and heavy precipitation, both characteristic features of atmospheric rivers. And atmospheric rivers hang low in the troposphere, the lowest part of Earth’s atmosphere, making it even more difficult for satellites to spy on them.

During each flight mission, planes drop instruments called drop probes that collect temperature, humidity, wind, and other data as they drop. Since Nov. 1, hunters have flown 39 missions to atmospheric rivers, Wilson says.

In the western US, atmospheric rivers tend to arrive from January through March. But that’s not really the start of atmospheric river season in this region: atmospheric rivers make landfall in the Pacific Northwest early in the year, in late fall. One such storm devastated that region in November 2021, spawning a deadly series of floods and mudslides.

“That storm not only affected people, but also the economy,” says Ralph, causing “millennial floods that destroyed rail lines right in the middle of a serious supply chain problem.”

In the aftermath of that event, CW3E and its partners received funding to begin reconnaissance flights of the aircraft on November 1, two months before these missions began in the past.

How will climate change affect atmospheric rivers?

In addition to the data collection challenges to predict these storms, it’s also difficult to tease out the many factors that fuel them, from warm tropical waters to large-scale weather patterns such as the El Niño Southern Oscillation. It’s also uncertain how a warming world will influence these storms, Ralph says.

“One thing to keep in mind is that the fuel for an atmospheric river is water vapor. It is pushed by the wind, shaped by the temperature gradient between the poles and the equator,” he says.

Atmospheric rivers are also often associated with extratropical cyclones, mid-latitude storms formed by the collision of cold and warm water masses. Such cyclones can interact with an atmospheric river, perhaps dragging it along. One such fast-forming “bomb cyclone” helped spur an atmospheric river that flooded California in January.

Global warming may have two potential offsetting effects on atmospheric rivers: Warmer air may hold more water vapor, which means more fuel for storms. But the poles are also warming faster than the equatorial regions, reducing the temperature difference between the regions and that can weaken the winds.

“But what we’re finding is that even with that reduced gradient, there are still times when cyclones can form,” says Ralph. And those storms are fed by increased water vapor. That, he says, could mean larger, longer-lasting atmospheric rivers in the future.

Some studies suggest that climate change won’t necessarily increase the number of atmospheric rivers, but could increase their variability, Wilson says. “We may have more frequent changes between very, very, very wet seasons and very, very, very dry seasons.” A warmer climate in general can mean that water is extracted from the soil more quickly.

That swing scenario is likely to make water management even more of a challenge in the western US, where atmospheric rivers are already both a blessing and a curse. Still, “we’re very hopeful,” says Wilson, that the data will ultimately help the region’s complicated water management, for example, by giving planners enough time to safely let water out of reservoirs before let them flood

The events also provide up to half of the region’s annual precipitation, bringing much-needed water to drylands and blanketing the high mountains with snowpack, another freshwater reservoir. This year’s storms “have done a lot to restore the dry landscape,” Ralph says, “greening” the landscape and refilling many smaller reservoirs.

But “drought It’s kind of complicated,” says Ralph (Serial number: 04/16/20). Historically low water levels in the largest reservoirs in the West, such as Lake Powell and Lake Mead, are not being replaced as quickly. “It will take more wet years like this to recover.”