By flying over atmospheric rivers, scientists aim to improve predictions

The term “atmospheric river” may sound airy and ethereal, but these massive, fast-moving, torrential storms can hit as hard as a freight train. Since December, the western United States has been hit by back-to-back atmospheric rivers, with the most recent flooding the state on March 15 and another expected to hit the state in the coming week. These powerful currents of water vapor arrive accompanied by strong winds, heavy rains and deep snow, causing floods, landslides and avalanches.

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

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A team of scientists is trying to change that. Over the past few months, they have flown more than three dozen storm reconnaissance missions. They launched dozens of weather balloons high into the stratosphere, each carrying instruments to measure temperature, humidity, atmospheric pressure and wind. And scientists have analyzed tons of data and run hundreds of computer simulations, all to predict when the next atmospheric river will arrive and how strong it will be.

The goal of this effort, the team says, is to improve forecasts, give people in the path of storms more time to prepare for flooding, and ultimately find ways to manage the storm. water during the region’s driest months.

It’s a big task, especially during this year’s seemingly incessant storm storms. “We’ve been hammered 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 pounded the western United States and Canada, dumping record rain and snowfall in the region. More than 121 billion metric tons of water fell on California alone, according to the US National Environmental Satellite Data and Information Service.

And that task is likely to become even more difficult, given the continuing 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 vapor, typically about 1,500 kilometers long and 500 kilometers wide (SN: 2/11/11). Currents form over warm ocean waters, often in the tropics, and meander across the sky, carrying huge 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 over land, they can release this water as rain or snow.

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

In 2013, he and his colleagues established the Center for Western Weather and Water Extremes, or CW3E, at the Scripps Institution of Oceanography in La Jolla, California. The group then created the first weather model suitable for predicting atmospheric rivers on the west coast of the United States. This year, the team also created a river atmospheric intensity scale, classifying events by their size and the amount of water they carry.

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

The data collected by the planes fills an important information gap, explains Anna Wilson. She is a Scripps atmospheric scientist who also manages field research for CW3E. Weather balloons are the workhorses of weather forecasting, but they are launched above the earth, and “it’s important to see what happens before [an atmospheric river] lands,” Wilson said.

Satellites can provide valuable atmospheric data about the ocean, but they usually cannot see through clouds and heavy precipitation, two characteristics of atmospheric rivers. And atmospheric rivers hang low in the troposphere, the lowest part of Earth’s atmosphere, making it even harder for satellites to spy on them.

During each flight mission, planes drop instruments called dropsondes that collect temperature, humidity, wind, and other data as they fall. Since Nov. 1, the fighters have flown 39 atmospheric river missions, Wilson says.

In the western United States, atmospheric rivers tend to arrive from January to March. But it’s not really the start of atmospheric river season in this region: atmospheric rivers make landfall in the Pacific Northwest earlier in the year, in late fall. One such storm devastated this region in November 2021, causing a deadly series of floods and landslides.

“This storm didn’t just hurt people, it hurt the economy,” Ralph said, causing “millennia-old flooding that destroyed rail lines right in the middle of a serious supply chain issue.” .

As a result of this event, CW3E and their partners received funding to begin reconnaissance flights of the aircraft on November 1, two months earlier than such missions have begun in the past.

How will climate change affect atmospheric rivers?

In addition to the difficulties of collecting data to forecast these storms, it is also difficult to disentangle the many factors that fuel them, from warm tropical waters to large-scale weather patterns such as the El Niño Southern Oscillation. The influence of a warming world on these storms is also uncertain, says Ralph.

“One thing to keep in mind is that the fuel of an atmospheric river is water vapor. It is driven by the wind, formed 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, possibly dragging it along. One of these fast-forming “bomb cyclones” helped spur an atmospheric river that flooded California in January.

An atmospheric river laden with water vapor (dark blue-green) swirls around drier air (brown) as it heads toward the west coast of the United States on January 4. The storm brought high winds and heavy rain, and caused flooding and downed power lines. Bluer colors indicate more water vapor per area of ​​the atmosphere. Lauren Dauphin/NASA Earth Observatory An atmospheric river laden with water vapor (dark blue-green) swirls around drier air (brown) so that she is heading for the west coast of the United States on January 4. storm brought high winds and heavy rain, and caused flooding and downed power lines. Bluer colors indicate more water vapor per area of ​​the atmosphere.Lauren Dauphin/NASA Earth Observatory

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

“But what we find is that even with this reduced gradient, there are still times when cyclones can form,” says Ralph. And these storms feed on the increase in water vapor. This, he says, could mean larger and more sustainable atmospheric rivers in the future.

Some studies suggest that climate change won’t necessarily increase the number of atmospheric rivers, but it 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 sucked out of the ground faster.

This see-saw scenario is likely to make water management even more difficult in the western United States, where atmospheric rivers are already both a blessing and a curse. Still, “we’re hopeful,” says Wilson, that the data will help the region’s complex water management, such as giving planners enough time to safely evacuate water from reservoirs before they are not flooded.

The events also provide up to half of the region’s annual rainfall, bringing much-needed water to the parched lands and supplementing the snowpack in the high mountains, another reservoir of fresh water. This year’s storms “have done a lot to restore the dryness of the landscape,” Ralph says, “greening” the landscape and filling in many smaller reservoirs.

But “drought is a complicated thing,” Ralph says (SN: 4/16/20). Historically low water levels in large western reservoirs, such as Lake Powell and Lake Mead, are not being replaced so quickly. “It will take more wet years like this to recover.”

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