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By flying over atmospheric rivers, scientists aim to improve forecasts

TechScienceBy flying over atmospheric rivers, scientists aim to improve forecasts
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The term “atmospheric river” may sound airy and ethereal, but these massive, fast-moving, drenching storms can hit as hard as a freight train. Since December, the US West has been slammed with back-to-back-to-back atmospheric rivers, most recently inundating the state on March 15 and a series of storms expected to affect the state in the coming week. and forecast. These powerful streams of water vapor are accompanied by strong winds, heavy rain and thick snow, causing floods, landslides and avalanches.

As big as they are, it’s surprisingly difficult to see these storms coming. One week warnings are about what the best forecasters can do right now.

A team of scientists is trying to change that. In the past few months, he has flown more than three dozen reconnaissance missions in storms. They have launched dozens of weather balloons into the stratosphere, each carrying instruments, to measure temperature, humidity, air pressure and wind. And scientists have crunched piles of data and run hundreds of computer simulations, all to predict when the next atmospheric river is going to hit and how intense it is likely to be.

The goal of this effort, the team says, is to improve predictions, give people more time to prepare for flooding along a storm’s path, and ultimately find ways to manage water for the region’s dry months. Have to find

It’s a mammoth task, especially during this year’s seemingly relentless barrage of storms. “We’re marked 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 consistently affected the western United States and Canada, bringing record rain and snow to the entire region. According to the US National Environmental Satellite Data and Information Service, more than 121 billion metric tons of water fell in California alone.

And the task is likely to be even more challenging, given the uncertainty over how atmospheric rivers will shift in intensity and frequency as the planet warms.

rivers in the sky

are atmospheric rivers long, narrow bands of condensed water vaporTypically about 1,500 km long and 500 km wide (Sn: 2/11/11, Currents form on warm ocean water, often in the tropics, and snake through the sky, transporting huge amounts of water. An atmospheric river can, on average, transport up to 15 times the amount of water at the mouth of the Mississippi River. When these storms make landfall, they can drop that water in the form of rain or snow.

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

In 2013, he and his colleagues created the Center for Western Weather and Water Extremes, or CW3E, at the Scripps Institution of Oceanography in La Jolla, California. The group then built the first weather model to predict atmospheric rivers over the US West Coast. This year, the team also created a atmospheric river intensity scaleRanking events based on size and how much water they contain.

To improve their forecasts of landfall and intensity, the team collects data from drifting ocean buoys, weather balloons and aeroplanes. The group also enlisted the assistance of the US Air Force. storm hunter – most famous for flying in the eyes of tropical cyclones from June to November – to conduct aerial reconnaissance (Sn: 5/18/12,

The data collected by the planes fills an important information gap, says 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 on the ground, 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 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, the planes drop instruments called dropsondes that collect temperature, humidity, wind and other data as they fall. Since November 1, says Wilson, Predators have flown 39 missions into atmospheric rivers.

In the US West, atmospheric rivers occur 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 in late fall, early in the year. One such storm devastated the area in November 2021, triggering a deadly series of floods and landslides.

“That storm not only hurt people, but it hurt the economy,” says Ralph.

Following that event, CW3E and its partners received funding to begin aerial reconnaissance flights on 1 November, two months earlier than these missions had begun in the past.

How will climate change affect atmospheric rivers?

In addition to the data-gathering challenges in predicting these storms, it is also difficult to disentangle the many factors that feed them, from warm tropical waters to large-scale weather patterns such as the El Niño Southern Oscillation. Ralph says it is also uncertain what effect a warming world will have on these storms.

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

Atmospheric rivers are also often associated with extreme tropical cyclones, which are mid-latitude storms formed by the collision of masses of cold and warm water. Such cyclones may interact with an atmospheric river, perhaps pulling it along with them. One such rapidly forming “bomb cyclone” helped set in motion an atmospheric river that drenched California in January.

A water vapor-filled atmospheric river (dark blue-green) swirls around dry (brown) air as it flows toward the US West Coast on Jan. 4. The storm brought strong 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 ObservatoryA water vapor-filled atmospheric river (dark blue-green) swirls around dry (brown) air as it flows toward the US West Coast on Jan. 4. The storm brought strong 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 could potentially have two adverse 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, reducing the temperature difference between the regions and allowing the winds to weaken.

“But what we’re finding is that even with that low shield, there are times when cyclones can form,” Ralph says. And those storms are blocking the increase in water vapor. This, he says, could mean larger and longer-lasting atmospheric rivers in the future.

Some studies suggest that climate change will not increase the number of atmospheric rivers, but it may increase their variability, Wilson says. “We could have more frequent transitions between very, very, very wet weather and very, very, very dry weather.” A warmer climate in general can mean that water is sucked out of the soil more quickly.

That seesaw scenario is likely to challenge water management even more in the US West, where atmospheric rivers are already both a blessing and a curse. Still, “we’re very hopeful,” Wilson says, that the data will eventually aid in the complex water management of the region, such as giving planners enough time to safely pump water out of reservoirs before they flood.

The events also provide about half of the region’s annual rainfall, bringing much-needed water to the burning lands and aloft snowpack in the high mountains, another reservoir of freshwater. This year’s storms have “done much to restore drought,” says Ralph, “greening” the landscape and refilling many small reservoirs.

But “Dry is a complicated thing,” says Ralph (Sn: 4/16/20, Historically low water levels in large reservoirs in the West, such as Lake Powell and Lake Mead, have not changed so quickly. “It will take more wet years like this to recover.”

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