The name “atmospheric storm” may sound airy and ethereal, but these storms are massive, fast-moving, hard-hitting storms that can hit a freight train. Since December, the US West has been hit by back-to-back tornadoes, the most recent one flooding the state on March 15, and another forecast to hit the state in the coming week. These rivers of strong steam with strong winds, heavy rains and snow, flash floods, landslides and avalanches.
Great as they are, these storms seem surprisingly tough to come by. A weekly warning of the best forecasters can do right now.
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A team of scientists is trying to change that. In recent months, more than three dozen expeditions have flown into the storm. They launched dozens of weather balloons into the stratosphere, carrying individual instruments to measure temperature, humidity, air pressure and wind. And scientists have turned their oars of data and run hundreds of computer simulations, all to predict when the next atmospheric current is going to come and how strong it is going to be.
The goal of this effort, the team says, is to improve predictions, give people more time to prepare for flooding during storms, and ultimately find ways to manage water during the region’s drier months.
It’s a big job, especially this time of year when the weather seems relentless. “We’re hit here: December, January, February, March,” says astronomer Marty Ralph. “This has been a long and active time.”
Just in December and January, nine atmospheric rivers hit the western United States and Canada unexpectedly, dumping record rain and snow across the country. More than 121,000 metric tons of water fell in California alone, according to the National Satellite Environmental Data and Information Service.
This task is likely to become more difficult, given the long-standing uncertainty over how atmospheric currents will change in intensity and frequency as the planet continues to warm.
Rivers in the sky
Atmospheric currents are long, tight links of condensed water vapor, usually about 1,500 kilometers long and 500 kilometers across.SN: 2/11/11). Rivers flow over the warm waters of the ocean, often in the tropics, and snake through the sky, carrying huge volumes of water. One atmospheric river, on average, can transport up to 15 times the water at the mouth of the Mississippi River. When these storms reach the ground, they can release that water as rain or snow.
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While atmospheric rivers can bring welcome water to an arid region, they are also the “primary, almost exclusive” cause of flooding on the US West Coast, says Ralph.
In 2013, he and colleagues created the Center for Western Climates and Extreme Waters, or CW3E, at the Writers’ Institution of Oceanography in La Jolla, Calif. The group then created the first weather model designed to predict atmospheric currents on the US West Coast. This year, the team also created an atmospheric river intensity scale, ranking things according to their size and how much water they carry.
To improve forecasts of disaster risk and intensity, the team collects data from ocean buoys, weather balloons and airplanes. The Air Force recruits a group of wind hunters – most famous for flying into the eyes of tropical cyclones from June to November – to conduct aerial reconnaissance (SN: 5/18/12).
The information collected from the planes fills a valuable gap, says Anna Wilson. The writer is an atmospheric scientist who also manages the CW3E field research. Weather balloons are the official forecast of the weather, but they are thrown over the ground and “It is important to see what happens in advance. [an atmospheric river] “It makes a difference,” Wilson said.
Satellites can provide valuable atmospheric data on the ocean, but they generally cannot see through clouds and heavy precipitation, the characteristic features of both atmospheric streams. Atmospheric currents in the troposphere, the lowest part of the Earth’s atmosphere, have prostrated, making it more difficult for satellites to explore.
During each mission flown, the plane drops instruments called hydrosondes that collect temperature, humidity, wind and other data. Since Nov. 1, hunters have flown 39 missions in atmospheric streams, Wilson says.
In the Western US, atmospheric flows tend to occur from January through March. But this is not really the beginning of atmospheric river season in this region: atmospheric rivers in the Pacific Northwest make landfall earlier in the year, in late fall. One such storm devastated that region in November 2021, causing a series of deadly floods and landslides.
“This storm is not only going to hurt people, it’s going to hurt the economy,” Ralph says, causing “thousand-year flooding that destroys straight rail lines in the middle of the supply chain, a serious issue.”
In the event of that repair, CW3E and their partners received the money to begin the plane’s reconnaissance flights on November 1, two months before these missions began in the past.
How does climate change affect atmospheric flows?
In addition to the data-gathering challenges for forecasting these storms, it’s also difficult to disentangle the many factors that feed into them, from warm tropical waters to the high-quality weather patterns of the El Niño Southern Oscillation. How global warming is driving these storms is also uncertain, says Ralph.
It is worth noting that the fuel of the atmospheric stream is water vapor. It is driven by the wind, formed by the temperature gradient between the poles and the equator,” he said.
Atmospheric currents are also often associated with extratropical cyclones, mid-latitude storms formed by the collision of cold and warm water masses. Such cyclones interact with an atmospheric stream, perhaps dragging it. One such rapidly forming cyclone is aided by the spur of an atmospheric river that soaked California in January.
Global warming can have two stabilizing effects on atmospheric currents: warmer air can hold more water vapor, which means more fuel for storms. But the poles are also heated faster than the equatorial regions, which reduces the difference in temperature between the regions, and which can weaken the winds.
“But what we’re finding is that even with that reduced gradient, there are still times when cyclones can form,” Ralph says. And those storms fasten the phases of the vapors of the waters. It could mean larger and longer-lasting atmospheric currents in the future, he says.
Some studies suggest that climate change will not necessarily increase the number of atmospheric rivers, but increase their variability, Wilson says. “We can have more frequent shifts between very, very wet times and very, very dry times. Warmer areas can generally mean that water is being drained from the ground more quickly.
That speculative scenario is likely to make water management even more of a challenge in the US West, 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 country’s complex water management, giving planners enough time to safely get water out of reservoirs before they flood.
The event also provides as much as half of the annual precipitation of the region, bringing much-needed water to the arid lands, and by lowering the snow in the highest mountains, another source of fresh water. This year’s weather “did a lot to restore the drought landscape,” says Ralph, “greening” the landscape and filling in many smaller reservoirs.
But drought is a complicated matter, says Ralph.SN: 4/16/20). Historically, water levels in larger reservoirs in the West, such as Lake Powell and Lake Mead, have not been restored as quickly. “It’s going to take more wet years for this one to heal.”
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