The storm raged over California for more than five days. As the powerful atmospheric river made landfall, fierce winds and heavy rains tore trees from their roots, turned streets into rivers and poured mud into homes.
In addition to chaos, the storm also brought opportunities. Scientists were ready, both on land and in flight, to deploy instruments that measure atmospheric rivers like this one. They released tools from airplanes, equipped with small parachutes, or floated them up from the ground, attached to balloons, directly into the path of the storm.
These small but powerful devices provide important information that will help improve weather forecasts as the climate crisis makes already powerful storms more dangerous.
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Atmospheric rivers have long been important features of weather systems in the western US and are vital for replenishing the state’s reservoirs and snowpack. But the strong systems that carry water across the Pacific Ocean are filled with enough moisture to rival the currents at the mouth of the Mississippi River—and often many times more—and often cause the most devastating floods.
Warming oceans strengthen storms, making them deadlier and more expensive. This week’s storms killed nine people, caused an estimated $11 billion in damage and economic losses, and dumped half of Los Angeles’ annual rainfall on the city within days.
Now scientists are trying to better understand these systems before they get worse. The work greatly increases the accuracy of weather forecasts, giving water managers more time to plan and giving communities earlier warnings to prepare, long before the clouds above darken. But there is much more to learn about these systems, especially as their dangers increase.
Research on these airborne water vapor plumes, originating in the tropical Pacific Ocean, has grown dramatically in the thirty years since “atmospheric rivers” were given their name. But predictions about where a storm will make landfall can still be off by hundreds of miles, and it’s difficult to predict how certain storms will progress.
Scientists are trying to understand the layered and complicated conversations taking place between the ocean, atmosphere and land, and hope to gain stronger insights into how, when and where storms will strike.
“The more we learn, the more we realize we need more data on this,” said Maike Sonnewald, leader of the computational climate and ocean group at UC Davis.
Sonnewald, an oceanographer who uses computer science to understand climate and long-term weather forecasting, added that recent developments in the satellite era have helped paint a picture of the interaction between the ocean and the atmosphere. Metaphorically speaking, that photo still has too few pixels.
“We don’t necessarily have high enough resolution to be able to model specific things,” she added, explaining that the dynamic nature of the ocean – and how easily small shifts can cause big changes in the models – poses predictive challenges. entails.
“The climate is changing – we are making the Earth hotter – and that is known. It is the details that are difficult to distinguish,” says Sonnewald. A warming atmosphere can hold exponentially more water vapor, and warming temperatures at the ocean’s surface will evaporate more quickly, so scientists can easily predict how things will worsen. It is more difficult to know when and where.
Within the storm systems
In the days before the storm hit, it was clear that a blow was coming. As it approached, officials had enough information to ready resources and warn residents.
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Global models that scientists rely on to make forecasts are good at “detecting that there is a potential for an impactful storm at least several days in advance,” says Alex Lamers, the Weather Prediction Center’s warning coordination meteorologist, but details are not necessary. shape until the storm is very close.
“The details really matter: the exact location where it intersects the coast, which mountain ranges it affects, the angle at which the wind affects the mountains and uphill areas,” Lamers said.
Satellites can only go so far in filling gaps in ocean information. “The Pacific Ocean is vast and there aren’t many actual weather observations,” he said.
That’s why a team of scientists led by Martin Ralph, the founding director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, began taking measurements directly from the storm systems themselves.
Since 2016, the Atmospheric River Reconnaissance (AR Reconnaissance) program has relied on U.S. Air Force “hurricane hunter” aircraft that drop a small cluster of instruments known as dropsondes, which can relay findings as they fall through the clouds into the ocean below. .
Each dropsonde is attached to a small parachute and floats through the clouds into the ocean – a journey that takes about 20 minutes – while key observations are fed back to scientists on board. Air temperature, pressure, water vapor and wind speed are all collected by the dropsondes, like an “MRI for an atmospheric river,” says Ralph, allowing researchers to look inside the system instead of having to rely on satellite images.
During a series of strong atmospheric river storms that hit California in 2023, the dropsondes helped push some heavy precipitation forecasts forward by about 12%, a feat that researchers say would have taken eight additional years using traditional data collection methods .
“If we misrepresent the atmospheric river in the model — how strong it is, where it is, its structure, how much water it has — those errors will lead to errors in the forecast over time,” Ralph said. , which explains the importance of going to the specific location of an atmospheric river to measure it.
Along with the parachuting dropsondes, traditional weather balloons released from the ground during the storms, called radiosondes, help complete the picture. “They are very different approaches, but both are necessary to have the most effective forecasting system,” Ralph said.
The team also uses new technology, including a method called Airborne Radio Occultation (ARO), invented by a Scripps geophysicist and atmospheric scientist, Jennifer Haase. While dropsondes take measurements while falling vertically beneath the aircraft, ARO uses sensors attached to the side of an aircraft that take measurements horizontally. They can infer properties such as moisture and temperature by measuring how much GPS signals break as they move through the atmosphere, helping scientists paint a more complete picture of a coming storm.
The first flights equipped with ARO were deployed this winter and were able to collect data at a distance of up to 300 km from an aircraft.
Mitigating the worst disasters
Flood risks are increasing in California and other parts of the arid American West, and having more accurate information will be essential to mitigating the worst disasters.
“Given the amount of warming we’ve seen so far, we expect major precipitation events to be about 10% more intense than they were before greenhouse gases were added to the atmosphere,” said Alex Hall, an atmospheric physicist and climate scientist at UCLA. .
“The scary thing is that if you look into the future to the point where we have twice as much warming as today, you have events that are 20% more intense, and entirely new types of events that don’t even exist now.”
For Ralph, this reality is a call to arms.
“As these storms will evolve and change over time, our development of AR exploration and associated instruments to measure and predict atmospheric rivers will be able to keep up,” he said. “This is really a climate mitigation, so that people can better adapt to what is happening.”