by The storm raged over California for more than five days. As the mighty atmospheric river came ashore, furious winds and torrential downpours uprooted trees, turned streets into rivers and sent mud into homes.
Along with the chaos, the storm brought opportunity. Scientists were ready, on land and in flight, to deploy instruments that measure atmospheric rivers like this. They dropped instruments 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.
Related: Flooding in California: how atmospheric rivers became a state of emergency
Atmospheric rivers have long been important features of weather systems across the western US and are critical to replenishing the state’s reservoirs and snowpack. But filled with enough moisture to rival flows at the mouth of the Mississippi—and often more—the strong systems that carry water across the Pacific Ocean often have the most destructive floods.
Warming oceans are fueling the storms, making them deadlier and more expensive. This week’s storms killed nine people, caused an estimated $11bn in damage and economic loss, and dumped half of Los Angeles’ annual rainfall on the city in just a few days.
Now, scientists are racing to better understand these systems before they deteriorate. The work is greatly improving the accuracy of weather forecasts, giving water managers more time to plan and earlier warnings for communities to prepare, long before clouds rise overhead, but there is much more to learn about these systems, especially as the dangers from them grow.
Research into these airborne plumes of water vapor drawn from the tropical Pacific Ocean has grown exponentially in the three decades since “atmospheric rivers” got their name. But where a storm will make landfall can still be predicted by hundreds of miles, and it is difficult to predict how particular storms will play out.
Scientists are working to make sense of the complex interactions between the ocean, atmosphere and land, and hope to gain stronger insights into how, when and where storms strike.
“The more we learn, the more we recognize that we need more data on this,” said Maike Sonnewald, head of the climate and ocean computational group at UC Davis.
Sonnewald, an oceanographer who uses computer science to gain insights into the climate and long-range weather forecasts, said recent advances in the satellite age have helped paint a picture of how the ocean and atmosphere interact. . Metaphorically, that picture has too few pixels.
“We don’t necessarily have a high enough resolution to model specific things,” she said, explaining that the dynamic nature of the ocean — and how easily small shifts can cause big changes in the models — creates predictive challenges.
“The climate is changing – we’re making the world warmer – and we know that. The details are hard to identify,” Sonnewald said. A warming atmosphere can hold more water vapor and heated ocean surface temperatures will evaporate faster, so scientists can easily predict how things will get worse. It is more difficult to know when and where.
Within the storm systems
In the days leading up to the storm, it was clear it was going to pack a punch. As it got closer, officials had enough information to marshal resources and warn residents.
Related: Yes, the Los Angeles River is very full. But he’s just ‘doing his job’
Global models that scientists rely on to issue forecasts are good at “sniffing out the possibility of an impact storm at least several days in advance,” said Alex Lamers, warning coordination meteorologist for the Weather Prediction Center, but it doesn’t take he the details. shape until the storm is close enough.
“The details really don’t matter – the exact location where it’s crossing the coast, the mountain ranges that are affecting it, the angle that the winds are affecting the mountains and uplands,” he said. Lammers.
Satellites can only go so far to fill information gaps over the ocean. “The Pacific Ocean is a huge expanse and not many weather observations are made there,” 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 making measurements directly from inside the storm systems themselves.
Since 2016, the atmospheric river recon (AR recon) program has relied on US air force “hurricane hunter” planes that drop a small cluster of instruments, called dropsondes, that can transmit results as they drop through the clouds and into the ocean below. .
Fixed by a small parachute, each droplet floats through the clouds and into the ocean – a journey that takes about 20 minutes – while relaying important observations back to the scientists on board. The dropsondes collect air temperature, pressure, water vapor and wind speed, like an “MRI for an atmospheric river”, according to Ralph, which enables researchers to look inside the system rather than relying on satellite images.
During a series of strong atmospheric river storms that hit California in 2023, the dropsondes helped advance some heavy precipitation forecasts by about 12%, a feat that researchers believe would have taken an additional eight years using traditional data collection methods.
“If we get the atmospheric river wrong in the model – how strong it is, where it’s located, its structure, how much water there is – those errors will cause errors in the forecast over time in the future,” said Ralph , explaining the importance of going to a specific location of an atmospheric river to measure it.
Along with the parachute dropsondes, traditional weather balloons released from the ground during storms, called radiosondes, help to complete the picture. “They are very different approaches but both are needed to have the most effective forecasting system,” said Ralph.
The team is also using new technology, including a method called airborne radio occultation (ARO), invented by Scripps geophysicist and atmospheric scientist, Jennifer Haase. While dropsondes take measurements while falling vertically below the aircraft, ARO uses sensors attached to the side of an aircraft that take measurements horizontally. They can infer properties like humidity and temperature by measuring how much GPS signals change as they move through the atmosphere, helping scientists paint a more complete picture of an incoming storm.
The first ARO-equipped flights were deployed this winter, and were able to collect data up to 186 miles (300km) from an aircraft.
Mitigating the worst disasters
Flood risks are increasing in California and other parts of the arid American west, and more accurate intelligence will be needed to mitigate the worst disasters.
“Given the amount of warming we’ve seen so far, we expect large precipitation events to be about 10% more intense than before greenhouse gases were added to the atmosphere,” said Alex Hall, atmospheric physicist and scientist climate of UCLA. .
“The scary thing is, if you look into the future to the point where we have twice as much warming as we have today, you have events that are 20% more intense, and completely new classes of events that don’t even exist now. “
For Ralph, this reality is a call to arms.
“As those storms progress and change over time, our development of AR recon and related tools to measure and predict atmospheric rivers will be able to keep up with that,” he said. “This is really climate mitigation so people can better adapt to what’s happening.”
