Tracking atmospheric rivers could transform California reservoir levels

Atmospheric rivers can wreak havoc on the West Coast. These “rivers in the sky” stream enormous amounts of moisture from the tropics to western North America — double the flow of the Amazon River, on average.

This moisture can produce downpours that cause widespread flood damage. From 1978 to 2017, this damage amounted to $1.1 billion per year according to a 2022 study. But atmospheric rivers are also crucial for life in California.

“It’s where we get the water supply,” said Anna Wilson, field research manager for the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography.

Atmospheric rivers refill reservoirs and build up snowpack that steadily supplies the state with water. They have also ended about 30% to 40% of California droughts, but tracking these deluges has historically been challenging. Atmospheric river landfall forecasts can be off by hundreds of miles, uncertainty that confounds reservoir operators deciding whether to release water in anticipation of flooding.

The Atmospheric River Reconnaissance program aims to improve forecasts via a trio of aircraft, which have been collecting data since November. The information, collected from inside atmospheric rivers, contributes to mathematical models used in weather forecasts.

“You can’t really get this data any other way,” said John James, Yuba Water Agency’s director of resource planning, who is working to incorporate this data in water operations.

Taking samples from the sky

The AR Recon program started in 2016 and has expanded in scope ever since. This year’s campaign launched two months earlier than in previous years, a change driven by last year’s weather. In late October 2021, the Bay Area was drenched when a bomb cyclone tapped into a category 5 atmospheric river and produced record-breaking rain.

The program’s includes planes operated by the National Oceanic and Atmospheric Administration Aircraft Operations Center and the US Air Force Reserve 53rd Weather Reconnaissance Squadron.

NOAA’s Gulfstream IV is stationed in Honolulu, near the birthplace of atmospheric rivers like the “Pineapple Express,” which ferries moisture thousands of miles from Hawaii to California. After taking off, the plane can reach an altitude of 41,000 feet — a few miles above what a commercial jet flies — in just 20 minutes.

“By using the aircraft’s unique height and speed advantage, we can cover more ground more quickly, which translates into more data, which people can put to use more quickly,” said Captain Jason Mansour, a NOAA Corps officer and aircraft commander of the NOAA Gulfstream IV.

Master Sgt.  Chris Becvar, a loadmaster for the 53rd Weather Reconnaissance Squadron, opens a dropsonde tube in preparation to load it during a flight.

Master Sgt. Chris Becvar, a loadmaster for the 53rd Weather Reconnaissance Squadron, opens a dropsonde tube in preparation to load it during a flight.

Staff Sgt. Kristen Pittman/US Air Force

The aircraft looks like a business jet, but carries a suite of scientific instruments, including Doppler radar in the tail and a special weather radar in the nose, which led to a Muppet-inspired nickname.

“The nose cone on Gonzo is a little bigger and a little differently shaped than your average Gulfstream jet,” said James Carpenter, a flight director with the NOAA Aircraft Operations Center.

During flights, Carpenter monitors the radar and provides weather updates to the pilots, like Mansour. He also coordinates communication with the rest of the flight crew, a flurry of activity that lasts for the seven-and-a-half hours of a typical flight.

As the aircraft soars over atmospheric rivers, the crew repeatedly deploys dropsondes, scientific instruments that collect data as they descend by parachute through the top of the atmospheric river to the ocean below. As the tube-shaped instruments fall, they capture details about temperature, pressure, water vapor and winds. The data then feeds into the system used by global weather prediction models.

Capt.  Jason Mansour, the aircraft commander, and Lt.  Cmdr.  Rick DeTriquet at the controls of NOAA's Gulfstream IV during an atmospheric river mission.

Capt. Jason Mansour, the aircraft commander, and Lt. Cmdr. Rick DeTriquet at the controls of NOAA’s Gulfstream IV during an atmospheric river mission.

Lt. jg Nicolas Osborne/NOAA Corps

“Once we know how much moisture is in this particular system, we can then extrapolate and say, ‘Hey, based on these actual readings, this is what the West Coast can expect,’” Mansour said.

Improved data also addresses the uncertainty that forecasters face when trying to pinpoint where atmospheric rivers will make landfall and which areas will see the heaviest precipitation.

“Our data is helping to fine-tune that, so they can predict that better,” said Lt. Col. Ryan Rickert, a flight meteorologist with the 53rd Weather Reconnaissance Squadron of the Air Force Reserve Command.

Gulfstream IV, nicknamed Gonzo, takes off from Florida's Lakeland Linder Regional Airport, home of the NOAA Aircraft Operations Center.

Gulfstream IV, nicknamed Gonzo, takes off from Florida’s Lakeland Linder Regional Airport, home of the NOAA Aircraft Operations Center.

Lt. Kevin Doremus/NOAA

The Air Force’s WC-130J aircraft fly a tad lower, but still far above atmospheric rivers hovering within a mile or two of the ocean surface. Like their NOAA counterparts, the flight crews drop dropsondes that peek inside atmospheric rivers. The team also deployed 50 buoys in the Pacific Ocean in November — half off the West Coast and half northwest of Hawaii — to gather even more information about atmospheric rivers as they stream toward the western US

“Trying to fill observation gaps within and around atmospheric rivers over the ocean is the mission of AR Recon,” said Wilson.

Impacts on water supplies

The data from the Atmospheric River Reconnaissance program could also transform how officials operate California’s reservoirs.

“In the past, we really managed the reservoirs based on the calendar,” James said.

Reservoir operators follow a storage and release schedule based on past weather. But these operating procedures have generally remained unchanged since a dam was first constructed. In the case of New Bullards Bar Dam, that was over 50 years ago.

In the intervening decades, climate change has transformed weather patterns in the West. By adhering to schedules reflecting the world decades ago, reservoir operators may not be leaving enough space to contain a deluge from an extreme atmospheric river. Conversely, water may be needlessly released during a drought.

Capturing and storing water from atmospheric rivers will be more and more important as California increasingly swings between wet and dry conditions in a warming world. Incorporating weather forecasts, which have become more reliable since dams were constructed, could address these issues.

“We’re looking at ways to further optimize our reservoirs to both retain water when there are no storms in the forecast,” James said. “And release water when we see large storms in the forecast.”

The Yuba-Feather pilot program is a joint effort involving the Yuba Water Agency’s New Bullards Bar Reservoir and the California Department of Water Resources’ Lake Oroville Reservoir. The agencies are investigating how to utilize such forecasts in water supply operations.

The strategy has already proven successful at Lake Mendocino, according to a 2021 report. Using forecasts to inform operations increased the water supply at the reservoir by nearly 20% during Water Year 2020.

In order for such reservoir operations to be successful, forecasters can’t be unsure where atmospheric rivers will make landfall.

“If they don’t know the (atmospheric river) is going to hit smack dab in the reservoir, and then all of a sudden it does and they didn’t let any water out, now you’re talking about dams possibly breaking and all kinds of other huge impacts to people,” Rickert said.

The data from the AR Recon flights will help forecasters and reservoir operators be better prepared for whatever atmospheric rivers bring.

“It’s important for both sides of the story,” Rickert said.

Jack Lee (he/him) is a San Francisco Chronicle staff writer. Email: [email protected]