As fires rage, California centre rushes to understand their remedy: Atmospheric rivers, US
At the US Centre for Western Weather and Water Extremes, researchers feel the urgency as they examine connections between West Coast precipitation and a devastating wildfire season, which has yet to conclude. The centre, part of Scripps Institution of Oceanography in La Jolla, California, has unlocked many secrets of atmospheric rivers; airborne jets of tropical moisture that can break droughts and quell fires but also unleash raging floodwaters. Under the leadership of Director Marty Ralph, the centre measures the strength of atmospheric rivers on a one-to-five scale and there is now a greater understanding of how their presence or absence determines so much, such as whether vegetative fuels will be primed to ignite when fire comes too close. “It’s been an exciting time,” Ralph said. “I mean, there’s so much potential for better weather prediction on the West Coast … to impact decisions that matter.” But looming in the background of this important research are the reports from Ralph’s friends and colleagues across the West who have been severely affected by the siege of blazes. “It’s been tragic,” he said. Climate scientist Dan Cayan, who works with Ralph, added, “The [fires’] different impacts and symptoms certainly keep us in touch with reality, and underpin the need for the sort of work we do.” California has seen its worst wildfire season on record in 2020, with about 4,2 million acres burned, more than double the acreage in the previous record-breaking year.
The work done by the centre has revealed just how crucial atmospheric rivers are to California, which is both the most populous US state and the country’s top agricultural producer. These phenomena deliver 25 to 50 percent of the water supply in key areas, with tens of billions of dollars in annual benefits. But atmospheric rivers also contribute to more than 90 percent of the region’s major flood events, at an average cost of $1 billion per year.
“It's just remarkable how that one concept can explain a lot of the action in the Western United States,” Cayan said.
With 30 September 2020 marking the final day of the 2019-20 water year, a look at recent history turns up evidence of the ties between atmospheric rivers and ripe conditions for wildfire.
During the 2018-19 water year, California saw the landfall of 36 atmospheric rivers, with seven of those considered strong or greater (Category 3 and above). But in the 2020 water year, the Golden State saw 20 percent fewer atmospheric rivers (29) and there was just one strong atmospheric river event.
For the entire West Coast, the 2019 water year experienced 41 atmospheric rivers, 11 of them strong, while the water year 2020 experienced 40, with seven strong.
The effect of less atmospheric river activity can be found in California’s water year 2020 rainfall totals, if not the dried-out fuel for its fires. Water years provide a better measurement of precipitation than do calendar years because the October-September span begins when fall does rather than starting up in early winter.
San Diego, thanks to a once-in-a-decade rainy spring, overshot its annual average rainfall of 10,3 inches by nearly one-third. In downtown Los Angeles, this year’s total nearly bull’s eyed the average of a little under 15 inches, the product of extreme swings between dry and wet.
To the north, though, the rain gauges at the airports of San Francisco, San Jose, Oakland and Napa went empty far more than usual: All received half or even less of their climatological averages. Only slightly better, yet still well under the norm, was the annual precipitation in Eureka, Modesto, Redding, Sacramento and Stockton.
This prolonged dry stretch, never before in San Francisco’s recorded weather history had it endured a rainless February, meant that Northern California’s forests didn’t get the moisture needed to dampen potential fuel.
Ralph noted that the centre’s research has determined that when it comes to the effects of rain and snow on fire season, the question is not just how much but also when. “If we get a lot of precipitation in the winter, it can grow a lot of undergrowth, which creates more fuel for the fire, potentially. And then in the fall, if we get good storms early enough, they can really reduce the fire risk and shorten the fire season,” Ralph said.
“But if the productive storms don’t show up early enough or they are delayed, the fire season is extended; things get drier and the risks go up.” That’s been the case late this season in Southern and Central California, which have endured periods of dangerous fire weather into December.
Does the past hold any hope for the fires soon to be doused by an atmospheric river? It’s a definite maybe. Meteorology researcher Chad Hecht said the first event of the season is not always a big producer of precipitation. Moreover, the winter outlook from the National Weather Service calls for below-average precipitation in the southern half of California and for drought conditions to worsen.
Of course, it’s a mixed blessing when torrential rain falls on fire-scorched earth, as dangerous mudslides can ensue.
Ralph said that the centre has become the hub for atmospheric river knowledge by “building a critical mass of scientists and engineers and applications, experts working together in a focused way” on an issue that hadn’t been studied in depth. The effects of atmospheric rivers in California had been recorded for decades, and past researchers had made a few stabs at formal inquiry, but only now are they being probed from angles ranging from their responsibility for Bay Area landslides to their presence in the Antarctic.
Said Ralph, “It doesn’t take a scientist to realize if we have a better sense of what’s coming in the next day or a few days or even months, there’s so many things that can be done to take advantage of that information, whether it’s to anticipate the end of fire season … to understanding whether the storm that just filled your reservoir will be followed by a dry period that won’t create a flood risk.”
Source: The Washington Post