Atmospheric rivers are wet and windy by nature—they move vast amounts of water vapor through the sky in long, narrow bands. These features, such as the recurring Pineapple Express events that carry moisture from around Hawaii to the West Coast of the United States, often supply needed precipitation, but they can also bring flooding rains and wind damage. Seemingly similar atmospheric rivers, though, can result in very different amounts of precipitation and wind on land, puzzling scientists and complicating forecasting efforts.
In new research looking at data from the past few decades, scientists found that each atmospheric river comes in one of four “flavors”—wet, windy, wet and windy, or neutral—depending on whether it is moisture or wind dominated. The new classification scheme should help researchers discover new insights into how atmospheric rivers affect weather on the U.S. West Coast and elsewhere, according to Katerina Gonzales, a climate scientist and graduate student at Stanford University.
“Different flavors of atmospheric rivers have different impacts,” said Gonzales, who presented the research at AGU’s Fall Meeting 2019 in San Francisco, Calif. This realization “could give us better clues for what we’re going to get from future storms and climate change.”
West Coast Flavors
Atmospheric rivers are responsible for 30%–40%, on average, of the annual precipitation on the U.S. West Coast, with most of this coming from only a few events. Some strong atmospheric rivers carry up to 15 times as much water as the average flow at the mouth of the Mississippi River. The frequency of atmospheric river events is expected to increase with climate change, according to the U.S. Global Change Research Program’s Fourth National Climate Assessment, released in 2017.
Individual atmospheric river events are generally characterized using a metric called integrated vapor transport (IVT), which accounts for water volumes and winds to assess the total amount of water vapor moving through the atmosphere in a river. But it is not uncommon for storms with similar IVT values to have different impacts on land in terms of precipitation and wind speeds, which could be the difference between muggy rains and dry, blustering gales.
The researchers wanted to find a better way of classifying atmospheric rivers to understand their diverse behavior. “You need two ingredients to get moisture transport,” Gonzales said. “We’re losing that nuance when we only look at IVT of atmospheric rivers.”
Gonzales and her team analyzed previously published IVT data for every day that the West Coast experienced an atmospheric river between 1980 and 2015. They then parsed the IVT values into moisture and wind components. When the researchers plotted those components against one another, they found that they could categorize atmospheric rivers with similar IVT values as having high moisture and low wind (wet), low moisture and high wind (windy), high moisture and high wind (wet and windy), or average levels of each (neutral).
The researchers also found that “windy” atmospheric rivers not only showed larger extremes in surface wind speeds but also resulted in higher average precipitation both in the Pacific Northwest and in California.
Gonzales said she was surprised by the result at first. “I would have expected the moisture-dominant atmospheric rivers to [result in] more precipitation,” Gonzales said. But she now thinks that “wet” storms may actually be “wind limited” and that “moisture is allowed to see its full potential” as precipitation in wind-dominant atmospheric rivers when clouds are forcibly blown against coastal mountain ranges, causing the moisture to condense into rain or snow.
Ingredients for Improving Storm Forecasts
Researchers are realizing that IVT alone is no longer enough to understand the behavior of atmospheric rivers, said Gang Chen, an atmospheric scientist at the University of California, Los Angeles, who was not involved with the new research. “This study in particular tries to separate the effects of moisture and wind…and I think this separation could be useful” in providing “a better way to look into the total effect” of atmospheric river events, Chen said.
Gonzales and her colleagues hope they can use their classifications to forecast the flavors of future atmospheric river events. Preliminary data suggest that the windy atmospheric rivers that tend to dump more rain are associated with deep troughs of low-pressure systems offshore, but further research is required to build predictive models. She said she imagines a future in which water managers and public safety officials can be warned further in advance of when such storms are expected to hit land.
—Ariana Remmel (@science_ari), Science Communication Graduate Student, University of California, Santa Cruz