Explainer: What are atmospheric rivers and bomb cyclones?

Rainstorms slam northern California
View of the Golden Gate Bridge in San Francisco as rainstorms known as "atmospheric river" slam northern California, U.S., January 5, 2023. REUTERS/Carlos Barria

Jan 5 (Reuters) - Atmospheric rivers are storms akin to rivers in the sky that dump massive amounts of rain and can cause flooding, trigger mudslides and result in loss of life and enormous property damage.

One such storm — along with a bomb cyclone — was battering California on Thursday, killing a child and knocking out power to tens of thousands.

Atmospheric rivers can carry up to 15 times the volume of the Mississippi River, according to the National Oceanic and Atmospheric Administration. They appear as a trail of wispy clouds that can stretch up to hundreds of miles.

This is the third atmospheric river storm, and the strongest, to hit California since last week, according to meteorologist Bob Oravec of the National Weather Service.

Research predicts that climate change will cause atmospheric rivers to become 25% longer and 25% wider and to carry more water. This could make managing water supply much harder as moderate atmospheric rivers, which can be beneficial for water supplies, will be less frequent, and strong ones could become more calamitous.

Bomb cyclones are often associated with atmospheric rivers and typically form in winter when cold and warm air masses collide. Also called "explosive cyclogenesis” or a “weather bomb", a bomb cyclone is a low-pressure system that experiences a fall in pressure of 24 millibars in 24 hours.

Carl Schreck, a weather scientist at North Carolina State University, has said atmospheric rivers can supercharge a cyclone's engine.

Scientists say warmer air and water temperatures from climate change can lead to more evaporation for storms to absorb and dump.

Massive storms are increasingly deluging California followed by periods of drought that increase the risk of wildfires.

Amid the cycles of wet and dry — both phenomena exacerbated by climate change — the state is experimenting with capturing the megastorm floods to ease destructive droughts.

The projects, known as a recharge system, turn unused fields into large ponds to hold water so that it can percolate into the porous rock and earth below, creating or restoring an aquifer rather than rushing to the sea.

The captured runoff is stored in underground aquifers. Unlike damming and diverting rivers, this system does not damage the environment.

In 2019, an atmospheric river nicknamed the "Pineapple Express" hit California. The water vapor from near Hawaii brought rain and triggered mudslides that forced motorists to swim for their lives and sent homes sliding downhill.

Reporting by Lisa Shumaker in Nashville and Timothy Gardner in Washington; Editing by Josie Kao

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Lisa's journalism career spans two decades, and she currently serves as the Americas Day Editor for the Global News Desk. She played a pivotal role in tracking the COVID pandemic and leading initiatives in speed, headline writing and multimedia. She has worked closely with the finance and company news teams on major stories, such as the departures of Twitter CEO Jack Dorsey and Amazon’s Jeff Bezos and significant developments at Apple, Alphabet, Facebook and Tesla. Her dedication and hard work have been recognized with the 2010 Desk Editor of the Year award and a Journalist of the Year nomination in 2020. Lisa is passionate about visual and long-form storytelling. She holds a degree in both psychology and journalism from Penn State University.

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Timothy reports on energy and environment policy and is based in Washington, D.C. His coverage ranges from the latest in nuclear power, to environment regulations, to U.S. sanctions and geopolitics. He has been a member of three teams in the past two years that have won Reuters best journalism of the year awards. As a cyclist he is happiest outside. Contact: +1 202-380-8348