CA - Navigating the Storm: Atmospheric Rivers Now Ranked Like Hurricanes
A study has shown that atmospheric rivers, bands of water vapor intensified by climate change, can be ranked on a new intensity scale, similar to hurricanes. The research mapped global patterns of these events over 40 years, identifying hotspots for the most intense atmospheric rivers.
The findings will aid meteorologists and city planners in predicting and preparing for these potentially damaging weather events.
Atmospheric rivers, which are long, narrow bands of water vapor, are becoming more intense and frequent with climate change. A new study demonstrates that a recently developed scale for atmospheric river intensity (akin to the hurricane scale) can be used to rank atmospheric rivers and identify hotspots of the most intense atmospheric rivers not only along the U.S. West Coast but also worldwide.
Atmospheric rivers typically form when warm temperatures create moist packets of air, which strong winds then transport across the ocean; some make landfall. The intensity scale ranks these atmospheric rivers from AR-1 to AR-5 (with AR-5 being the most intense) based on how long they last and how much moisture they transport.
In part because some West Coast weather outlets are using the intensity scale, “atmospheric river” is no longer an obscure meteorological term but brings sharply to mind unending rain and dangerous flooding, the authors said. The string of atmospheric rivers that hit California in December and January, for instance, at times reached AR-4. Earlier in 2022, the atmospheric river that contributed to disastrous flooding in Pakistan was an AR-5, the most damaging, most intense atmospheric river rating.
The intensity of an atmospheric river depends on how long it lasts (typically 24 to 72 hours; horizontal axis) and how much moisture it moves over one meter each second (measured in kilograms per meter per second; vertical axis). While weaker atmospheric rivers can deliver much-needed rain, more intense storms are more damaging and dangerous than helpful. Credit: AGU, after Ralph et al. (2019).
The scale helps communities know whether an atmospheric river will bring benefit or cause chaos: The storms can deliver much-needed rain or snow, but if they’re too intense, they can cause flooding, landslides, and power outages, as California and Pakistan experienced. The most severe atmospheric rivers can cause hundreds of millions of dollars of damage in days in the western U.S.; damage in other regions has yet to be comprehensively assessed.
“Atmospheric rivers are the hurricanes of the West Coast when it comes to the public’s situational awareness,” said F. Martin Ralph, an atmospheric scientist at Scripps Institution of Oceanography at UC San Diego and a co-author on the new study. People need to know when they’re coming, have a sense for how extreme the storm will be, and know how to prepare, he said. “This scale is designed to help answer all those questions.”
Ralph and his colleagues originally developed the scale for the U.S. West Coast. The new study demonstrates that atmospheric river events can be directly compared globally using the intensity scale, which is how the researchers identified where the most intense events (AR-5) form and fizzle out, and how many of those make landfall.