Understanding science behind atmospheric rivers
An atmospheric river, or AR — an increasingly popular term in the world of weather — is a band of water vapor that develops over the ocean and is wrung out when it makes landfall. How potent are they?
The strongest atmospheric rivers carry as much water, in its gaseous form, as a Category 5 hurricane. A handful of atmospheric rivers can deliver half of California’s rainfall in a matter of days.
While vital to water supplies, atmospheric rivers can also lead to disaster. Dozens of atmospheric rivers hit California during the winter of 2016-17, causing flooding in San Jose that inundated hundreds of homes and leading to spillway damage at Oroville Dam that prompted officials to evacuate nearly 200,000 people downstream. What causes them?
Atmospheric rivers are formed when water vapor concentrates in the atmosphere, generally in the tropics or subtropics, and strong winds push the plumes across the ocean. As the name suggests, these atmospheric channels are long and narrow, like a river in the sky.
When an atmospheric river crosses land — and especially mountains — the water-laden air is forced upward, which cools the vapor and makes it heavier. The vapor then falls as rain or snow. Some of the systems are thousands of miles long, and result in days of precipitation.
Atmospheric rivers that hit California often originate around Hawaii, hence the nickname “Pineapple Express.” They have been known to contribute to major storms on the East Coast and in Europe as well. Why have we started hearing more about ARs?
The term atmospheric river was coined in 1998 by two MIT climate researchers, though scientists had earlier described conveyer belt-like water columns in the sky.
Only with evolving satellite observations in the 2000s, however, were the systems extensively studied and incorporated into popular speech. Can we forecast ARs?
Meteorologists can anticipate the systems as much as a week out. They’ve also improved their estimates of how much precipitation will fall and where.
Several Bay Area water agencies are partnering with the National Weather Service and university climate experts to install a region-wide radar system that will better monitor atmospheric rivers.
The Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography is also working to develop a rating system for the storms.