Environment & Science

Rivers of water in the air provide much of California's rain

An atmospheric river reaches the San Francisco Bay Area, Dec. 11, 2014.
An atmospheric river reaches the San Francisco Bay Area, Dec. 11, 2014.
University of Wisconsin

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Atmospheric rivers are bands of warm temperatures and strong winds that push large amounts of water vapor, often for thousands of miles. Scientists from government and university research institutions have deployed a force of research aircraft and an oceangoing vessel to better understand the phenomenon and how it results in precipitation for California. 

Scientists said that too little is known about atmospheric rivers and that better understanding would aid meteorologists' ability to predict weather. 

“We don’t understand enough about how atmospheric rivers transport water vapor and how the water vapor comes together in them. We also don’t know enough about how aerosols can change the amount of precipitation that can come out of an atmospheric river when it hits shore," said Marty Ralph, a research meteorologist and director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography. 

The need to understand atmospheric rivers is made apparent by the realization of how much water vapor exists in one at a given time. Ralph said that early estimates of the water vapor in atmospheric rivers put the equivalent at 10 times the amount of water emptied into the Gulf of Mexico by the Mississippi River. He said refined estimates have put that amount even higher.

“We had thought it was 10 Mississippis, as an average value for atmospheric rivers, but the average of cases we have now is 20 Mississippis,” Ralph said. "A handful of atmospheric rivers each year, provide about 30-50 percent of all the precipitation in Northern California.” 

Over the next month, four research aircraft from the National Oceanic and Atmospheric Administration, the Department of Energy and NASA will fly into atmospheric rivers hundreds of miles off the coast of California, taking a host of readings. A NOAA ship will also take measurements as part of the coordinated effort. 

“This experiment will double the number of samples we have, so we’ll have a much more accurate measure of how strong the atmospheric rivers are,” Ralph said.

Among other things, the project, titled CalWater 2015, will explore the role of air particles, natural or human induced, in precipitation from atmospheric rivers. Research has indicated that some pollution may inhibit precipitation by keeping large water droplets from forming at the bottoms of clouds. However, researchers said more work needs to be done to better understand potential human impacts on rain and snowfall. 

"We are only scratching the surface in terms of how aerosols influence the precipitation, because aerosols are of so many different types and so many different sizes,” said Ryan Spackman, flight operations scientist for CalWater 2015. “Every single water droplet or ice crystal, for instance snow, that falls from the sky has an aerosol particle at its center, so aerosols are key to understanding how much is going to fall, where it’s going to fall, how distributed the precipitation is, which is crucial for water supply understanding." 

Spackman said better understanding will allow water managers to anticipate where and when precipitation will fall, thus allowing them to make decisions to maximize preparedness. 

"In the state of California, there’s tremendous amount of variability in when that precipitation falls. It’s feast or famine, and that’s a very challenging problem for water managers – how much water to keep, when to let it go, and are we going to have enough," Spackman said. "This research is aimed at improving our abilities to forecast these events.”