Atmospheric River Research Gathers Steam

A pattern that channels atmospheric moisture into “rivers” of water vapor before making landfall is the subject of a recent article co-authored by a researcher at Scripps Institution of Oceanography, UC San Diego.

The atmospheric river phenomenon, one variant of which is popularly termed the “pineapple express,” is also working its way into the flood management plans of officials in California and other states on the West Coast. Mike Dettinger, a climate researcher with a joint appointment at Scripps and the U.S. Geological Survey, is the co-author of an August cover story about atmospheric rivers that appeared in Eos, a news publication issued by the American Geophysical Union. He said the phenomenon had gone largely unrecognized by science until the 1990s, when a confluence of analysis and technological advancements gave it shape.

“Since we’ve been studying them in the past 13 years, we’ve seen that atmospheric rivers, now often called ‘ARs,’ have been the major engine moving water vapor around the world outside the tropics,” said Dettinger.

Dettinger and co-author Marty Ralph of NOAA’s Physical Sciences Division in Boulder Colorado said that raising awareness of atmospheric rivers is imperative for improving flood management plans around the country, as well as to improving extreme precipitation forecasts and flood warnings in key areas.  They estimate that as much as 95 percent of vapor transport concentrates in filament-like atmospheric flows covering less than ten percent of the planet’s circumference outside tropical latitudes. The vaporized water carried by the wind in these narrow regions is equivalent to the amount of liquid water carried by seven to 15 Mississippi Rivers combined.

Atmospheric rivers, some of which form over the Pacific Ocean, and periods of low atmospheric pressure have combined throughout history to create record-breaking rainstorms and widespread flooding in California.  A similar Gulf of Mexico flow, sometimes called the “Mayan express,” was recently shown to have been the source of catastrophic flooding in Nashville, Tenn. in May 2010.

ARs were not recognized as a phenomenon until a team at MIT analyzed numerical weather models and noticed that water vapor around the world tended to move in narrow bands. Roughly that same year, satellite microwave radar technology advanced to the point where the rivers, which had been virtually invisible to previous satellite instruments, suddenly took clear shape.

Dettinger said other technological advancements have now made preparing for the effects of the next major river much more possible. With atmospheric rivers in mind, the California Department of Water Resources, NOAA, and Scripps are installing more than 30 soil moisture stations around the state, upgrading another 36 earthquake monitoring sites to also measure water vapor concentrations aloft in the atmosphere, and ten radar units that can track the snow level aloft during winter storms in real time. The “snow-level radars” run about $40,000 each and are about the size of large pizza pans, noted Dettinger, who added that they replaced much larger units that cost about 10 times as much.

Further phases of the California program known as Enhanced Flood Response and Emergency Preparedness (EFREP) will provide “atmospheric-river observatories” with modern observing methods including sophisticated upward-looking “wind profiling” radar that can monitor atmospheric rivers when they make landfall. Drones like those whose use was pioneered by the U.S. military, combined with manned aircraft, could in the future carry out weather reconnaissance missions that run the length of the Pacific.

The Eos article describes a number of scientific studies and projects applying AR findings that have emerged recently with a focus on this subject, said Ralph.

 “Between the development of better scientific understanding and the invention of new monitoring and prediction tools, ARs are becoming a well-recognized phenomenon that is to extreme weather and flooding for the U.S. West Coast states, what hurricanes are to the southeast and eastern U.S,” Ralph said.

Dettinger said that work will also be needed to be done to engage some mainstream meteorologists, whose classical training did not include information on rivers in the sky.  This has been addressed recently through formal training of National Weather Service forecasters as part of NOAA’s Hydrometeorology Testbed (HMT).

Scientists are striving “to get traditional meteorologists to see that these things are absolutely critical to understanding major winter storms outside the tropics,” Dettinger said.