Submitted via IRC for Bytram
New scale to characterize strength and impacts of atmospheric river storms
A team of researchers led by Scripps Institution of Oceanography at the University of California San Diego has created a scale to characterize the strength and impacts of "atmospheric rivers," long narrow bands of atmospheric water vapor pushed along by strong winds. They are prevalent over the Pacific Ocean and can deliver to the Western United States much of its precipitation during just a few individual winter storms.
They are the source of most of the West Coast's heaviest rains and floods, and are a main contributor to water supply. For example, roughly, 80 percent of levee breaches in California's Central Valley are associated with landfalling atmospheric rivers.
The scale, described Thursday in the February 2019 Bulletin of the American Meteorological Society, assigns five categories to atmospheric rivers (ARs) using as criteria the amount of water vapor they carry and their duration in a given location. The intention of the scale is to describe a range of scenarios that can prove beneficial or hazardous based on the strength of atmospheric rivers.
The scale was developed by F. Martin Ralph, director of the Center for Western Water and Weather Extremes (CW3E) at Scripps, in collaboration with Jonathan Rutz from the National Weather Service and several other experts. It ranks atmospheric rivers from 1 to 5 and creates the categories "weak," "moderate," "strong," "extreme," and "exceptional." It uses amounts of water vapor within an atmospheric river as its basis and a period of 24 to 48 hours as its standard measurement of duration. When an AR lasts in an area for less than 24 hours, it is demoted by one category, but if it lingers for more than 48 hours, it is promoted. This approach is based on research showing that a combination of strong water vapor transport with long duration over a location, is what causes the greatest impacts. Unlike the hurricane scale, recently criticized for not representing adequately the impacts of slow-moving lower-category hurricanes, the AR scale builds in duration as a fundamental factor.
[...] Ralph said that the scale could provide a crucial tool to officials with an operational need to assess flood potential in their jurisdictions before storms strike. Unlike other scales that focus primarily on damage potential, such as the Fujita scale for tornadoes or the Saffir-Simpson scale for hurricanes, the atmospheric river scale accounts not only for storms that can prove hazardous, but also for storms that can provide benefits to water supply.
"The scale recognizes that weak ARs are often mostly beneficial because they can enhance water supply and snow pack, while stronger ARs can become mostly hazardous, for example if they strike an area with conditions that enhance vulnerability, such as burn scars, or already wet conditions," say Ralph and co-authors in the paper appearing today in the February 2019 issue of the Bulletin of the American Meteorological Society. "Extended durations can enhance impacts."
F. Martin Ralph, Jonathan J. Rutz, Jason M. Cordeira, Michael Dettinger, Michael Anderson, David Reynolds, Lawrence J. Schick, Chris Smallcomb. A Scale to Characterize the Strength and Impacts of Atmospheric Rivers. Bulletin of the American Meteorological Society, 2019; DOI: 10.1175/BAMS-D-18-0023.1
