New Scale to Characterize Strength and Impacts of Atmospheric River Storms

  • by Will Holbert
  • Feb 6, 2019
  • Water News

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.

Atmospheric rivers (ARs) 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 % of levee breaches in California’s Central Valley are associated with landfalling atmospheric rivers.

The scale, described today in the February 2019 Bulletin of the American Meteorological Society, assigns five categories to atmospheric rivers 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.

Improving atmospheric river forecasting and using it to maximize water storage plays a central role in the concept behind Forecast Informed Reservoir Operations, or FIRO. A coalition of five ACWA member agencies are currently advocating for additional state funding to accelerate advances in FIRO research and capability.

“We’re on the verge of a historic transformation in understanding how atmospheric rivers can achieve multiple goals in water system operations, including drought relief, groundwater recharge and fisheries health,” said ACWA Executive Director Dave Eggerton. “This newest tool illustrates how we can better prepare for flood events, and maximize opportunities for extra storage.”

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. Unlike the hurricane scale, recently criticized for not representing adequately the impacts of slow-moving lower-category hurricanes, the atmospheric river scale builds in duration as a fundamental factor.

The scale ranks ARs as follows:
•    AR Cat 1 (Weak):  Primarily beneficial. For example, a Feb. 2, 2017 AR hit California, lasted 24 hours at the coast, and produced modest rainfall.
•    AR Cat 2 (Moderate): Mostly beneficial, but also somewhat hazardous. An atmospheric river on Nov. 19-20, 2016 hit Northern California, lasted 42 hours at the coast, and produced several inches of rain that helped replenish low reservoirs after a drought.
•    AR Cat 3 (Strong): Balance of beneficial and hazardous. An atmospheric river on Oct. 14-15, 2016 lasted 36 hours at the coast, produced 5-10 inches of rain that helped refill reservoirs after a drought, but also caused some rivers to rise to just below flood stage.
•    AR Cat 4 (Extreme): Mostly hazardous, but also beneficial. For example, an atmospheric river on Jan. 8-9, 2017 that persisted for 36 hours produced up to 14 inches of rain in the Sierra Nevada and caused at least a dozen rivers to reach flood stage.
•    AR Cat 5 (Exceptional): Primarily hazardous. For example, a Dec. 29 1996 to Jan. 2, 1997 atmospheric river lasted over 100 hours at the Central California coast. The associated heavy precipitation and runoff caused more than $1 billion in damages.

Ralph is considered a leading authority on atmospheric rivers, which were officially defined by the American Meteorological Society in 2017. Researchers have only begun to study atmospheric rivers in depth in the past two decades building on earlier research into extratropical cyclone structure and precipitation, especially in the United Kingdom. In that time, they have also come to understand how these events frequently make the difference between flood and drought years in key coastal regions around the world such as California.

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.

“This scale enables improved awareness of the potential benefit versus hazard of a forecast AR,” said co-author Michael Anderson of the California Department of Water Resources. “It can serve as a focal point for discussion between water managers, emergency response personnel and the research community as these key water supply and flood inducing storms continue to evolve in a changing climate.”

“Forecasters in the western U.S. have been using the concept of ARs in their forecasting for a few years now, and many have been looking for a way to distinguish beneficial from hazardous AR storms,” said Rutz.  “The scale was designed partly to meet this need, and it is anticipated that it will be used extensively.”

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