Atmospheric River Observatories
Atmospheric Rivers (ARs) are the regions of extratropical storms where high winds and water vapor are concentrated. These ingredients can combine to produce heavy rainfall upon landfall, especially over mountainous terrain. Most flooding events that occur along the west coast of North America in winter are associated with the landfall of AR conditions. ARs are visible in satellite images over the oceans, but their impact over land is not captured well by NOAA's current operational observing system.
Based on successes in the NOAA Hydrometeorology Testbed (HMT), the California Department of Water Resources and U.S. Department of Energy have invested in a "picket fence" of seven (7) long term coastal AROs ranging from Southern California to near the Canadian border.
In 2017, due to the success of the coastal AROs, California invested in the addition of two inland AROs: one to monitor moisture transport through the gap in coastal terrain existing in the San Francisco Bay area, and a second to monitor AR conditions near Lake Oroville.
About the AROs
Six (6) AROs are sited in California: Bodega Bay, Mckinleyville, Oroville, Point Sur, Santa Barbara, and Twitchell Island. Two (2) are sited in Oregon: Astoria and North Bend. One (1) is sited in Forks, Washington.
Each ARO monitoring station includes an advanced suite of instrumentation to monitor AR conditions and evaluate model forecast performance in real time, advance understanding of key physical processes associated with ARs, and guide future model development aimed at improving AR forecasts.
These instruments include:
- a Doppler wind profiler for measuring wind profiles and snow level aloft,
- a Global Positioning System receiver for measuring the column-integrated water vapor concentration, and
- a tower to measure meteorological conditions at the surface.
In addition to the AROs, PSL operates instrumentation at a number of other sites to study the microphysics of the precipitation and other characteristics of ARs. This Atmospheric River Observation Network consists of 57 total sites throughout California, Oregon, and Washington.
The combined ARO and ancillary site observations provide a more complete picture of ARs, helping researchers better understand the characteristics leading to extreme AR events that produce flooding, debris flows, and other negative impacts versus more benign AR events that replenish water supplies without adverse impacts.
Research improvements
Using satellite data in conjunction with observations from the AROs, PSL researchers have developed scientifically-based AR thresholds for water vapor content and upslope wind speed that are used to identify landfalling ARs. These thresholds are part of an AR water vapor flux tool developed by PSL along with partners from NOAA's Global Systems Laboratory and the National Weather Service San Francisco/Bay Area Weather Forecast Office.
The display from this award-winning tool allows forecasters to monitor the atmospheric forcings associated with ARs and to evaluate the performance of a weather forecast model's prediction of those forcings. The tool has been implemented at several sites in California, Florida, Mississippi, North Carolina, Oregon, and Washington.
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