Photo of ARO on Twitchell Island, at the east end of the San Francisco Bay Delta. (Credit: Clark King, NOAA)
Photo of ARO on Twitchell Island, at the east end of the San Francisco Bay Delta. (Credit: Clark King, NOAA)
Atmospheric Rivers (ARs) are the regions of extratropical storms where high
winds and water vapor are concentrated. These factors 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. To help fill
this important observing gap, scientists from ESRL's Physical Sciences
Division (PSL) developed the concept of an atmospheric river observatory
(ARO). Each ARO monitoring station includes a Doppler wind profiler for
measuring wind profiles and snow level aloft and a Global Positioning
System receiver for measuring the column-integrated water vapor
concentration. These are the primary variables in ARs that determine
whether an AR event produces a flood. In addition, the ARO consists of an
S-band precipitation profiling radar (S-PROF) and surface-based
disdrometers to study the microphysics of the precipitation, along with a
meteorological tower to monitor AR conditions near the surface.
Using satellite data in conjunction with observations from the AROs, PSL
researchers have developed scientifically-based AR thesholds 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 ESRL's Global Systems Laboratory and the
National Weather Service San Francisco/Bay Area Weather Forecast Office.
The award-winning display from this 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 ARO stations in California, Florida, Mississippi, North Carolina, Oregon, and Washington
since 2008. Based on successes in HMT, the California Department of Water Resources and U.S.
Department of Energy have invested in a "picket fence" of seven long term coastal AROs ranging
from Southern California to near the Canadian border. Recently California added 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.
