Atmospheric rivers, or ARs, are narrow regions in the atmosphere that transport large amounts of water vapor across the Pacific and other regions. In one day, an average AR transports an amount of water vapor equivalent to a foot of liquid water covering 10 million acres — an area roughly the size of Maryland. This is about seven times the average daily flow of water from the Mississippi River into the Gulf of Mexico. Only a portion of the water vapor transported by an AR is transformed into rain or snow; for example, about 20-40 percent in one AR crossing northern California makes it to the surface.
The importance of ARs was recently highlighted in a major emergency preparedness scenario led by the U.S. Geological Survey that focused on the possibility of a series of strong ARs striking California. That scenario showed that resulting flooding, wind and even mudslides could exceed damages brought on by Hurricane Katrina in 2005.
Results from WISPAR will include demonstration of new technology, contributions to the science of ARs, and, through collaborations with NOAA’s Winter Storms Reconnaissance Program — started by the National Weather Service in 1999 to collect observations to improve winter storm forecasts — the potential of offshore monitoring of ARs to aid in weather predictions.
While ARs are responsible for great quantities of rain that can produce flooding, they also contribute to beneficial increases in snowpack. A series of atmospheric rivers fueled the strong winter storms that battered the U.S. West Coast from western Washington to southern California from Dec. 10-22, 2010, producing 11 to 25 inches of rain in certain areas. The ARs also contributed to the snowpack in the Sierras, which received 75 percent of its annual snow by Dec. 22, the first full day of winter.
“We need to know more so we can better predict the exact timing, location and amounts of precipitation,” said Marty Ralph, head of the Water Cycle Branch at NOAA’s Earth System Research Laboratory in Boulder, Colo. “Although we have been studying atmospheric rivers since 2003, there are still things to be learned about the fate of ARs in a changing climate, such as do they get stronger or weaker, do they occur more or less frequently?”
NASA’s Global Hawk, which is operated by NASA’s Dryden Flight Research Center in southern California, will be equipped with sensors including an advanced water vapor sensor — the high-altitude monolithic microwave integrated circuit sounding radiometer, or HAMSR — created by NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and a new dropsonde funded by NOAA and developed by the National Center for Atmospheric Research in Boulder, Colo. The dropsondes will be launched from the Global Hawk and take temperature, wind and other readings as they descend through an AR.
Unmanned aircraft are being used in a variety of scientific studies because they are able to fly long distances, stay aloft for more than 24 hours and can travel at high and low altitudes that could be dangerous for humans.
Because microwave satellite techniques that monitor ARs over the oceans do not work well over land and no direct wind measurements are taken in ARs over the ocean, NOAA researchers are installing an observing network across California that will help monitor ARs as they strike the coast and move inland. The system will measure both winds and water vapor. Initial sites in this network provide data that can be compared with forecast models. This will allow for adjustments to model predictions of the strength and position of the ARs, conditions that are crucial to determining when and where the most extreme precipitation will occur.
NOAA’s Hydrometeorology Testbed has helped advance understanding of ARs and developed tools for use in their monitoring and prediction. Results from the Testbed are being used to implement a permanent network of modern observations across California jointly with the California Department of Water Resources and Scripps Institution of Oceanography. A related project with the California Energy Commission and Scripps is studying ARs in a changing climate. Each project derives benefit from the unique observations from WISPAR.
The California Department of Water Resources project, which focuses on enhanced flood response and emergency preparedness, also involves atmospheric and hydrologic modeling, specialized display systems and the development of decision support tools. Building on lessons learned by the pilot project in California, a smaller but similar set of instrumentation has also been installed in western Washington to aid forecasters in dealing with AR-driven extreme events in that region.
No comments:
Post a Comment