November 8, 2013
Atmospheric River Storms Could Help Predict Winter Snowfall
Lee Rannals for redOrbit.com - Your Universe Online
A new study says “atmospheric river” storms from the Pacific Ocean could eventually help scientists predict major winter snowfalls.Atmospheric rivers are short-lived wind tunnels that carry water vapor from tropical oceans to mid-latitude land areas. They are known to bring large quantities of rain and snow on California’s Sierra Nevada Mountains.
Researchers studied how two of the most common atmospheric circulation patterns in the Northern Hemisphere interact with atmospheric rivers. They found when those patterns line up in a certain way, they create a virtual freeway that leads the winds straight to the Sierras.
When an atmospheric river crosses the warm tropics in the Pacific, it becomes laden with water vapor. A moderate-sized atmospheric river contains as much water as the Mississippi River dumps into the Gulf of Mexico in an average week. Once the river comes ashore and stalls, the water falls as snow or rain.
"Atmospheric rivers are the bridge between climate and West Coast snow,” Bin Guan of the Joint Institute for Regional Earth System Science and Engineering, said in a statement. "If scientists can predict these atmospheric patterns with reasonable lead times, we'll have a better understanding of water availability and flooding in the region.”
Having a better scientific understanding of atmospheric rivers would help improve flood prediction. A single California atmospheric-river storm in 1999 caused 15 deaths and $570 million in damage.
Researchers used data from the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite, along with NOAA satellite data and snowpack data from the California Department of Water Resources. They looked at the snowy winter of 2010 through 2011 for the study, during which 20 atmospheric rivers made landfall.
The scientists compared the dates of the events in the study with the phases of the Arctic Oscillation (AO) and the Pacific/North American (PNA) teleconnection. These weather patterns stretch thousands of miles across the atmosphere and shape the climate of the mid-latitudes. Each pattern affects a different part of the Northern Hemisphere by seesawing between phases of lower-than-average and higher-than-average air pressure over various parts of the globe.
The team charted these phases throughout the winter of 2010 and 2011, and then looked at the period between 1998 to 2011. They found similar correspondence, with more atmospheric rivers occurring when both patterns were negative.
Guan said in the double-negative periods the high- and low-pressure systems associated with that phase in each pattern mesh to create a lingering atmospheric low-pressure system just northwest of California.
"I looked at 50 years of atmospheric data. Only five months had those phases of the PNA and AO occurring together for more than 15 days of the month," he said in a statement.