Study findings aid forecasters, water managers
The famed Pineapple Express touted by skiers in the Western U.S. may not be all it’s cracked up to be. Instead of bringing fresh powder, the the atmospheric river storms, as they’re technically known, more often bring snow-destroying rain to many areas.
A new study by NASA and several other research institutions took a close look at data from satellites and ground observations from 1998 through 2014 to show the connection between atmospheric river storms and rain-on-snow events. According to the study, the atmospheric rivers are two-and-a-half times more likely than other types of winter storms to result in destructive “rain-on-snow” events, which increase flood risks in winter and reduce water availability the following summer.
Atmospheric rivers are narrow streams of very humid air that flow thousands of miles from the warm subtropical Pacific Ocean to the West Coast of North America. When the warm, moist air flows up and over the Sierra Nevada and other mountain ranges it condenses to form precipitation.
Only 17 percent of West Coast storms are caused by atmospheric rivers, but those storms provide 30 to 50 percent of California’s precipitation and 40 percent of Sierra snowpack, on average. They have also been blamed for more than 80 percent of the state’s major floods.
“In California, atmospheric rivers tend to be the warmest winter storms we get. We wanted to understand what the connection was between these storms and rain-on-snow events,” said Bin Guan, lead author of the study, which is accepted for publication in the journal Geophysical Research Letters. Guan is affiliated with the Joint Institute for Regional Earth System Science and Engineering, a collaboration between NASA’s Jet Propulsion Laboratory in Pasadena, California, and UCLA.
The researchers quantified the difference between atmospheric river storms that cause rain-on-snow and those that do not. The rain-on-snow-producing atmospheric river storms were, on average, 4 degrees Fahrenheit (2 degrees Celsius) warmer than the others.
“That small difference in temperature often determines whether we gain snow or lose snow from a storm,” said Guan.
The researchers found that the warmer storms typically originate in the Pacific south of 25 degrees north latitude. The cases without rain-on-snow events came from farther north, outside the tropics.
The amount of snow that melts in these events depends on how warm the rain and air are and how much rain falls. But the researchers found that, on average, warmer storms generate about a quarter-inch (0.7 centimeter) of snowmelt (i.e. liquid water) for each day of rain, providing 20 percent of the water available for runoff in these events. In other words, as Guan explained, “The primary contribution to any flooding still comes from the rainfall, but the melting snow makes things 20 percent worse.”
Scientists from UCLA; the Scripps Institution of Oceanography, San Diego; and the Earth System Research Laboratory in Boulder, Colorado partnered in the study.