They found that changes to snow's brightness results in its melting weeks earlier in spring than with pristine snow. In addition, less mountain snow going into late spring means reduced runoff in late spring and summer. They will report their findings in an upcoming issue of the Journal of Geophysical Research -- Atmospheres.
Making Snowhills from Mountains
Researchers know that soot settles on snow. And like an asphalt street compared to a concrete sidewalk, dirty snow retains more heat from the sun than bright white snow. Qian and colleagues wanted to determine to what degree dark snow contributes to the declining snowpack.
To get the kind of detail from their computer model that they needed, the PNNL team used a regional model called the Weather Research and Forecasting model -- or WRF, developed in part at the National Center for Atmospheric Research in Boulder, Colo. Compared to planet-scale models that can distinguish land features 200 kilometers apart, this computer model zooms in on the landscape, increasing resolution to 15 kilometers. At 15 kilometers, features such as mountain ranges and soot deposition are better defined.
Recently, PNNL researchers added a software component to WRF that models the chemistry of tiny atmospheric particles called aerosols and their interaction with clouds and sunlight. Using the WRF-chem model, the team first examined how much soot in the form of so-called black carbon would land on snow in the Sierra Nevada, Cascade and Rocky Mountains.
Then the team simulated how that soot would affect the snow's brightness throughout the year. Finally, they translated the brightness into snow accumulation and melting over time.
"Earlier studies didn't talk about snowpack changes due to soot for two reasons," said atmospheric scientist and co-author William Gustafson. "Soot hasn't been widely measured in snowpack, and it's hard to accurately
|Contact: Mary Beckman|
DOE/Pacific Northwest National Laboratory