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Polluted Snow Hastens Runoff on the Cascade & Rocky Mountains

Soot Melts Snow and Creates Chaotic Chain Reaction
Soot from pollution causes winter snow to repeatedly warm and shrink. This continuous cycle sends snowmelt rushing down mountains as much as a month early, according to scientists at the Pacific Northwest National Laboratory ( People rely on the water from the water reservoirs of the Cascades and Rockies for farming, fisheries, and hydroelectricity, so the effect of pollution can be serious. The soot warms the mountain snow and surrounding air by up to 1.2 degrees Fahrenheit, which is enough to incite the snow to melt too early.
Scientist Yun Quian said, •If we can project the future ... how much water we'll be getting from the rivers and when ... then we can better plan for its many uses. Snowmelt can be up to 75 percent of the water supply, in some regions. These changes can affect the water supply, as well as aggravate winter flooding and summer droughts.• The Cascades have lost 60 percent of their snowpacks since the 1950s.
The soot-snow cycle starts when soot darkens snow it lands upon, which then absorbs more of the sun's energy than clean, pure white snow. The resulting thinner snowpack reflects less sunlight back into the atmosphere and further warms the area, continuing the snowmelt cycle.  This study was the first to examine the consequences of soot in particular, not the effect of greenhouse gases in general. The soot comes from diesel engines and power plants and the like.
Computer Technology Made Study Possible
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, Colorado. 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.
Big Snow Melt in Spring Means less Water for Summer

The scientists found that changes to snow's brightness results in its melting weeks earlier in spring than with unpolluted snow. In addition, less mountain snow going into late spring means reduced runoff in late spring and summer. Thanks to this study, soot"s dirty secret has finally been revealed.
By Neil Whitehall
Get Hydrology Jobs, Contributing Editor

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