UW study shows retreating snow cover could produce winter rains over Great Plains

UW study shows retreating snow cover could produce winter rains over Great Plains

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Rose Casey, of Madison, uses hiking poles as she walks on designated snowshoe and cross-country ski trails Monday at the UW Arboretum. "It's nice to be able to get out without the snow," Casey said. A new study from UW-Madison finds winter storms in the Great Plains become more intense and tend to fall as rain when there is no snow on the ground.

As the climate warms, Wisconsin will — not surprisingly — get less snow. But a new study from UW-Madison finds the retreating snow cover may result in more intense winter storms dumping rain on frozen ground.

Ankur Desai, a professor of atmospheric and oceanic sciences, came to Madison in 2007, which happened to be the snowiest winter on record, with nearly double the normal 50.9 inches.

Ankur Desai

Desai

Desai, whose Ecometeorology Lab studies how vegetation and physical features on the ground influence — and are influenced by — the atmosphere, started thinking about how we forecast snowstorms.

Meteorologists have long believed winter storms in North America tend to follow the snow line, where snow cover gives way to bare ground.

Snow traps heat in the ground and reflects sunlight, keeping the air above it cooler and drier. The air above uncovered ground tends to be warmer and more humid.

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A map shows the marked snowshoe and ski trails at the entrance to the UW Arboretum. Under current greenhouse gas emission levels, snow cover is expected to retreat over the coming century, leaving southern Wisconsin ground bare in 10 out of 12 months by 2100.

“Weather forms wherever there’s big gradients in temperature,” Desai said.

But with warming winters, Wisconsin’s snow line is expected to shift about 350 miles to the north by the end of this century, said Stephen Vavrus, a senior scientist with the Nelson Institute Center for Climatic Research and a contributor to the study.

By 2100, climate models predict only the northern half of the state will regularly be snow-covered in December, and the entire state will generally be bare in March.

Change in March snow cover

The median extent of 1-inch March snow cover for the years 1986-2005 is represented by the blue lines in this climate projection by Stephen Vavrus of the Nelson Institute Center for Climatic Research based on the Community Earth System model. The red lines show the anticipated snow cover in the years 2081-2100.

Given those changes, Desai set out to test whether a shifting snow line would bring storms with it.

Ryan Clare, one of Desai’s graduate students, ran 500 simulations using decades of weather data and computerized forecast models. Focusing on the Great Plains, Clare would go back in time to a few days before a storm and move the snow line to test whether it would affect the storm’s trajectory.

The results were surprising: While some storms moved north, the effect wasn’t uniform. But the storms consistently delivered more precipitation.

“Storms that are now over this moister, darker, warmer ground … are more intense,” Desai said. “But they’re also more likely to be slightly warmer, which means the increasing likelihood that you get rain instead of snow.”

Receding snowline

If that rain falls on frozen ground, it could create the potential for more runoff, erosion, flooding and other problems.

While it sounds counter-intuitive, a thick snowpack insulates the ground from cold air above, said Steven Loheide, a professor of civil and environmental engineering at UW-Madison. Without that blanket, Loheide said, the ground is more likely to freeze — and to a greater depth — during a cold snap.

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Dane County snowplows clear the Beltline on Jan. 23, 2019. A new UW-Madison study found that as the snow cover moves north in the coming century, winter storms will likely become more intense and deliver rain instead of snow.

“Understanding the multifaceted feedbacks between snow cover, frozen ground, infiltration and precipitation regimes is critical for predicting the ways in which our water resources will be affected by climate variability and changes in environmental conditions,” Loheide said.

Desai said the study, submitted for publication in the Journal of Climate, could also help meteorologists improve their winter storm forecasting.

“We’ve basically shown that this really common rule of thumb … is in actuality, you know, maybe not the best rule to follow,” Desai said. “There may be some things that we can continue on with these model simulations to better understand in a warmer climate with less persistent snow cover — which is what we’re seeing happening, especially in Wisconsin — how do these storms change.”

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