Science Story March 2023

Probing snow

When we last talked with Dr. Jessica Lundquist of the University of Washington (Spring 2022 Newsletter), her team was setting up all the instruments needed to measure snow sublimation, the transformation of the snowpack (a solid) into vapor. Last October, the team installed the instruments at RMBL. Then winter came and generously dropped all the snow the researchers could want and then some. Now, with the avalanche of data the team has gathered, it’s likely that scientists will know more about what happens to fallen snow than ever before.

Who needs this information? Only the 40 million people who depend on the Colorado River for drinking water, among other things. The mountain snowpack is by far the largest source of water for the river. We already know that snowfall in the mountains has diminished over the past decade. Yet the river’s water has decreased much more in proportion. How much water are we losing to sublimation? That’s the urgent question researchers want to answer in the Sublimation of Snow project, aptly shortened to SoS.

Dr. Lundquist’s team set up an extraordinary array of instruments at the Kettle Ponds, a particularly windy location. Where there’s dry air and wind, more snow vaporizes. Fresh snowfall makes a difference, too. New snow is light and fluffy, easily picked up by the wind and blown around. It also bounces along the surface, and lots of snow bouncing around equals a lot of snow vaporizing. If there isn’t new snow, the wind doesn’t pick up snow particles because they stick together.

Blowing snow is actually very important to measure. It’s also very hard to do, not just because it’s a moving target but because you have to observe the snow from every angle. Hence the plethora of sensors. There are four towers at the Kettle Ponds site, ranging from 10 to 20 meters tall, all taking meteorological measurements. There are snow pillows. Like giant bathroom scales, they measure the weight of the snow, which tells researchers how much water it holds. There are more than 20 anemometers, or eddy covariance sensors, measuring turbulence in the air. There are also various laser sensors, including LiDAR, that map not only the depth of the snowpack but also the location of snowflakes and the speed at which they’re dancing through the air.

Although plenty of other studies have been done on snow sublimation, Dr. Lundquist believes that her team may be the first to scientifically analyze everything that’s happening to the snow in one area. This matters because regardless of how much we improve our measurement of snow, “if we’re not measuring sublimation, we’re missing a potentially big part of the equation,” she says, “and that causes our conclusions to be wrong.”

Without accounting for snow sublimation, we can’t accurately predict how much of the snow that fell will end up in the Colorado River and ultimately in people’s homes in seven states.

Understanding snow sublimation is a highly complex science, and few understand this as well as Dr. Lundquist. Fortunately, with the efforts of scientists like her and more broad-based research, the snow’s secrets will not stay buried for long.

Jessica Lundquist, PhD, grew up in California and spent summers hiking in the Sierra Nevada. She received her PhD in Oceanography from Scripps Institution of Oceanography at U.C. San Diego. She was a CIRES postdoctoral fellow with the University of Colorado, Boulder and NOAA. She is a professor of Civil and Environmental Engineering at the University of Washington. Her research focuses on spatial patterns of snow and weather in the mountains and how those patterns are likely to affect streamflow and water resources in a changing climate.