Dust and Snow

 
High in the snowy San Juan Mountains, tiny particles have big implications


Tom Painter thinks a lot about dust, and it all started during a hike with his father.

Eight years ago, when the younger Painter was working on his Ph.D. in geography, he and his father spent a day hiking in the Maroon Bells near Aspen, Colo. "We were walking up the snowfields at about 5:30 in the morning, and I told my dad, ‘Let’s do an experiment,’ " he says. "I knew there was a layer of dust on the snow, so I scraped off an area about a foot square, and showed him how clean it was underneath. "Then we climbed the peak, and when we returned in the afternoon, you could see this column of white snow just extruding out of the surface." On top of that column, about a handsbreadth high, was the small, bright square he’d exposed in the morning.

The column hadn’t risen from the snowfield; the snow surrounding it had melted, because the dirty snowfield had absorbed more solar radiation than the clean square. Says Painter: "My dad started asking questions that I didn’t know the answer to."

Painter, now a researcher at the National Snow and Ice Data Center in Boulder, Colo., knew that dust changed the reflectivity — or albedo — of the snow surface. White snow reflects a lot of light, so it absorbs less energy and melts relatively slowly. Dirty snow reflects less light and melts more quickly. The faster snow melts, the sooner it exposes deeper dust layers, creating a feedback loop that further accelerates the melt.

What Painter didn’t know was where the dust in the Rockies came from, how often it arrived, or just how large a role it played in the melting of the snowpack. "So when I finished my dissertation a few years later, the first thing I did was to start looking at dust," Painter says. "It’s turned out to be a much bigger issue than I ever expected."

From the summit of Red Mountain Pass in southwestern Colorado, on a clear day in mid-April, the snow-covered slopes of the San Juan Mountains wink back from every direction. Here, a crew of hydrologists, chemists, snow scientists, and inveterate skiers is preparing to climb into Senator Beck Basin, high above the western side of the pass, to watch snow disappear. As University of Colorado graduate student Maureen Cassidy jokes, the team will "weigh and measure and smell and taste" the snowpack, looking at the role of dust in its seasonal exit.

In 2004, when Painter first started investigating the mysteries of dust, he and his collaborators gathered some preliminary data in Senator Beck Basin, using it to construct a computer model that compared the effects of temperature and dust on snowmelt. They found that stepping up air temperatures by a whopping 6 degrees Celsius would cause the basin’s snow to disappear five days earlier.

The impacts of dust were more dramatic. "We expected a difference of maybe five or six days," between the melting of dusty and dust-free snow, says hydrologist Andrew Barrett, also with the National Snow and Ice Data Center. Instead, they found that a single dust event could cause snow to melt away 18 days earlier than it would if there were no dust at all.

In the Western mountains, which act as the region’s water towers, the implications of these rough estimates are stark. If dust deposits in the mountains increase for any reason, the researchers surmise that snowmelt will both happen faster and finish sooner, leading to bigger and earlier peak flows in streams and rivers. That would cause headaches for water managers, who would need to store the rush of water, and it would perhaps result in more frequent or serious summer water shortages. The dance between dust and snow, the scientists realized, needed closer examination.

So Painter and Barnett, along with Chris Landry, the director of the Center for Snow and Avalanche Studies in Silverton, Colo., expanded their study and began gathering a suite of detailed data. Each week during the spring, Landry and others dig a series of pits in the snow — sometimes more than seven feet deep — to study how the snowpack changes as dust layers are exposed to the surface. They also download hourly measurements of snow reflectivity from a state-of-the-art meteorological station, and hourly data on streamflow from a gauge at the bottom of the basin.

During their mid-April outing, the crew digs additional snow pits, and also measures the depth of the snow throughout the basin with thin aluminum probes. With these and other datasets, the researchers plan to build a sophisticated model of dust, climate, snowmelt and runoff, one that can be adapted to other basins and ranges.