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Petrzelka, Jennifer ¹ ; Williams, Mark ² ; Fassnacht, Steve ³ ; Zukiewicz, Lucas ⁴

¹ Department of Geography and Institute of Arctic and Alpine Research, University of ŷ���ڱ���Ƶ, Boulder
² Department of Geography and Institute of Arctic and Alpine Research, University of ŷ���ڱ���Ƶ, Boulder
³ Watershed Science Program, ŷ���ڱ���Ƶ State University, Fort Collins, CO
⁴ Watershed Science Program, ŷ���ڱ���Ƶ State University, Fort Collins, CO

The presence of dust on the snow’s surface has been shown to decrease albedo by as much as 40% compared to a clean snow cover. Thus dust on snow affects snow surface radiative fluxes, generally resulting in earlier snowmelt compared to “clean” snowpacks. Less understood is how dust on snow events may affect infiltrating meltwater movement in the snowpack. An aeolian dust event with a distinctive red color that was deposited on February of 2006 at Niwot Ridge in the ŷ���ڱ���Ƶ Front Range provides opportunity to investigate how dust on snow may change the surface characteristics of snow and meltwater movement through the snowpack. High-resolution digital imagery (spatial resolution of 0.043mm) of the “red” and “clean” snow surfaces showed that the surface roughness of “clean” snow was about 4 times that of “red” snow, and that the range in surface roughness of “clean” snow was about 16 times that of “red”. Thus, dust on snow events appear to have caused the snow surface during snowmelt to be smoother and more even when compared to adjacent “clean” snow surfaces. Concurrent analyses of ice columns showed large differences between “red” snow and “clean” snow. Ice columns found in clean snow had a mean depth of 60cm, which was significantly longer (p=0.0001) compared to the mean depth of 16.7cm in red snow. The shape also varied, with ice columns in clean snow having a uniform width along the entire length, whereas those found in red snow were widest at the snow surface and decreased in width with depth.

The presence of dust on the snow surface appears to change the spatial distribution of the energy balance at the mm to cm scale. The lower albedo associated with red snow increases melt and thus liquid water content at the surface resulting in larger snow grains as water causes snow grains to coalesce. Larger snow grains decrease scattering and increase absorption of radiation, leading to a more uniform distribution of solar radiation. In contrast, hollows in the clean snow trap incident sunlight more efficiently than peaks. This means that the hollows melt faster than the peaks leading to greater surface roughness when compared to “red” snow. The movement of liquid water through the snowpack in “red” snow thus appears to be more uniform, decreasing the size of preferential flowpaths that give rise to ice columns. Thus the variation in ice column morphology between red and clean snow gives insight into how dust deposition events may affect the energy balance, subsequent melt and meltwater flowpaths in snow covered areas. The impact of dust on snow events will become increasingly important as climate change may result in more frequent and intense drought and thus more dust available for transport.

Fassnacht, S.R, Williams, M.W., Corrao, M.V., 2009, Changes in the surface roughness of snow from millimetre to metre scales: Ecological Complexity, v. 6, p. 221-229.

Tiedje, T., Mitchell, K.A., Lau, B., Ballestad, A., Nodwell, E., 2006, Radiation transport model for ablation hollows on snowfields: Journal of Geophysical Research, v. 111, F02015.

Williams, M.W., Rikkers, M., Pfeffer, T., 2000, Ice Columns and Frozen Rills in a Warm Snowpack, Green Lakes Valley, ŷ���ڱ���Ƶ, U.S.A.: Nordic Hydrology, v. 31, p. 169-186.