Blowing Snow at a Colorado Alpine Site: Measurements and Implications

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Abstract
The size and frequency of blowing snow particles were measured by a photoelectric device during two winters at Niwot Ridge, an alpine site in the Front Range, Colorado. Blowing snow occurred over 50% of the time, with 95% of the days in January having blowing snow. Over 30 blowing snow events occurred each winter, with each event averaging 36 h long. Visibility estimates during blowing snow were derived from information on blowing snow and wind speed. During 80% of the hours with blowing snow, short-term visual ranges were estimated to be less than 50 m. During blowing snow events, median visibility was in the 150- to 200-m range. Visual ranges less than 20 m occurred during precipitation and during some days with clear skies. As daily precipitation increased above amounts of 10 mm, maximum minimum ranges decreased in a linear fashion. An operational system for visibility monitoring and its implications for winter travel at high elevations are discussed. During the winters of 1973/74 and 1974/75, mass transport ranged above 480 g m-1 s-1, but averaged 25 g m-1 s-1 under typical blowing snow conditions. The mass of water annually transported as blowing snow was equivalent to streamflow discharge during 3 1/2 d of peak flows at an alpine basin adjacent to the study site. Snow source depletion and the variations in snow surface hardness, topography, and the transport process itself combined to reduce the usefulness of wind speed-transport formulas. Sublimation losses probably range between 30 and 51% of precipitation for the 2-yr period or the equivalent of 4.5 .times. 106 to 7.6 .times. 106 m3 of water annually. These calculations are based on equations developed on the high plains; their applicability to the Colorado alpine is uncertain.