Snow saltation threshold measurements in a drifting-snow wind tunnel

Wind tunnel measurements of snowdrift in a turbulent, logarithmic velocity boundary layer have been made in Davos, Switzerland, using natural snow. Regression analysis gives the drift threshold friction velocity (ut), assuming an exponential drift profile and a simple drift to friction velocity relationship. Measurements over 15 snow covers show that ut is influenced more by snow density and particle size than by ambient temperature and humidity, and varies from 0.27 to 0.69 m s -1 . Schmidt's threshold algorithm and a modified version used in SNOWPACK (a snow-cover model) agree well with observations if small bond sizes are assumed. Using particle hydraulic diameters, obtained from image processing, Bagnold's threshold parameter is 0.18. Roughness lengths (z0) vary between snow covers but are constant until the start of drift. Threshold roughness lengths are proportional to u 2 t. The influence of macroscopic objects on the roughness length is shown by the lower values measured over the smooth and flat snow surface of the wind tunnel (0.04 � z0 � 0.13 mm), compared to field measurements. Mean drifting-snow grain sizes for mainly new and partly decomposed snow are 100-175 mm, and independent of surface particle size. This paper describes the results of a series of experiments investigating the form of the wind velocity boundary layer over a snow surface, with and without drift. This information is essential to help calculate the energy balance at the snow surface and also to assess mass movement of snow by wind. The process of drift is important both over relatively flat terrain, such as Antarctic regions, where it contributes to mass balance, and also in mountainous terrain, where accumulation on steep slopes can contribute to the danger of avalanches. The velocity boundary layer over a surface is often de- scribed by a log-law, where the wind speed at a particular height is a function of the surface friction velocity, u� , and aerodynamic roughness length, z0 (Stull, 1988). The param- eters uand z0 are then used to model the fluxes of heat and water vapour from the surface, which are important, not only for snowpack development (Marks and Dozier, 1992; Lehn- ing and others, 2002b), but also for mass-balance calculations (Cline, 1997; Liston and Sturm, 1998; Box and others, 2004). Aerodynamically rough, solid surfaces have roughness lengths independent of u� (Schlichting and Gersten, 2003). The same does not always hold over granular surfaces, such as snow or sand; above the threshold friction velocity, ut, particles will be entrained and transported by drift. At low wind speeds, this drift will be predominantly saltation, where particles follow parabolic paths over the surface (Bagnold, 1941, p. 10-37). These drifting particles increase the aerodynamic roughness of the surface. Owen (1964) summarized earlier research relating to the aerodynamic effects of drifting particles, and developed a theoretical framework to explain this effect. Together, these suggest two