Transport by Saltation

Snow particles that are too heavy to be suspended in the airflow and too light to remain creeping on the surface, tend to move in short, ballistic trajectories above the snow surface. The phenomenon is known as saltation because of the casual, dancing behaviour of the particles. The figure below illustrates a typical snow particle trajectory path which is about 1 cm high and 5 to 10 times longer than the height (Kind, 1986). It is also found that maximum saltation heights are in a range from 10 to 100 cm (Mellor, 1965).

The transport mechanisms of saltation are rather complicated. It is convenient to simplify the problem by decomposing the wind induced forces resolved into lift, drag and gravity (White, 1986). Forces that act parallel to the relative particle velocity are represented by a drag force and consist of form drag and surface drag. Form drag is a result of the pressure difference in the front and back of the particle and friction drag is caused by viscous skin friction on the particle surface. Drag is a function of particle porosity, shape, size, relative location of the particles, velocities and fluid properties.

Snow Particle in a typical trajectory movement.

Lift is the resulting force normal to the relative velocity between particle and air stream, and this force is essential in initiating particle motions from the surface. Particles seem to lift vertically from the surface even in laminar flows, without vertical components of air velocity. This effect is due to a pressure distribution on the surface of the particle and will usually occur for a short time while the particle is close to or on the surface. Snow particles will fall down and impact the surface as the lift decreases with height and gravity remains.

Impacting particles seem to lose most of their energy to dissipation at the snow surface, but some momentum contributes to initiate surface grains in new saltation motions. Kind (1986) claims that momentum transferred through the acceleration/ impacting process is responsible for most of the surface shear stress, at wind speeds above two times the threshold value.

Trajectory lengths, form and direction might vary because of particle interactions and other particle properties, but the net flow rate of saltating particles will be in the direction of the mean flow. For typical snow drift conditions, saltation is probably the most important transport factor, and Kind (1986) estimated that saltation is the most important transport mode for velocities less than 15 m/s.

There are many factors that affect the behaviour of the saltation layer. In addition to the described lift and drag, there is a complicated interaction between particles of different sizes, which certainly will involve statistical analysis. Larger particles in trajectory will also be affected by spinning effects, thus making the situation even more complicated. It is not possible to model the enormous number of snow particles in a real snow drift, but studying particle physics gives us valuable information about the drift phenomenon.

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