In a rather simplified characterization, snow will be eroded in areas where the wind is strong and transported downstream until the transport efficiency decreases, so that the air flow can no longer maintain the upstream vertical drift profile. However, numerical modelling of snow drift calls for a more detailed description of the phenomenon.
Particle transport modes during snow drift is commonly classified as creep, saltation and suspension (Kim et al., 1991; Tabler, 1988). “Creep” refers to particle motion along the surface, “saltation” to particles in jumping trajectories above the surface and “suspension” applies to suspended particle transport in the air flow, see figure below.
Illustration of the transport regions above a horizontal snow surface.
A natural mixture of snow consists of particles of a wide range of sizes. In most situations particle transport is due to the presence of all of the three previously mentioned transport modes. The large particles will either become immobile or be transported by creeping on the surface. Intermediate particles will basically remain in the saltation layer while small light particles will go into suspension. The amount of transport in each mode depends on wind speed, snow properties and distribution of particle size and shape.
The heights of the different transport regions in the figure above are only meant as guiding values, since the transport processes are very much connected and no sharp borders exist.
Snow particles in wind transport are usually fragments of the origin snow crystals. Snow particles tend to fall into smaller pieces if they are affected by winds during, or soon after snowfall. Drifting particles gradually become smaller and more rounded due to abrasion and evaporation. The different classes of particles will appear with rather monodisperse size distribution in the respective transport layers. A typical saltating particle will, according to Iversen (1979), have a diameter between 100 to 150 mm and a density from 500 to 900 kg/m3.
Snow fall velocity
Mellor and Radok (1960) suggested that the snow fall velocity for blown particles is between 0.3-1.0 m/s, depending on various snow properties. In numerical modelling the snow fall velocity is often considered to be a constant due to constant gravity effect in the governing transport equations. However, the snow deposition becomes rather discontinuous or non-streamlined. However, using a variable fall velocity which also considers the snow phase concentration seems to eliminate some of these non-continuous effects.
The application of variable snow fall velocities can be used to consider effects such as hindered settling, a resistance of the downward movement due to particle interactions. Particle interactions will increase with concentration and is probably most important in the saltation layer.