Buoyancy effects induced by drifting snow particles

Snowdrift is one of the many manifestations of two-phase flows in which the inherently stably stratified drift-density profile acts to destroy turbulence. This can be quantified by using an appropriate “particle” Richardson number, equivalent to thermally stratified flow. This Richardson number is proportional to the mean fall velocity of the particles (averaged over the particle-size spectrum) and the drift density and therefore depends strongly on height above the surface. It exhibits a maximum close to the surface, where drift densities are largest. It then decreases to minimum values at intermediate heights, above which Richardson numbers increase with height. The particle Richardson number and the associated decrease in turbulent exchange coefficient depend strongly on wind speed, particle-eddy exchange coefficient and mean particle radius It is found that particle-buoyancy effects in snowdrift are non-negligible for large wind speeds and large particle-eddy exchange coefficients.