| Blowing snow events are frequent in winter at high latitudes. The physical processes of blowing snow include the saltation of snow particles bouncing along the surface and the suspension of snow particles controlled by turbulence and the gravitational settling, as well as the sublimation of the particles and its impact on the size spectrum, plus the turbulent diffusion of the water vapor released and heat required during sublimation. Our basic model, PIEKTUK, comes in fetch- or time-dependent forms, simulating the evolution, with time or fetch, of a column of blowing snow lifted from a horizontally homogeneous snow-covered surface, once the threshold wind speed for saltation has been exceeded. Three spectral, time-dependent blowing snow models, PEIKTUK-T, WINDBLAST and SNOWSTORM, developed independently at York University/CANADA, Leeds University/UK and Utrecht University/The Netherlands, have been inter-compared under standard conditions to validate the physics implementation and numerical aspects. In all models, sublimation leads to an increase in mixing ratio and a decrease in temperature and the vertically integrated sublimation rate generally decreases with time after an initial phase. However, if the feedback of sublimation on the environment is not considered in the model, the predicted sublimation rate can be much higher. Model results are also compared with field data from the British Antarctic Survey's 2nd STable Antarctic Boundary Layer Experiment (STABLE2) conducted at Halley in 1991 and from Schmidt's blowing snow experiments at Laramie, Wyoming, USA in 1974. In the basic models, particle settling velocity is assumed to be equal to spherical particle terminal velocity in still air and the particle diffusion coefficient is set equal to that for momentum. The results from model comparisons with field data show that the basic model has insufficient capability to diffuse enough particles upwards or to hold the particles in suspension. Although it can be corrected by increasing the particle diffusion coefficient, our belief is that it is the assumed settling velocity of blowing snow particles that is in error. The settling velocity given by solving the particle motion equation in a turbulent spectrum is smaller than the terminal velocity. Particle irregularity will also reduce the settling velocity. The settling velocity of blowing snow particles has been obtained from observational data by fitting an ideal power law profile for each particle size bin. The settling velocity obtained in this way is generally smaller than the spherical particle terminal velocity. However, the power law may fail for smaller particles because of an unbalanced state between gravitational settling and turbulent diffusion. By using the modified settling velocities, model results are greatly improved. |
