Parameterizing snow saltation: the exponential decay of the mass flux profile and its relation to the saltation dynamics

Drifting snow events lead to snow redistribution and enhance snow sublimation. Taking into account the importance of snow sublimation to the correct prediction of the surface mass balance, a general consensus has emerged within the scientific community on the importance of correctly representing the aeolian transport of snow in large scale models. Nevertheless, few attention has been given to the description of snow saltation – the motion of snow particles in approximately the first 10 cm above the ground. The saltation layer feeds the upper regions of the atmosphere with suspended particles and, therefore, considerably influences the predictions of drifting snow concentration. In order to improve our understanding of snow saltation dynamics and the quality of the parameterizations used in large scale models, numerical simulations combining Large Eddy Simulations with a full description of the particle-fluid-bed interactions were performed. In particular, we focused on the exponential decay of the particle mass flux vertical profile in the saltation layer – a commonly observed feature in both wind tunnel and field experiments and a widely used assumption in saltation models. The inverse of the exponential decay constant is expected to be proportional to the height of the saltation layer, which is generally assumed to increase with the square of the fluid friction velocity, following wind tunnel experiments developed with snow. However, recent field measurements developed over sand-covered surfaces revealed that the saltation layer height is invariant with the fluid friction velocity. From numerical simulations of snow saltation, we found that these conclusions are not contradictory. In fact, they reveal two distinct dynamic regimes of the saltation system: while at low wind velocities, typical of wind tunnel experiments, a quadratic increase of the saltation layer height with the fluid friction velocity is seen, at high wind velocities, frequently found in the field, the saltation layer height does not vary with the fluid friction velocity. Following these findings, adjustments to the exponential decay constant can be proposed. In addition, this work shows the potential of detailed models to improve our understanding of the saltation system and foster the development of more physically-based snow saltation parameterizations.