The influence of drifting and blowing snow on surface mass and energy exchange is difficult to quantify due to limitations in both measurements and models, but is still potentially very important over large areas with seasonal or perennial snow cover. We present a unique set of measurements that make possible the calculation of turbulent moisture, heat, and momentum fluxes during conditions of drifting and blowing snow. From the data, Monin–Obukhov estimation of bulk fluxes is compared to eddy-covariance-derived fluxes. In addition, large-eddy simulations with sublimating particles are used to more completely understand the vertical profiles of the fluxes. For a storm period at the Syowa S17 station in East Antarctica, the bulk parametrization severely underestimates near-surface heat and moisture fluxes. The large-eddy simulations agree with the eddy-covariance fluxes when the measurements are minimally disturbed by the snow particles. We conclude that overall exchange over snow surfaces is much more intense than current models suggest, which has implications for the total mass balance of the Antarctic ice sheet and the cryosphere.

Wind erosion of snow covered surfaces is frequently observed in alpine and polar regions. Snow transport leads to the formation of bedforms, intensifies snow sublimation and modifies the microstructure of surface snow layers. Moreover, the interaction between the wind field and the complex topography creates regions of enhanced snow erosion and deposition, which greatly contributes to snow height heterogeneity. In alpine regions, these processes are of great importance for water management and avalanche risk assessment (Lehning et al., 2008). In Antarctica, snow transport is enhanced by the katabatic winds, dominating large areas from the inner plateau to the coast, and clouds of blowing snow particles with a height of hundreds of meters can be observed (Palm et al., 2017).The aeolian transport of snow occurs at different heights above the ground. The terms drifting snow and blowing snow are commonly used to indicate, respectively, the movement of snow particles close to the surface (up to ∼2 m height) and the movement of smaller snow particles transported at high elevations. Three transport modes (creep, saltation and suspension) are commonly distinguished during snow transport events (Bagnold, 1941). The rolling and sliding of snow grains along the surface is defined as creep. Creeping particles are typically too large and heavy to be lifted by the flow. During drifting snow events, their motion is mainly driven by impacting particles. Saltation refers to the ballistic motion of particles close to the ground. Particles in saltation generally hit the ground with enough kinetic energy to hop again (rebound) or eject other particles on the bed (splash). They are mostly concentrated in the first 10 cm above the surface and the ensemble of saltating particles constitute the saltation layer. Suspension refers to the motion of smaller snow particles transported above the saltation layer. They mainly follow the wind flow and travel great distances before being deposited on the ground or sublimate.At low wind speeds, the mass flux in saltation is greater than the mass flux of suspended particles (Gordon et al., 2009;Nishimura & Nemoto, 2005). At high wind speeds, snow transport in suspension becomes relevant and is currently simulated in mesoscale models by advection-diffusion equations (

Wind erosion of snow covered surfaces is frequently observed in alpine and polar regions. Snow transport leads to the formation of bedforms, intensifies snow sublimation and modifies the microstructure of surface snow layers. Moreover, the interaction between the wind field and the complex topography creates regions of enhanced snow erosion and deposition, which greatly contributes to snow height heterogeneity. In alpine regions, these processes are of great importance for water management and avalanche risk assessment (Lehning et al., 2008). In Antarctica, snow transport is enhanced by the katabatic winds, dominating large areas from the inner plateau to the coast, and clouds of blowing snow particles with a height of hundreds of meters can be observed (Palm et al., 2017).The aeolian transport of snow occurs at different heights above the ground. The terms drifting snow and blowing snow are commonly used to indicate, respectively, the movement of snow particles close to the surface (up to ∼2 m height) and the movement of smaller snow particles transported at high elevations. Three transport modes (creep, saltation and suspension) are commonly distinguished during snow transport events (Bagnold, 1941). The rolling and sliding of snow grains along the surface is defined as creep. Creeping particles are typically too large and heavy to be lifted by the flow. During drifting snow events, their motion is mainly driven by impacting particles. Saltation refers to the ballistic motion of particles close to the ground. Particles in saltation generally hit the ground with enough kinetic energy to hop again (rebound) or eject other particles on the bed (splash). They are mostly concentrated in the first 10 cm above the surface and the ensemble of saltating particles constitute the saltation layer. Suspension refers to the motion of smaller snow particles transported above the saltation layer. They mainly follow the wind flow and travel great distances before being deposited on the ground or sublimate.At low wind speeds, the mass flux in saltation is greater than the mass flux of suspended particles (Gordon et al., 2009;Nishimura & Nemoto, 2005). At high wind speeds, snow transport in suspension becomes relevant and is currently simulated in mesoscale models by advection-diffusion equations (

<p>Drifting snow is a multi-scale process. It is composed of particles rolling and sliding along the surface, particles in saltation following short ballistic trajectories in the first 10 cm above the surface and particles in suspension at higher regions of the atmosphere. <span>Drifting snow is currently represented in </span><span>some</span><span> regional and mesoscale atmospheric models by taking into account its effect on snow height, snow sublimation </span><span>and</span><span> snow densification.</span> Snow saltation is a sub-grid process in these models and is therefore parameterized. However, the current parameterizations are based on limited field and wind tunnel measurements and do not take into account the effect of the bed characteristics, as grain size, inter-particle cohesion and snow density, on the saltation dynamics.</p><p>In order to improve the current saltation models, <span>we conducted </span>wind tunnel <span>experiments using natural snow at the</span> WSL Institute for Snow and Avalanche Research SLF to measure the kinematics and shape of particles in saltation. The wind tunnel is located at 1670 m above sea level, has a cross section area of 1x1 m<sup>2</sup> and a total length of 14 m. Naturally deposited snow is collected in trays after each snowfall and transported to the tunnel without disturbing the snowpack. <span>We used a high speed camera, aquiring images at 5 kHz with backlighting provided by an LED to capture images of saltating snowflakes. We measured wind speed </span>with an array of pitot tubes <span>positioned 2-10 cm above the snowbed</span>. <span>We additionally measured </span>the density and hardness of the snow cover before the experiments using a box density cutter and a Snow Micro Pen (SMP), respectively. W<span>e process the</span> images with a<span> 2D</span> Particle Tracking Velocimetry (PTV) algorithm <span>allowing us to </span>obtain Eulerian and Lagrangian statistics of the kinematic quantities as well as estimates of the snowflake characteristics like size, aspect ratio and orientation. In addition, by assuming a constant particle density, <span>we derive </span>particle mass flux profiles.</p><p>The results show that the particle size distribution in saltation can indeed be characterized by a lognormal or a gamma distribution. From the analysis of the particle streamwise velocity profiles, it is clear that the assumption of a constant particle speed inside the saltation layer (common in simple saltation models) might not be a good approximation even for low friction velocities. We will present in how far we can assess the influence of the snow properties on mass flux and saltation dynamics as a basis to validate recent model results on the influence of inter-particle cohesion for example. Moreover, this data set will contribute to the development of new parameterizations for snow saltation mass flux and streamwise velocity that would take into account the effect of snow density and hardness.</p>

Abstract. Drifting and blowing snow are important features in polar and high mountain regions. They control the surface mass balance in windy conditions and influence sublimation of snow and ice surfaces. Despite their importance, model representations in weather and climate assessments have high uncertainties because the associated physical processes are complex and highly variable in space and time. This contribution investigates the saltation system, which is the lower boundary condition for drifting and blowing snow models. Using a combination of (previous) measurements and new physics-based modeling with Large Eddy Simulations (LES), we show that the prevailing parameterizations that describe the saltation system in atmospheric models are based on contradictory assumptions: while some scaling laws are typical of a saltation system dominated by aerodynamic entrainment, others represent a saltation system controlled by splash. We show that both regimes can exist, depending on the friction velocity. Contrary to sand saltation, aerodynamic entrainment of surface particles is not negligible. It is important at low wind speeds, leading to a saltation height and near surface particle velocity which increase with the friction velocity. In a splash dominated saltation regime at higher friction velocities, the saltation height and near surface particle velocity become invariant with the friction velocity and closer to what is observed with sand. These findings are accompanied by a detailed description of the theoretical, experimental and numerical arguments behind snow saltation parameterizations. This work offers a comprehensive understanding of the snow saltation system and its scaling laws, useful for both modelers and experimentalists.