[1] This article presents a measurement of electrification generated by wind-blown sands in a field wind tunnel and a numerical methodology to simulate the effect of electrification on the sand saltation movement after the mutual couple interaction between the sand movement and the wind flow is taken into account. The measured data of electric charge on the ''uniform'' sands in the wind-tunnel tests show that the sign of electric charge, either negative or positive, is mainly dependent on the diameter size of sand particles, i.e., negative charge is gained when the diameter is smaller than 250 mm and positive charge is obtained if the diameter is larger than 500 mm, and that for both ''uniform'' and mixed sands, the average charge-to-mass ratio decreases with increasing the wind velocity, and increases with height from sand bed. Meanwhile, the measurement of electric field in wind-sand cloud related to the electric charge displays that the magnitude of electric field increases generally as the wind velocity and the height increase, and the direction of the field is always upwardly vertical to the Earth's surface, which is opposite to that of the fair-weather field. In order to exhibit the effect of electrification on sand saltation movement, a theoretical model by considering the mutual coupling interaction between wind flow and sand movement is proposed after the electric force exerted on the moving sands is considered. Through solving the nonlinear coupling dynamic equations by a proposed program, the effect of electrification on sand saltation motion, e.g., trajectory, is discussed quantitatively. After that, its effect on wind-sand transport flux, sand ejecta flux, and wind profile is also displayed. The results show that the prediction for the Bagnold's kink is good agreement with the measurement in literature.

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Snow is one of the most dynamic natural elements on the Earth's surface, and the variations in its distribution in time and space profoundly affect the hydrological cycle, climate system, and ecological evolution as well as other natural processes. Most previous studies have paid less attention to the process determining the distribution of snow on the ground as a result of the effect of nonuniform mountain wind on the trajectories of snow particles. In this paper, we present a numerical study on the falling snow deposition process involving snow particles of mixed grain sizes over complex terrain. A three‐dimensional large‐eddy simulation code was used to predict the wind field by considering the fluid‐solid coupling effect, and the Lagrangian particle tracking method was employed to track the movement of each tracking snow particle. The grid resolution and model parameters were determined by the best fit with the field experiment, and the coupling effect between snow particles and wind field was found to be nonnegligible when the drifting snow occurred. In general, the preferential deposition on a single ridge showed a tendency from windward slope toward leeward slope with the increasing advection, while it was hard to describe the snow distribution over complex terrains with a unified deposition model due to the interaction of surrounding topographies and different atmospheric stabilities, and the particle tracking approach was substantially suitable for this issue. Our study significantly improved the understanding of the evolution of snow distributions at high levels of resolution.

Abstract. Particle size distribution of dust at emission (dust PSD) is an essential quantity to estimate in dust studies. It has been recognized in earlier research that dust PSD is dependent on soil properties (e.g. whether soil is sand or clay) and friction velocity, u∗, which is a surrogate for surface shear stress and a descriptor for saltation-bombardment intensity. This recognition has been challenged in some recent papers, causing a debate on whether dust PSD is “invariant” and the search for its justification. In this paper, we analyse the dust PSD measured in the Japan Australian Dust Experiment and show that dust PSD is dependent on u∗ and on atmospheric boundary-layer (ABL) stability. By simple theoretical and numerical analysis, we explain the two reasons for the latter dependency, which are both related to enhanced saltation bombardment in convective turbulent flows. First, u∗ is stochastic and its probability distribution profoundly influences the magnitude of the mean saltation flux due to the non-linear relationship between saltation flux and u∗. Second, in unstable conditions, turbulence is usually stronger, which leads to higher saltation-bombardment intensity. This study confirms that dust PSD depends on u∗ and, more precisely, on the probability distribution of u∗, which in turn is dependent on ABL stability; consequently, dust PSD is also dependent on ABL. We also show that the dependency of dust PSD on u∗ and ABL stability is made complicated by soil surface conditions. In general, our analysis reinforces the basic conceptual understanding that dust PSD depends on saltation bombardment and inter-particle cohesion.

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