| 08 Dec 2025
Near-threshold aeolian sand transport: Effects of boundary layer flow conditions
Abstract. Boundary layer thickness is a critical factor in aeolian sand transport, as it governs the scale of energy-containing turbulent structures, yet its specific mechanisms remain inadequately quantified. Previous studies have established the role of turbulence in particle entrainment but often overlook systematic variations in boundary layer thickness. This study aims to clarify how boundary layer thickness modulates wall-shear stress fluctuations, threshold wind velocities, sand flux, and particle kinematics. We use the three-dimensional large-eddy simulation coupled with a saltation model to investigate these interactions. Results reveal that increased boundary layer thickness enhances extreme-value probability density of wall-shear stress and significantly lowers impact entrainment and rebound thresholds—the latter dropping to less than 50 % of conventional wind-tunnel values. Sand transport response is velocity-dependent: at low velocities, transport rises markedly with thickness under fluid-driven entrainment; the effect diminishes at moderate velocities; and at high velocities, transport scales proportionally with thickness under splash-dominated entrainment. Moreover, thicker boundary layers intensify near-bed particle activity, elevating particle velocities and concentrations, reducing intermittency, increasing saltation height, and enlarging mean and variance of airborne particle diameters. These findings elucidate how boundary layer thickness modulates aeolian sand transport via turbulence–particle interactions, offering key insights for improving atmospheric and climate models and advancing the physics of turbulence-driven sediment transport in atmospheric boundary layer.
This study accurately addresses a long‑standing yet insufficiently quantified issue in aeolian physics—the role of boundary layer thickness. Using a large‑eddy simulation–saltation coupled model, it systematically reveals how boundary layer thickness modulates turbulent structures and thereby significantly affects the physical mechanisms of near‑threshold particle entrainment, transport flux, spatial distribution, and grain‑size characteristics. The conclusions provide a clear physical explanation for the discrepancies between wind tunnel and field observations, and offer direct guidance for improving dust emission parameterization schemes in climate models. The paper features a clear structure, sound methodology, and comprehensive data. It is recommended for acceptance after minor revisions. Below are several suggestions for the authors to consider during revision:
1) To reduce computational costs, the study employs the approach where each numerical particle represents multiple physical particles (lines 166-168), with the representative ratio varying widely (from 50 to 2000) depending on the boundary layer thickness and friction velocity. This is a practical strategy. Please briefly explain the potential impact of this assumption on the key results and its validity, especially under near-threshold conditions characterized by low particle concentration and high representative ratios.
2) The friction velocity typically refers to a parameter of the airflow itself, whereas the saltation friction velocity or effective friction velocity often accounts for the feedback from sand particles. Please briefly clarify the specific meaning of the saltation friction velocity used in this paper: is it the bed shear velocity under particle-laden conditions (i.e., the friction velocity that incorporates particle feedback), or is it derived through a specific formulation?
3) The text mentions classic models such as the cubic law of Bagnold (1941) and the quadratic relationship of Creyssels et al. (2009), and points out that near the threshold state, the relationship between sand transport rate and shear stress follows different patterns (exponential or power law). It is recommended to quantitatively compare the fitted relationships obtained in this study with those from existing research.
4) In the text, the transport intensity is defined as a key metric linking microscopic mechanisms to macroscopic flux, and its variations with height and boundary layer thickness are presented (Fig. 4b). The authors are requested to provide a clear mathematical definition or calculation formula for the transport intensity in the main text (e.g., at line 257), as this would significantly enhance the interpretability and reproducibility of the results in Fig. 4(b).
5) Lines 387-389 are slightly cumbersome in syntax. It is suggested to revise them into a clearer structure.