Modern dune fields are valuable sources of information for the large-scale analysis of terrestrial and planetary environments and atmospheres, but their study relies on understanding the small-scale dynamics that constantly generate new dunes and reshape older ones. Here, we designed a landscape-scale experiment at the edge of the Gobi desert, China, to quantify the development of incipient dunes under the natural action of winds. High-resolution topographic data documenting 42 mo of bedform dynamics are examined to provide a spectral analysis of dune pattern formation. We identified two successive phases in the process of dune growth, from the initial flat sand bed to a meter-high periodic pattern. We focus on the initial phase, when the linear regime of dune instability applies, and measure the growth rate of dunes of different wavelengths. We identify the existence of a maximum growth rate, which readily explains the mechanism by which dunes select their size, leading to the prevalence of a 15-m wavelength pattern. We quantitatively compare our experimental results with the prediction of the dune instability theory using transport and flow parameters independently measured in the field. The remarkable agreement between theory and observations demonstrates that the linear regime of dune growth is permanently expressed on low-amplitude bed topography, before larger regular patterns and slip faces eventually emerge. Our experiment underpins existing theoretical models for the early development of eolian dunes, which can now be used to provide reliable insights into atmospheric and surface processes on Earth and other planetary bodies.
Gobi is a type of desert pavement in Asia over which aeolian saltation generates atmospheric dust. However, the high-frequency behavior of aeolian saltation over gobi is still poorly understood. In this study, coupled high-frequency observations of three-dimensional (3-D) wind speed (U) and saltation particle count rate in a vertical array of Sensit sensors were performed in a gobi region of Milan County, southern Xinjiang, China. The results revealed that aeolian saltation over a gobi surface exhibited an intermittent phenomenon, and the level of the intermittency was governed by the relative wind strength (s), which is consistent with that of sand surfaces. The threshold wind speed (U t ) decreased exponentially with increasing transport activity parameter (AP), and a prominent decrease in U t was examined when the AP values reached 0.7; comparatively, for sand surfaces, a linearly declining trend occurred over the whole AP range of 0-1. The saltating particles over the gobi surface were distributed in layers of 0.08-0.30 m with an average value of 0.20 ± 0.04 m, which was significantly larger than that of sand surfaces. The relationship between the instantaneous sand transport rate (Q) and U over the gobi surface was also relatively poor and could be improved by lagging Q by 2 s. The mechanism for the collision between saltating particles and the gobi bed, which is rigid and complex in texture, was significantly different from that of the soft and deforming sand bed, causing the difference in aeolian saltation between gobi and sand surfaces.
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