Wind‐blown sand dunes are both a consequence and a driver of climate dynamics; they arise under persistently dry and windy conditions, and are sometimes a source for airborne dust. Dune fields experience extreme daily changes in temperature, yet the role of atmospheric stability in driving sand transport and dust emission has not been established. Here, we report on an unprecedented multiscale field experiment at the White Sands Dune Field (New Mexico, USA), where by measuring wind, humidity and temperature profiles in the atmosphere concurrently with sediment transport, we demonstrate that a daily rhythm of sand and dust transport arises from nonequilibrium atmospheric boundary layer convection. A global analysis of 45 dune fields confirms the connection found in situ between surface wind speed and diurnal temperature cycles, revealing an unrecognized climate feedback that may contribute to the growth of deserts on Earth and dune activity on Mars.
Early‐stage bedforms develop into mature dunes through complex interactions between wind, sand transport, and surface topography. Depending on varying environmental and wind conditions, the mechanisms driving dune formation and, ultimately, the shape of nascent dunes may differ markedly. In cases where sand availability is plentiful, the emergence and growth of dunes can be studied with a linear stability analysis of coupled transport and hydrodynamic equations. Until now, this analysis has only been applied using field evidence in unidirectional winds. However, in many areas of the world and on other planets, wind regimes are more often bimodal or multimodal. Here, we investigate field evidence of protodune formation under a bimodal wind regime by applying linear stability analysis to a developing protodune field. Employing recent development of the linear stability theory and experimental research, combined with in situ wind, sediment transport, and topographic measurements during a monthlong field campaign at Great Sand Dunes National Park, Colorado, USA, we predict the spatial characteristics (orientation and wavelength) and temporal evolution (growth rate and migration velocity) of a protodune field. We find that the theoretical predictions compare well with measured dunefield attributes as characterized by high‐resolution Digital Elevation Models measured using repeat terrestrial laser scanning. Our findings suggest that linear stability analysis is a quantitative predictor of protodune development on sandy surfaces with a bimodal wind regime. This result is significant as it offers critical validation of the linear stability analysis for explaining the initiation and development of dunes toward maturity in a complex natural environment.
more » « less- NSF-PAR ID:
- 10368378
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 125
- Issue:
- 11
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract Gravity‐driven grainflows on aeolian dunes are important agents of sand transport on Earth and Mars but have been the focus of few field studies. We present results from the first comprehensive field study to evaluate hypotheses posed by previous studies: (a) grainflow frequency depends on the sand transport rate; (b) grainflow magnitude is independent of sand transport rate; and (c) grainflow speed depends on its area. A barchan slipface was monitored with a terrestrial laser scanner and video camera, with measurements of wind speed and sand transport rate. More than 1,600 grainflows were detected and measured. Key findings support the first hypothesis, refute the second hypothesis, and support the third hypothesis. We also found that grainflow speeds measured in laboratory studies are substantially slower than comparable examples measured in this field study, and the grainflow speed and area relationships found for field and laboratory data are significantly different.
-
Abstract Wherever a loose bed of sand is subject to sufficiently strong winds, aeolian dunes form at predictable wavelengths and growth rates. As dunes mature and coarsen, however, their growth trajectories become more idiosyncratic; nonlinear effects, sediment supply, wind variability and geologic constraints become increasingly relevant, resulting in complex and history-dependent dune amalgamations. Here we examine a fundamental question: do aeolian dunes stop growing and, if so, what determines their ultimate size? Earth’s major sand seas are populated by giant sand dunes, evolved over tens of thousands of years. We perform a global analysis of the topography of these giant dunes, and their associated atmospheric forcings and geologic constraints, and we perform numerical experiments to gain insight on temporal evolution of dune growth. We find no evidence of a previously proposed limit to dune size by atmospheric boundary layer height. Rather, our findings indicate that dunes may grow indefinitely in principle; but growth depends on morphology, slows with increasing size, and may ultimately be limited by sand supply.more » « less
-
more » « less
Abstract Sand patches are one of the precursors to early stage protodunes and occur widely in both desert and coastal aeolian environments. Here we show field evidence of a mechanism to explain the initiation of sand patches on non‐erodible surfaces, such as desert gravels and moist beaches. Changes in sand transport dynamics, directly associated with the height of the saltation layer and variable transport law, observed at the boundary between non‐erodible and erodible surfaces lead to sand deposition on the erodible surface. This explains how sand patches can form on surfaces with limited sand availability where linear stability of dune theory does not apply. This new mechanism is supported by field observations that evidence both the change in transport rate over different surfaces and in situ patch formation that leads to modification of transport dynamics at the surface boundary.
-
more » « less
Abstract Notwithstanding the large number of studies on bedforms such as dunes and antidunes, predicting equilibrium bedform type and geometry for a given flow regime, sediment supply and caliber remains an open problem. Here, we present results from laboratory experiments specifically designed to study how upper regime bedform type and geometry vary with sediment supply and caliber. Experiments were performed in a sediment feed flume with flow rates varying between 5 and 30 l/s and sand supply rates varying between 0.6 and 20 kg/min. We used both uniform and non‐uniform sands with geometric mean diameters varying between 0.22 and 0.87 mm. Analysis of our data and data available in the literature reveals that the ratio of total (bedload plus suspension) volume transport rate of sediment to water discharge
Q s/Q wplays a prime control on upper regime equilibrium beds. Equilibrium bedforms transition from washed out dunes (lower regime) to downstream migrating antidunes (upper regime) forQ s/Q wbetween 0.0003 and 0.0007. For values ofQ s/Q wgreater than 0.0015, the bedform length increases withQ s/Q w. At these high values ofQ s/Q w, equilibrium in fine sand is characterized by upstream migrating antidunes, cyclic steps, and significant suspended load. In experiments with coarse sand, equilibrium is characterized by plane bed with bedload transport in sheet flow mode. Standing waves form at the transition between downstream migrating antidunes and upstream migrating bedforms.
