Macroscopic Flow Disequilibrium Over Aeolian Dune Fields

Aeolian dune fields are self‐organized patterns formed by wind‐blown sand. Dunes are topographic roughness elements that impose drag on the atmospheric boundary layer (ABL), creating a natural coupling between form and flow. While the steady‐state influence of drag on the ABL is well studied, nonequilibrium effects due to roughness transitions are less understood. Here we examine the large‐scale coupling between the ABL and an entire dune field. Field observations at White Sands, New Mexico, reveal a concomitant decline in wind speed and sand flux downwind of the transition from smooth playa to rough dunes at the upwind dune‐field margin, that affects the entire ∼10‐km ‐long dune field. Using a theory for the system that accounts for the observations, we generalize to other roughness scenarios. We find that, via transitional ABL dynamics, aeolian sediment aggradation can be influenced by roughness both inside and outside dune fields. Plain Language Summary Just as how cyclists opt to have smooth skin to gain a speed advantage, near‐surface winds in the atmosphere are faster over smooth topography. Dunes slow winds down because they are rough, but these features are also shaped by the winds. When wind passes over a boundary between smooth and rough surfaces, it cannot instantaneously slow down; instead, there is a transition zone where it decelerates. This area of transition can exist at the edge of a dune field if the land upwind of it is relatively smooth. We observe this at White Sands Dune Field, and generalize the results with theory. Aside from helping to explain the striking patterns we see on planetary surfaces, this result is useful because dunes are often used to interpret wind conditions. Key Points Wind and sediment flux decrease downwind from the leading edge of White Sands Dune Field A Lagrangian 1‐D model explains how nonequilibrium flow induced by surface roughness transitions produces observed dynamics Results are generalized using the 1‐D model for other roughness scenarios