Greedy parabolics: Wind flow direction within the deflation basin of parabolic dunes is governed by deflation basin width and depth

摘要

Parabolic dunes are 'U' or 'V'-shaped aeolian landforms that form on pre-existing sand deposits. Their morphology consists of an upwind deflation basin, bordered by often vegetated trailing arms and a downwind depositional lobe. The orientation of parabolic dunes is commonly attributed to the prevailing or resultant wind direction. Consequently, the orientation of parabolic dunes stabilised by vegetation growth has been used as a proxy for wind direction during past climates in several studies. However, the ability or extent of parabolic dune morphology to steer incident wind flow parallel to the orientation of the landform, and thus migrate in the direction of the current landform orientation rather than prevailing wind direction, is unknown. By numerically modelling wind flow within the deflation basin of eight parabolic dunes, we demonstrate for the first time that wind flow direction within the deflation basin of a parabolic dune is highly controlled by the depth and width of the deflation basin. The greater the depth-width ratio of the landform (i.e. the deeper and narrower the deflation basin), the greater the degree of flow steering relative to the axis orientation of the landform. These results demonstrate that future studies must exercise caution when using parabolic dune orientation as a direct proxy for prevailing wind direction, especially where parabolic dunes have a relatively high deflation basin depth-width ratio, as the deflation basin of these landforms may continue to migrate in an antecedent wind direction.