Airflow up the stoss slope of sand dunes: limitations of current understanding

The windward (stoss) side of a sand dune acts as a streamlined obstacle in the path of the wind. Continuity principles necessitate compression of the flow field up the stoss slope of a dune, and shear stress must progressively increase as the flow accelerates. Measurements in transverse flow over thirteen dunes at Padre Island, Texas, the Algodones, California, and White Sands, New Mexico, confirm that velocity profiles on the stoss slope are not log-linear, and that flow acceleration occurs very close to the surface within an internal boundary layer. As a consequence, in the overlying flow where measurements have historically been made, an overall decrease in shear stress occurs up the slope. The actual shape of the velocity profiles, and the identification of the appropriate segment of the profile from which to derive the shear stress that drives saltation represent major problems not approachable by traditional means.