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Emergence and growth of sand dunes results from the dynamic interactionbetween topography, wind flow and sediment transport. While feedbacks betweenthese variables are well studied at the scale of a single and relatively smalldune, the average effect of a periodic large-scale dune pattern on atmosphericflows remains poorly constrained, due to a pressing lack of data in major sandseas. Here, we compare local measurements of surface winds to the predictionsof the ERA5-Land climate reanalysis at four locations in Namibia, both withinand outside the giant linear dune field of the Namib Sand Sea. In the desertplains to the north of the sand sea, observations and predictions agree well.This is also the case in the interdune areas of the sand sea during the day.During the night, however, an additional wind component aligned with the giantdune orientation is measured, in contrast to the easterly wind predicted by theERA5-Land reanalysis. For the given dune orientation and measured wind regime,we link the observed wind deviation (over 50\textdegree) to the daily cycle ofthe turbulent atmospheric boundary layer. During the night, a shallow boundarylayer induces a flow confinement above the giant dunes, resulting in large flowdeviations, especially for the slower easterly winds. During the day, thefeedback of the giant dunes on the atmospheric flow is much weaker due to thethicker boundary layer and higher wind speeds. Finally, we propose that theconfinement mechanism and the associated wind deflections induced by giantdunes could explain the development of smaller-scale secondary dunes, whichelongate obliquely in the interdune areas of the primary dune pattern.

Local Wind Regime Induced by Giant Linear Dunes: Comparison of ERA5-Land Reanalysis with Surface Measurements