Field evidence for the upwind velocity shift at the crest of low dunes

TL;DR

This study provides field evidence that wind velocity peaks upwind of dune crests, supporting hydrodynamical models and elucidating mechanisms behind dune formation and growth.

Contribution

First field measurements confirming the upwind velocity shift at dune crests and its agreement with hydrodynamical linear analysis predictions.

Findings

Wind velocity peaks upwind of dune crest

Agreement with hydrodynamical linear analysis

Implications for dune initiation and growth

Abstract

Wind topographically forced by hills and sand dunes accelerates on the upwind (stoss) slopes and reduces on the downwind (lee) slopes. This secondary wind regime, however, possesses a subtle effect, reported here for the first time from field measurements of near-surface wind velocity over a low dune: the wind velocity close to the surface reaches its maximum upwind of the crest. Our field-measured data show that this upwind phase shift of velocity with respect to topography is found to be in quantitative agreement with the prediction of hydrodynamical linear analysis for turbulent flows with first order closures. This effect, together with sand transport spatial relaxation, is at the origin of the mechanisms of dune initiation, instability and growth.