flow and the turbulent layer of the eddy after the brink. The brink separates windward side and slip face. In this case the brink and the crest do not coincide which means in this case that this dune has not yet reached a stationary state.

The dune model {#sec:model}
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The model described here can be seen as a minimal model including the main processes of dune morphology. As predecessor of the model described here the work of revealed interesting new insights into dynamics and formation. The model is now extended to a two dimensional shear stress calculation (longitudinal and lateral direction), a full sand bed and different boundary conditions. Nevertheless the wind is restricted to be constant and unidirectional in time. In this article the model is used to simulate transverse dune fields. In every iteration the horizontal shear stress ${\boldsymbol{\rm \tau}}$ of the wind, the saltation flux ${\boldsymbol{\rm q}}$ and the flux due to avalanches are calculated. The time scale of these processes is much smaller than the time scale of changes in the dune surface so that they are treated to be instantaneous. In the calculation of the surface evolution a time step of 3–5 hours is 