strongly with respect to time.

The slip face of the first dune is missing or is short due to the short evolution time (Figure \[fig:transv\_evo3\]). An observation of this absence for example at a coast where the sea provides a certain amount of sand supply was not found in the literature. The simulation depicted here and any other simulation of dune fields with lengths of 1 km to 4 km do not show a system that reaches a stationary state. According to older dune fields and less climate changes produce larger transverse dunes. The smaller slope at the brink found in this modeling agrees also with qualitative observations. In the model a dune height of $100$ m is reached in roughly $50$ years. Estimates have predicted some $10,000$ years to develop a $100$ m dune. Probably this large difference can be explained by a smaller average wind velocity, changes of wind direction over longer periods and changes in sand supply and climate acting on real dunes. The so called memory (the time to build a dune beginning with a plain sand bed) is related to the ratio of height of the dune and annual rate of sand flux $H/Q_{ann}$ [@Cooke93]. 