  The wavelength at which a dune pattern emerges from a flat sand bed is
controlled by the sediment transport saturation length, which is the length
needed for the sand flux to adapt to a change of wind strength. The influence
of the wind shear velocity on this saturation length and on the subsequent dune
initial wavelength has remained controversial. In this letter, we present
direct measurements of the saturation length performed in a wind tunnel
experiment. In complement, initial dune wavelengths are measured under
different wind conditions -- in particular after storms. Using the linear
stability analysis of dune formation, it is then possible to deduce the
saturation length from field data. Both direct and indirect measurements agree
that the saturation length is almost independent of the wind strength. This
demonstrates that, in contrast with erosion, grain inertia is the dominant
dynamical mechanism limiting sediment transport saturation.
