  Modern dune fields are valuable sources of information for the large-scale
analysis of terrestrial and planetary environments and atmospheres, but their
study relies on understanding the small-scale dynamics that constantly generate
new dunes and reshape older ones. Here we designed a landscape-scale experiment
at the edge of the Gobi desert, China, to quantify the development of incipient
dunes under the natural action of winds. High-resolution topographic data
documenting 42~months of bedform dynamics are examined to provide a spectral
analysis of dune pattern formation. We identified two successive phases in the
process of dune growth, from the initial flat sand bed to a meter-high periodic
pattern. We focus on the initial phase, when the linear regime of dune
instability applies, and measure the growth rate of dunes of different
wavelengths. We identify the existence of a maximum growth rate, which readily
explains the mechanism by which dunes select their size, leading to the
prevalence of a 15~m-wavelength pattern. We quantitatively compare our
experimental results to the prediction of the dune instability theory using
transport and flow parameters independently measured in the field. The
remarkable agreement between theory and observations demonstrates that the
linear regime of dune growth is permanently expressed on low-amplitude bed
topography, before larger regular patterns and slip faces eventually emerge.
Our experiment underpin existing theoretical models for the early development
of eolian dunes, which can now be used to provide reliable insights into
atmospheric and surface processes on Earth and other planetary bodies.
