How do barchan dunes migrate across a desert?

Barchan dunes are crescent-shaped sand dunes that migrate across deserts through a continuous process of erosion and deposition driven by wind blowing predominantly from one direction.12

As wind moves across the desert, it carries sand grains primarily through saltation (a jumping motion). When this wind encounters a sand patch, it deposits sand on the windward side, which has a gentle slope. As the dune grows, the leeward slope becomes increasingly steep until the wind can no longer follow the surface contour and separates from it, creating a "dead zone" in the lee.3

Sand brought up the windward slope falls into this sheltered area. Once the leeward slope reaches the angle of repose for dry sand—approximately 32 degrees—it becomes a slip face. At this point, the dune reaches a state of equilibrium; it moves forward as a whole as sand is eroded from the windward side and deposited on the leeward slip face. Smaller dunes generally migrate faster than larger ones because they require less sand to reconstitute their form.3