This collection contains supporting information for M. Y. Louge, A. Valance, J. Fang, S. Harnett, F. Porte-Agel, P. Chasle, Evolution of turbulent boundary conditions on the surface of large barchan dunes: anomalies in aerodynamic roughness and shear velocity, aeolian thresholds and the role of dune skewness, J. Geophys. Res. Earth Surface (2023). It consists of one .xlsx Excel workbook and six .csv worksheets, with contents summarized in the readme.txt file. Wind friction is the engine that erodes sand dunes, relentlessly pushing them over roads, houses and infrastructure. Our records of wind speed on crescent-shaped mobile dunes challenge conventional understanding of this process. Using field measurements and models, we show that the highest friction occurs where the gentle upward dune surface abruptly gives way to a steeper avalanching downward slope. Our data also reveals that the `aerodynamic roughness', a measure of wind friction on sand, contradicts existing models based on historical data for turbulent pipe flow. Because numerical simulations are used to predict flow over landforms that are inaccessible to detailed measurements, we validate them against data on a large dune. Our observations imply that, to achieve greater fidelity, simulations should subdivide the fluid neighborhood of the dune more finely, and revisit how they treat aerodynamic friction on its surface. Although our work involved large desert dunes, we expect these suggestions to apply more broadly to atmospheric, fluvial or submarine landforms that are surrounded by rougher terrain or that feature sudden changes in slope.