On the Mechanisms of Excessively Large Drag Coefficient Under Low‐Wind Conditions

Abstract Observations show that the drag coefficient ( C D ) increases rapidly as wind speed decreases under low‐wind conditions, contradicting Monin‐Obukhov similarity theory (MOST). Analysis of multi‐year station data reveals that wind speed has a stronger influence on this anomalous C D increase than atmospheric stability. High‐resolution WRF‐LES simulations demonstrate that while moderate winds conform to MOST, the constant‐flux‐layer assumption fails under low winds: momentum flux varies by 378% within 25 m. Applying MOST under these conditions yields height‐increasing C D , explaining the excessively large C D in observations. Additionally, turbulent kinetic energy (TKE) is significantly lower under low winds, and the mean momentum flux reverses to upward. Further analysis identifies hundred‐meter‐scale eddies causing counter‐local‐gradient transport, with spectral analysis confirming their dominance in momentum flux. The near‐surface momentum flux is primarily generated by pressure perturbations and transported upward via vertical gradient terms.