Drag Coefficients in Light Wind

Abstract

Recent developments of observational techniques have enabled us to measure wind velocities down to the extremely low wind speed region. The present authors have made turbulent flux measurements near the surface in various conditions and found that the surface drag coefficient increases with decreasing wind speed in the low wind speed region below a few meters per second without regard to the stability. The inflection occurs at higher wind speed than expected in transition from smooth flow. The detailed analysis suggests that this may be caused by the residual turbulence by local temperature fluctuation under calm conditions.