Mechanisms for Wind Direction Changes in the Very Stable Boundary Layer

AbstractLarge, rapid, and intermittent changes in wind direction were commonly observed in low–wind speed conditions in the very stable boundary layer during the phase 2 of the Project Sagebrush field tracer study. This paper investigates the occurrence and magnitude of these wind direction changes in the very stable boundary layer and explores their associated meteorological factors. The evidence indicates that these wind direction changes occur mainly at wind speeds of less than 1.5 m s−1 and are associated with momentum and sensible heat fluxes approaching zero in low–wind shear conditions. This results in complete vertical decoupling. They are only weakly dependent on the magnitude of turbulence. The magnitude of the wind direction changes is generally greatest near the surface, because of the greater prevalence of low wind speeds there, and decreases upward.