Nocturnal wind-directional shear in complex terrain

In the weak-wind nocturnal boundary layer, the wind direction near the surface may vary substantially with height. The directional shear is caused by transient modes and shallow, terrain-induced motions. This study examines the large directional shear for nocturnal data collected in the lowest 10 m from three field programs. The directional shear in two broad valleys with high sidewalls is dominated by transient motions within the valley cold pool. In contrast, the directional shear within a shallow narrow valley is dominated by terrain-induced down-valley drainage flows, in spite of the fact that the magnitude and height of the valley sidewalls are much smaller than the other two valleys. The relative contribution of the directional shear to the total vector shear is examined as a function of wind speed and stability. In this article, we examine directional shear near the surface in the stable boundary layer. With weak-wind stable conditions, strong-wind directional shear is commonly generated by transient modes, density currents (such as thin cold air drainage) and other unknown mechanisms. In this photo, winds are more significant, several metres per second, yet strong directional shear survives due to persistent northerly flow at the surface (right to left) and overlying transient easterly flow above 3 m. The fully developed turbulence at the surface diffuses machine-generated fog into the overlying easterly flow.