Transition in Atmospheric Turbulence Structure from Neutral to Convective Stability States

Abstract

The magnitude and structure of the turbulence eddies in the atmosphere surface layer directly impacts the operation of wind turbines through the temporal variabilities in shaft torque and in blade, shaft bending moments that alter power production and reduce reliability due to premature component failure. The scales, strengths and detailed structure of daytime atmospheric turbulence is strongly dependent on the relative contribution of buoyancy-driven vertical motions from surface heating to shear driven by a geostrophic wind at the mesoscale. In this study, we apply spectral LES at high resolution and low dissipation to describe and quantify the turbulence structure over the range of stability states that characterize the transition from the purely shear-driven neutral boundary layer to the moderately convective atmospheric boundary layer, driven by both shear and buoyancy. Whereas we anticipated a transition that is rapid but monotonic in the stability parameter –zi/L (zi is the boundary layer depth and L the Monin-Obukhov length scale), our simulations indicate a particular transitional state with extraordinarily strong streamwise coherence in streamwise and vertical velocity when the stability state is characterized by – zi/L  1. Visualizations indicate that this particular turbulence structure is quite differnet from slightly less or more unstable boundary layers. The degree of streamwise coherence decreases quantitatively more rapidly towards the neutral limit (-zi/L 1.5) states. We scrutinize these observations and hypothesise that the special stability state with abnormally strong coherence could create unusually strong variations in wind turbine loadings.