A novel method to quantify atmospheric stability

Atmospheric stability significantly influences the characteristics of a wind resource and strongly affects wind turbine power production and structural loads. Stability is governed by the thermal structure of the atmospheric boundary layer (ABL). Unstable ABLs are convective and increase turbulence, but reduce vertical wind shear, while stable ABLs reduce turbulence and increase wind shear. In neutral ABLs mechanical effects of terrain and roughness dominate. The Monin-Obukhov length (MOL) and Richardson numbers are the recommended methods to quantify atmospheric stability. These methods require advanced and expensive measurements of temperature differences or 3D-sonic covariances, not installed on standard wind power masts. This study presents a novel methodology to quantify stability based only on standard wind measurements. At high wind speeds turbulence and wind shear converge to the neutral ABL response in a given direction while the relative deviations of shear and turbulence at lower wind speeds strongly correlate with stability. When normalized by the neutral shear and turbulence, these deviations directly quantify stability. Here we present a novel method to estimate MOL directly from the ratio of normalized shear and turbulence. The proposed method to quantify stability from standard wind measurements, provides an important input to advanced wind flow modelling of stability effects to improve the accuracy of predicted power production and structural loads.