Correction of Wind Profiles Due to Thermal Stratification

Abstract

In this chapter the influence of thermal stratification on the wind profile is investigated in detail. The statistical distribution reveals that non-neutral situations occur rather often and on average the stratification is slightly stable. Classifying measured vertical wind profiles into different stability classes shows that absolute wind speeds and, in particular, the gradients strongly depend on the degree of stability of the lower boundary layer. Moreover, the Monin– Obukhov length L determined from atmospheric data according to the deBruin method proves to be robust enough to serve as a stability parameter in general situations despite the rather restrictive assumptions under which L is theoretically derived. Hence, using L together with empirical functions to correct the well-known logarithmic profile due to thermal stratification leads to major improvements in transferring the wind speed to hub height for heights up to 80 m. Though the prediction of the degree of thermal stratification by the numerical weather prediction system Lokalmodell of the German Weather Service has a rather low accuracy, the investigations for two wind farms show that taking thermal stratification into account signifi- cantly improves the wind power forecast.