Posters

Abstract

The drag forces exerted by vegetation on boundary layer winds are crucial when modelling wind speed and turbulence intensity over a wind farm site. The default approach in Large-Eddy Simulation (LES) and mesoscale models is to use a surface roughness length that depends on the type of vegetation. For highly forested sites, however, the surface roughness approach does not give satisfactory results as it is found to overestimate wind speeds. A canopy parametrisation differs from the surface roughness approach by exerting a quadratic drag force on all model levels that are impacted by the trees and not just the lowest model level. However, since a canopy parameterization depends on details of the vegetation, global databases of tree heights and leaf area index (LAI) are needed to apply this method with global coverage. In this research we show how a new data source of global tree height can be used together with a digital surface model to estimate the required parameters for the canopy parameterization. We present results over a number of forested sites, with a special emphasis on the Hyytiälä measurement station located in a heavily forested location in southern Finland with tree heights around 30m. In general, the results show that the canopy parameterization strongly reduces wind speed bias compared to the traditional surface roughness approach. Typical changes in average wind speed are in the range of 0.5 m/s. For turbulence intensity, the results provide a more mixed picture: for some sites, the turbulence intensity is overestimated with the new canopy parameterization. The overestimation in turbulence intensity can partly be explained by the lower average wind speeds, but the canopies also generate turbulence by introducing an inflection point in the wind speed profile. Further work will focus on removing the causes of the turbulence overestimation, a refinement of the seasonal dependencies of leaf area index as well as on a better understanding of how trees affect the thermodynamic coupling between soil and atmosphere.