Velocity statistics and spectra over a forested site measured with a tall mast

In the large expansion of wind power it becomes necessary to use also non-ideal sites for the placement of turbines. Such sites may have a complex terrain in terms of surface elevation as well as being forested. The atmospheric boundary layer is assumed to be severely different as compared to the one over flat, low-vegetation areas, which changes the mean velocity distribution as well as the turbulence intensity, thereby negatively affecting both the power production and loads on the turbines. In this study we use data from a 140 m tall mast in a forest in South-Eastern Sweden, where a unique measurement campaign with sonic anemometers has been running since November 2010 for 16 months. The sonic anemometers give the three velocity components with a frequency resolution of about 10 Hz. The site is covered by approximately 20 m high trees and a 40 degree sector, representative of an approximately homogeneous forest flow, is selected for the analysis of the velocity statistics. The screening of the results indicates the presence of a constant stress layer up to 3-5 canopy heights from the ground. An evaluation of the turbulence statistics in this layer and the levels above is presented. In addition, the spectra are evaluated and compared with the commonly used turbulence models.