Turbulence above offshore wind farms measured by aircraft

The flow inside and around large offshore wind farms may range from smaller and shorter structures present due to the mechanical turbulence generated by wind turbines to larger, and longer structures indicative of the mesoscale flow. In this study, we explore the variation in turbulence structures above large offshore wind farms located in the North Sea using the data obtained from a research aircraft. The aircraft was flown upstream, downstream, and above the wind farm clusters. We evaluate the normalized autocorrelation of the wind velocity component in the mean wind direction for transverse separations and the rate of energy dissipation in the inertial subrange for neutral and stable stratification. During the neutral stratification, there is high ambient turbulence in the atmosphere and large values of the energy dissipation rate. The intensity of small-scale turbulence structures is increased above and downstream of the wind farm and it dominates the mesoscale flow structures. While in stable stratification, mesoscale flow structures are not only dominant upstream of the wind farm but also in the downstream flow. Apart from the atmospheric stratification, the wind farm layout also affects the turbulent structures, the rate of energy dissipation, and time for wake recovery.