Utility-scale wind turbines operate in dynamic flows that can vary significantly over time scales from less than a second to several years. To better understand the inflow to utility-scale turbines on time scales from seconds to minutes, the National Renewable Energy Laboratory installed and commissioned two inflow measurement towers at the National Wind Technology Center near Boulder, Colorado, in 2011. These towers are 135 m tall and instrumented with sonic anemometers, cup anemometers, wind vanes, and temperature measurements to characterize the inflow wind speed and direction, turbulence, stability and thermal stratification for two utility-scale turbines. In this paper, we present variations in mean and turbulent wind parameters with height, atmospheric stability, and as a function of wind direction that could be important for turbine operation, and for the persistence of turbine wakes. Wind speed, turbulence intensity, and dissipation are all factors that affect turbine performance. Our results show that these all vary with height across the rotor disk, demonstrating the importance of measuring atmospheric conditions that influence wind turbine performance at multiple heights in the rotor disk, rather than relying on extrapolation from conditions measured at lower levels.

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