Identical Twin Towers for Studies of Natural Turbulence and Wind Farm Boundary Layers

A twin-tower network, with continuous measurements of turbulent fluxes of momentum and heat, mean wind speed, temperature, and relative humidity at six levels (120m, 80m, 40m, 20m, 10m, and 5m), has been constructed to enable concurrent measurements of natural turbulence and wind-farm boundary layers.  Data from both towers, including sonic anemometer data, became available starting in summer 2016.  Identical towers separated by 22 km allow us to accumulate simultaneous data inside and outside a mid-continent utility-scale wind-farm on a landscape of otherwise homogeneous terrain and land use (cropland).  For wind from an 65^o sector from W to SSW the two towers are not influenced by flow from any turbines.  For outdoor turbulence experiments we are usually limited to use of spatial averages or time averages for determining statistical properties of the atmosphere (Stull, 1988). However, our twin-tower configuration allows for use of an ensemble average, albeit a two-member ensemble, for studying turbulence properties from this 65^o sector.

The tall towers offer a unique platform for other measurements in an agroecological environment.  A hyperspectral camera will capture a time and spatial history of conditions over uniform fields of crops over the entire growing season.  Other agriculturally relevant properties such as concentrations, dispersion, and vertical profiles of insects, pollen, and spores can be observed, and the changes in these properties due to wind farms can be assessed.

Operating utility-scale wind turbines introduce turbulence over a range of scales, with the largest scales being comparable with the rotor diameter.  By introducing a distinct spectral signature, turbines in a wind farm establish a boundary layer with surface fluxes and dispersive conditions distinctly different from the natural boundary layer. As land area devoted to wind farms continues to rapidly expand, characteristics of wind-farm boundary layer conditions will be needed to understand the ecological and agro-ecological aspects of this significant landscape modification.  For winds from prevailing directions at this site, our twin-tower measurement facility offers a unique capacity to compare simultaneous measurements of the natural boundary layer with the distinctly different boundary layer of a wind farm.