Nocturnal Surface-Layer Observations of Turbulence Measurements at a Wind-farm in Central Iowa

Due to the need for renewable sources of energy, there has been substantial interest in developing wind power as a viable resource. Thus, while wind power is estimated to provide only 2% of the nation's current energy needs, its contribution is projected to reach 20% by 2030. It is also anticipated that much of the onshore production of wind power will occur in a swath stretching from Oklahoma and Texas through the Upper Midwest (Minnesota, Iowa, Illinois, Nebraska, and Kansas) and into North and South Dakota. This region is also home to a significant portion of the U.S. production of wheat, corn and soybeans. In light of the growing trend of integrating wind turbines into agricultural production areas, concern has been expressed regarding the potential impacts of this practice on crop yields. Specifically these concerns focus on the effects of perturbations in the temperature, carbon dioxide concentrations and air flow in the surface-layer over croplands. A study was conducted during the summer of 2010 in central Iowa in which four 10 m eddy covariance towers were located along a transect located in a wind farm that was planted with corn. One tower was positioned upwind of the wind turbines while the other three were located downwind of either one or two lines of wind turbines. This provided a unique opportunity to study the effects of wind turbines on the downwind microclimate above crop canopies. Sonic anemometers were at 6 m AGL with additional ancillary measurements of temperature and humidity. In this study, the turbulent regimes under nocturnal (stable) conditions during periods when the turbines were operating and when the turbines were shut down were compared. The results of a suite of time series, spectral, and wavelet analyses of the wind velocity components and temperature suggest that the wind turbines may cause perturbations in the downwind turbulent conditions in some, but not all, cases. The potential causes and implications of this will also be discussed.