Observed thermal impacts of wind farms over northern Illinois

This paper assesses  impacts of three wind farms in northern Illinois using land surface temperature (LST) data from the Moderate Resolution Imaging Spectro radiometer  (MODIS) instruments onboard the Terra and Aqua satellites for the period 2003-2013.  Changes in LST between two periods (before and after construction of the wind turbines)  and between wind farm pixels and nearby non-wind-farm pixels are quantified. An areal  mean increase in LST by 0.18-0.39 °C is observed at nighttime over the wind farms, with  the geographic distribution of this warming effect generally spatially coupled with the  layout of the wind turbines (referred to as the spatial coupling), while there is no apparent  impact on daytime LST. The nighttime LST warming effect varies with seasons, with the strongest warming in winter months of December-February, and the tightest spatial  coupling in summer months of June-August. Analysis of seasonal variations in wind speed and direction from weather balloon sounding data and Automated Surface Observing  System hourly observations from nearby stations suggest stronger winds correspond to  seasons with greater warming and larger downwind impacts. The early morning soundings  in Illinois are representative of the nighttime boundary layer and exhibit strong temperature  inversions across all seasons. The strong and relatively shallow inversion in summer leaves warm air readily available to be mixed down and spatially well coupled with the turbine.  Although the warming effect is strongest in winter, the spatial coupling is more erratic and  spread out than in summer. These results suggest that the observed warming signal at nighttime is likely due to  the net downward transport of heat from warmer air aloft  to the surface, caused by the turbulent mixing in the wakes of the spinning turbine rotor blades.