Wind Farm Wakes Affect Long-Term Climate Observations at the ARM Central Facility

Individual wind turbines generate wakes, regions of slower and more turbulent air downwind of the turbine. Clusters of wind turbines, or wind farms, will also generate larger wind farm wakes, creating wind speed deficits and surface temperature impacts downwind. Satellite measurements of surface winds near offshore wind farms have observed wind farm wakes extending 50 km downwind; aircraft measurements of wakes have also confirmed wind speed deficits extending at least 45 km downwind of wind farms. In situ measurements within wind farms have identified surface temperature impacts within a few km of individual wind turbines.

While these far downwind impacts may sound impressive, wakes such as these are not persistent features of the environment near wind farms. The length and magnitude of a wind farm wake is variable, depending on the inflow wind speed, inflow wind direction, and atmospheric stratification. Stably-stratified conditions, with very low levels of ambient turbulence, lead to longer wakes. In fact, wakes may not even be detectible during daytime convective conditions if ambient turbulence is vigorous enough to erode the wake within the first few rotor diameters downwind.

Therefore, assessing the impact of local wind farms on long-term climatological measurements requires carefully assessing flow conditions. At the same time, nearly 1000 wind turbines are located within a 50km radius of the Dept. of Energy’s Atmospheric Radiation Measurement (DOE ARM) program’s Central Facility near Lamont, OK. We employ both long-term measurements from the ARM CF and simulation studies using the WRF Wind Farm Parameterization to assess the impacts of wakes on measurements at the CF (red dot in Fig 1a,b).

For the simulations, we select a 4-day time period (8-12 Oct 2012) with strong variations in daytime stability, winds varying between 5 and 15 m s-1 at 100 m, and changing wind directions to sample possible wind farm wake variability. We consider simulations with three sets of turbines: the 2011 map of no turbines, the 2016 map of 571 wind turbines (blue dots in Fig 1a), and the 2019 map of 977 wind turbines (blue dots in Fig 1b). Fewer than 30 turbines were erected during the measurement collection period of 8-12 Oct 2012, but extensive wind and temperature profile measurements were available, enabling validation of the simulations with no wind turbines during southerly flow when the few turbines were located 35 km to the north of the ARM CF.

For the simulations without wind turbines, the simulated winds profile matches the observations well, especially in capturing the magnitude and height of the southerly nocturnal low-level jet. In the simulation incorporating the set of turbines installed by 2016 (571 turbines), the location of the ARM CF experiences wind speed deficits at 100 m between 0.2 and 2.75 m s-1, with the stronger deficits occurring at night in southwesterly flow conditions (Figure 1d,e). The simulation with the 2019 map of 977 turbines, with the addition of 406 more turbines, some of which are within 5km of the ARM CF, predicts just slightly larger wind speed deficits during southwesterly conditions (Figure 1d,e), and much larger wind speed deficits in southerly conditions. This result indicates that nightly wind speed deficits are be expected at the ARM CF from turbines 30 km away from the ARM CF.

We can also examine the near-surface flux measurements at the ARM CF before and after the installation of the 2017 wind farm, 3.5 km south of the CF. We focus on southerly flow conditions, and segregate daytime and nighttime measurements of turbulent kinetic energy. While daytime turbulent kinetic energy at 4m above the surface shows no statistically significant impact due to the installation of the wind farm, nocturnal TKE increases by 20% after the installation of the wind farm. Analysis of surface temperature measurements is ongoing.

The presentation will highlight the variability of wind turbine wakes and will summarize likely impact on surface and elevated measurements at the ARM CF.