In situ observations of the influence of a large onshore wind farm on near-surface temperature, turbulence intensity and wind speed profiles - Figure 1.
Figure 1. Overview of the wind farm, including close-up views of the NE and SW locations, and schematic of the unwaked (#1, black), SW farm-waked (#2, red), SW direct-waked (#3, blue), NE farm-waked (#4, green) and NE direct-waked (#5, magenta) wind direction bins. The frequency with which flow greater than 4 ms−1 was observed in the five directional sectors during 04/04/2012–05/20/2012 is 6.9, 16.6, 7.9, 14.2 and 1.8%, respectively.
Observations of wakes from individual wind turbines and a multi-megawatt wind energy installation in the Midwestern US indicate that directly downstream of a turbine (at a distance of 190 m, or 2.4 rotor diameters (D)), there is a clear impact on wind speed and turbulence intensity (TI) throughout the rotor swept area. However, at a downwind distance of 2.1 km (26 D downstream of the closest wind turbine) the wake of the whole wind farm is not evident. There is no significant reduction of hub-height wind speed or increase in TI especially during daytime. Thus, in high turbulence regimes even very large wind installations may have only a modest impact on downstream flow fields. No impact is observable in daytime vertical potential temperature gradients at downwind distances of >2 km, but at night the presence of the wind farm does significantly decrease the vertical gradients of potential temperature (though the profile remains stably stratified), largely by increasing the temperature at 2 m.