Probability distribution of TI at the (a) SW and (b) NE locations during the day (solid) and night (dashed) for 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 sectors

<p><strong>Figure 3.</strong> Probability distribution of TI at the (a) SW and (b) NE locations during the day (solid) and night (dashed) for 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 sectors.</p> <p><strong>Abstract</strong></p> <p>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 (<em>D</em>)), 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 <em>D</em> 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.