Hub-height velocity (a), wind direction (b), shear (c), and TI for the SW (solid) and NE (dashed) locations, and (e) hub height (80 m, solid) and near-surface (02 m, dashed) potential temperatures for the SW (red) and NE (blue) locations versus time of day (LST) on 05/16–05/17/2012

<p><strong>Figure 6.</strong> Hub-height velocity (a), wind direction (b), shear (c), and TI for the SW (solid) and NE (dashed) locations, and (e) hub height (80 m, solid) and near-surface (02 m, dashed) potential temperatures for the SW (red) and NE (blue) locations versus time of day (LST) on 05/16–05/17/2012. The dashed line in panel (b) indicates the extent of wind sector #3 where the SW mast is impinged upon by the adjacent wind turbine. The color vertical lines in frame (a) show the times for which vertical profiles are shown in figure S5 (available at <a href="http://stacks.iop.org/ERL/8/034006/mmedia" target="_blank">stacks.iop.org/ERL/8/034006/mmedia</a>).</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.