agate_ t1_j66q62p wrote on January 28, 2023 at 2:35 AM

This engineering question needs a meteorologist! The answer lies in the turbulent mixing of the lower atmosphere.

The planetary boundary layer is the turbulent layer of air near the ground. Turbulence in this layer mixes air near the ground with air higher up. This mixes all sorts of air properties from top to bottom of the layer: for example, humid air near the ground is mixed with drier air higher up, making the ground-level air less humid and the upper air more humid.

The layer also mixes momentum, or air speed. The air at ground level -- down among the grass blades -- isn't moving at all, but it's moving very fast several kilometers up. Turbulent mixing transfers momentum across the planetary boundary layer just like humidity, making the ground-level air go faster and slowing down the air higher up.

And now for the kicker: the amount of turbulence in the layer depends on solar heating. When the ground is heated by the sun, hot air rises. The rising plumes of air increase the turbulent mixing in the planetary boundary layer and cause it to extend higher up.

So, during the day, mixing in the boundary layer is more intense, so more slow-moving air at ground level is stirred up to the height of the wind turbine blades, so they experience slower wind speeds. At night, the PBL doesn't carry slow-moving air up to the turbines, so they get the full force of the upper-level winds.

You may have noticed that for you as a human, nights seem to be calmer, and it's windier during the day, which is the opposite of what wind turbines feel. This is the same effect in reverse! You're so close to the ground that you don't feel much wind unless turbulence in the planetary boundary layer brings it down to your height.

kilotesla t1_j696fnt wrote on January 28, 2023 at 5:25 PM

Thanks, that's a fantastic explanation of a really interesting phenomenon!

I wanted to see a graph of it and found this plot of wind speed by hour of the day at different heights above the ground, and it very clearly shows exactly what you're describing, with the neutral point between the two trends at about 150 m. Not all wind turbines are tall enough to be above that, but I'm not sure how representative that particular data is, and the height of a wind turbine tower is often augmented by putting it on a hill.

agate_ t1_j6a7c2x wrote on January 28, 2023 at 9:38 PM

Neat, I hadn’t seen the data presented that way before. Your graph also shows an important difference between land and sea: because the sea surface is usually smoother than the land (no trees and hills), there’s less turbulent mixing and the boundary layer is usually thinner. So the neutral point you’re talking about is much lower at sea.

The weaker, thinner boundary layer also means the overall wind speed is much higher at sea, which is why people go to the trouble of building offshore wind turbines.