Friction Force

From Wind Energy wiki

Friction is another force that affects wind. As a force of opposition, friction works to decrease the speed of moving objects. In the atmosphere, friction is maximized near the surface and decreases with height until it becomes negligible.

The amount of friction applied to overlying moving air depends on the nature of the surface. Air flowing over smooth surfaces, such as water bodies, will encounter much less friction than air mving over surfaces with great topographic relief (mountainous areas). Likewise, vegetation, hills, pasture, buildings, and road all impart varying amounts of friction to moving air. With such variety, it is impossible to describe and understand the precise effects of friction across even small spaces. This makes it impossible to account fully for friction and all its effects in mathematical equations that estimate future air motions in relation to wind turbines.

Despite its difficulty, friction msut be accounted for in forecasts because it not only slows air, but it also indirectly alters the trajectory. Remember that both the Coriolis effect and the centrifugal force alter wind trajectories and both are dependent on wind speed. Faster-moving objects experience greater Coriolis and centrifugal deflection. If friction is applied, speed is reduced. If speed is reduced, so is the amount of trajectory deflection from those two forces. Thus, the varying amount of friction with height causes wind direction to shift with height.