Friction is largest in the atmospheric boundary layer, the air layer nearest the ground.
Friction always slows down the wind, and therefore also reduces the Coriolis force.
The magnitude of frictional force is determined by the roughness of the surface and the speed of the wind. The force of friction over still water or an ice pond is small. Trees in a forest strongly oppose the wind, so the frictional force of forests is large. Meteorologists consider the effects of friction when dealing with the air within 20 to 200 meters of the surface. In the surface layer friction acts to slow the wind, and thus always acts opposite to the wind direction.