Does air in the atmosphere suffer friction in some way due solely to the planet's rotation? I mean, if you took a rotating planet with an atmosphere (not being influenced or heated by its star, its nucleus or any other element), would the air get some friction only due to the planet's rotation? Does this slow down the planet's rotation?
If an atmosphere were suddenly dumped onto a rotating planet, the planet would slightly slow down, some of the angular momentum is transferred from the planet to the atmosphere, to the point that the atmosphere rotates at the same speed as the planet. After which point, the planet would no longer slow down, as there is no air in the vacuum of space for the atmosphere to drag against and transfer any momentum to.
Due to conservation of angular momentum, only some weather effect accelerating the air faster than and in the direction of the planet rotation could actually slow the planet down.
The angular kinetic energy will not just disappear for no reason.
In the absence of weather, the atmosphere rotates right along with the planet and so neither exerts any net torque or drag on the other.
I do not know if the air mass movements and mixing effects of weather produce any net drag on the surface of the earth; perhaps the experts here can weigh in on this.