I know next to nothing about Rocketry and Space Flight. I assume that the majority of the force generated by a rocket's engine is simply the result of throwing gasses out of the bottom very fast, and relying on Newton III.
But when the rocket is near enough to the ground for the exhaust to be meaningful interacting with the surface (e.g. during take off, or during SpaceX's booster landings) does the engine thrust increase to any significant degree? (perhaps due some of the gaseous particles hitting the ground and then bouncing back upwards?)
If yes, can you give any indication of HOW much? Is it enough that launch/landing mechanics need to take it into account?, or is it no more significant than, say, air densities/pressures varying due to temperature or breezes.