Ground Effect: Lift is a function of low pressure on top of the blade, and high pressure underneath the blade. When hovering close to the ground, downwash creates an artificially high pressure beneath the blade, thus increasing the rotor systems efficiency.
Recirculation: If hovering in ground effect close to a solid structure like a hangar, the downwash attempting to escape horizontally from beneath the helicopter may encounter the solid structure and be forced upward. As the downwash rises, it can get drawn back into the rotor system, creating a circular airflow with an ever increasing velocity. The downward velocity of this airflow into the rotor system reduces the angle of attack, thus decreasing lift, and can force the helicopter into a hard landing.
Thoughts welcomed.
Hmm, the thoughts moved more to Zzzzz due to Ohmygawditis.
But you are not quite right on ground effect, it is not a pressure increase below the disc, rather it is a change of direction of the downwash, which reduces the induced drag, giving the disc system apparently more power, or the ability to hover with less collective pitch.
And recirculation, which I have never experienced or seen in 45 years of choppering, should have a different result from what is commonly accepted. People say that because the lift is reduced on the side next to the obstacle, be it a hangar or a cliff, the aircraft should be drawn into that obstacle.
But the loss of lift happens at one point, and with phase lag (glad you didn't talk of precession) the result should happen 90 degrees or so later. This would make the aircraft drift PARALLEL to the obstacle.
Riddle me that one.