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From Wikipedia (and referenced on that page):
"The phenomenon of 'wing in ground effect' is caused by the ground 'interrupting' the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the performance of the aircraft while it is experiencing the ground effect."
Ground effect reduces drag, and hence increases L/D, giving a gain in performance. Its not a "lift increaser", its a "drag decreaser".
Be careful about "downwash" here. There are two definitions, one the downward motion of air behind the wing, the other is due to induced drag of the wingtip vortices which reduces local angle of attack.
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