Some ground effect theory explain increase in lift with reduction in downwash,which cause increase in effective AoA.
One theory mention only increase in static pressure under the wing "cushion effect", because wing partialy block airlfow.
When comapre wing in ground effect(h/c=2.5%) and wing out of ground effect(OGE),both at geometric AoA = 6°, at this diagram we can see how biggiest change in Cp is done at bottom side of wing, change in Cp at top side of wing is very small,almost negligible.That mean bottom side is "key", side that make a difference.
It is well known when wing increase AoA, change in Cp is allways higher on top side,on bottom side change in Cp is small.
Is effective AoA increased at wing in ground effect, or if effective AoA is increased why change in Cp is not higher at top side compare to change in Cp at bottom side?
In other words, I dont see connection with "downwash/increased eff. AoA theory" with this diagram. But maybe I am wrong?
Unfortunately diagram show airfoil(wing with isolated end effects) test in wind tunnel, we can assume that same pressure distribution is at wing.
Cp= pressure coefficents, x/C= percent of chord lenght, h= height from ground to trailing edge, c= chord line length
h/c= smaller number,closer to ground