I didn't use a wind tunnel, just noticed this effect on
landing and takeoff in a 172. I suppose one could
trim for min sink, and then with no stick input watch
a plane or glider fly into ground effect.

From there you should see the nose initially drop. Due
to oscillation, it may come up again, but I always saw
an initial drop.

I saw the reverse when flying out of ground effect, although
it was also subtle because it was masked by the
osscillations a bit. Once out of ground effect, the nose
would pitch up, and the horn would be just chirping for
the stall. I'm guessing this is why the PTS soft field
power technique is to "remain in ground effect while accelerating
to Vx or Vy, as appropriate" after the wheels come off the ground.

As pointed out, this is a dynamic effect, and combined with
oscillation, pretty subtle.