Displacement, but exclusively in the +Z direction

I have a subdivided cube and I would like the top to have a rippled, displaced effect, but I want the displacement (I’m using Noise) to be exclusively in the +Z direction. IOW, I don’t want any of the displacement occurring towards the inside of the cube.

How does one control the displacement in this fashion?

Thanks!

Got it: the MIDLEVEL parameter in the Displacement Modifier is the key to restricting the displacement from one side to the other. In this case, displacing to the exterior, midlevel=0 , when using a positive Strength value.