Yes, just to be clear about the specific thing that's happening here: Large displacements that cross render bucket boundaries can be clipped with default settings. Increasing AA makes bucket sizes smaller due to more memory used per pixel (I believe), and thus the clipping behavior changes. Setting manually, larger bucket sizes may help as well, without increasing render time as Ulco mentions, but the Displacement Tolerance setting is indeed for this exact problem.
Incidentally, AA15 is a really high value for what your scene looks like. Higher AA values are really only necessary when objects (especially trees and other vegetation) are visible. For terrain and clouds only, AA values above 4 are seldom necessary.
- Oshyan