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Accurate prescription of ground-level boundary conditions is a key requirement for large-eddy simulation (LES) of atmospheric boundary-layer (ABL) flow. When the lower boundary is a homogeneously rough surface, the Monin–Obokhov (MO) similarity theory is a well-tested approach for specification of the boundary fluxes. This approach requires an empirically calibrated hydrodynamic roughness length to represent the aggregate effects of all flow-resisting surface details at unresolved scales, and a displacement height to represent a vertical shift in the velocity profiles. For cases in which the surface height distribution varies slowly enough in the horizontal direction that it is spatially resolvable in the LES, but where the height itself is small enough to fall below the first vertical grid point, special challenges arise. First, it is shown herein that prescription of a horizontally varying displacement height, in the …