  We model the superfluid flow of liquid helium over the rough surface of a
wire (used to experimentally generate turbulence) profiled by atomic force
microscopy. Numerical simulations of the Gross-Pitaevskii equation reveal that
the sharpest features in the surface induce vortex nucleation both
intrinsically (due to the raised local fluid velocity) and extrinsically
(providing pinning sites to vortex lines aligned with the flow). Vortex
interactions and reconnections contribute to form a dense turbulent layer of
vortices with a non-classical average velocity profile which continually sheds
small vortex rings into the bulk. We characterise this layer for various
imposed flows. As boundary layers conventionally arise from viscous forces,
this result opens up new insight into the nature of superflows.
