  In this work we show that the physics reach of a long-baseline (LBL) neutrino
oscillation experiment based on a superbeam and a megaton water Cherenkov
detector can be significantly increased if the LBL data are combined with data
from atmospheric neutrinos (ATM) provided by the same detector. ATM data are
sensitive to the octant of $\theta_{23}$ and to the type of the neutrino mass
hierarchy, mainly through three-flavor effects in e-like events. This allows to
resolve the so-called $\theta_{23}$- and sign($\Delta m^2_{31}$)-parameter
degeneracies in LBL data. As a consequence it becomes possible to distinguish
the normal from the inverted neutrino mass ordering at $2\sigma$ CL from a
combined LBL+ATM analysis if $\sin^2 2\theta_{13} \gtrsim 0.02$. The potential
to identify the true values of $\sin^2 2\theta_{13}$ and the CP-phase
$\delta_{cp}$ is significantly increased through the lifting of the
degeneracies. These claims are supported by a detailed simulation of the T2K
(phase II) LBL experiment combined with a full three-flavor analysis of ATM
data in the HyperKamiokande detector.
