  We analyze sensitive, sub-arcsecond resolution ALMA Science Verification
observations of CO emission lines in the protoplanetary disk hosted by the
young, isolated Ae star HD 163296. The observed spatial morphology of the 12CO
J=3-2 emission line is asymmetric across the major axis of the disk; the 12CO
J=2-1 line features a much less pronounced, but similar, asymmetry. The J=2-1
emission from 12CO and its main isotopologues have no resolved spatial
asymmetry. We associate this behavior as the direct signature of a vertical
temperature gradient and layered molecular structure in the disk. This is
demonstrated using both toy models and more sophisticated calculations assuming
non-local thermodynamic equilibrium (non LTE) conditions. A model disk
structure is developed to reproduce both the distinctive spatial morphology of
the 12CO J=3-2 line as well as the J=2-1 emission from the CO isotopologues
assuming relative abundances consistent with the interstellar medium. This
model disk structure has tau=1 emitting surfaces for the 12CO emission lines
that make an angle of about 15 degrees with respect to the disk midplane.
Furthermore, we show that the spatial and spectral sensitivity of these data
can distinguish between models that have sub-Keplerian gas velocities due to
the vertical extent of the disk and its associated radial pressure gradient (a
fractional difference in the bulk gas velocity field of approximately greater
than 5%).
