  Recent ALMA molecular line observations have revealed 3-D gas velocity
structure in protoplanetary disks, shedding light on mechanisms of disk
accretion and structure formation. 1) By carrying out viscous simulations, we
confirm that the disk's velocity structure differs dramatically using vertical
stress profiles from different accretion mechanisms. Thus, kinematic
observations tracing flows at different disk heights can potentially
distinguish different accretion mechanisms. On the other hand, the disk surface
density evolution is mostly determined by the vertically integrated stress. The
sharp disk outer edge constrained by recent kinematic observations can be
caused by a radially varying $\alpha$ in the disk. 2) We also study kinematic
signatures of a young planet by carrying out 3-D planet-disk simulations. The
relationship between the planet mass and the "kink" velocity is derived,
showing a linear relationship with little dependence on disk viscosity, but
some dependence on disk height when the planet is massive, e.g. $10 M_J$. We
predict the "kink" velocities for the potential planets in DSHARP disks. At the
gap edge, the azimuthally-averaged velocities at different disk heights deviate
from the Keplerian velocity at similar amplitudes, and its relationship with
the planet mass is consistent with that in 2-D simulations. After removing the
planet, the azimuthally-averaged velocity barely changes within the viscous
timescale, and thus the azimuthally-averaged velocity structure at the gap edge
is due to the gap itself and not directly caused to the planet. Combining both
axisymmetric kinematic observations and the residual "kink" velocity is needed
to probe young planets in protoplanetary disks.
