  We use a multi-dimensional hydrodynamics code to study the gravitational
interaction between an embedded planet and a protoplanetary disk with emphasis
on the generation of vortensity (potential vorticity) through a Baroclinic
Instability and subsequent development of Rossby-Wave-Instability (RWI). It is
found that the generation of potential vorticity is very common and effective
in non-barotropic disks through the Baroclinic Instability, especially within
the coorbital region. Our results also complement previous studies by
\citet{KLL03} that non-axisymmetric RWIs are likely to develop at local minima
of potential vorticity distribution that are generated by the interaction
between a planet and a inviscid barotropic disk. This second instability
appears to be very common and robust, regardless of the equation of state,
initial density distribution, and rotational law of the disk. The development
of RWIs results in non-axisymmetric density blobs, which exert stronger torques
onto the planet when they travel in the vicinity of the planet. As a result of
that, large amplitude oscillations are introduced to the time behavior of the
total torque acted on the planet by the disk. In our current simulations, RWIs
do not change the overall picture of inward orbital migration but bring in a
non-monotonic behavior to the migration speed. As a side effect, RWIs also
introduce interesting structures into the disk. These structures may help the
formation of Earth-like planets in the Habitable Zone or Hot Earths interior to
a close-in giant planet.
