  The Zombie Vortex Instability (ZVI) occurs in the dead zones of
protoplanetary disks (PPDs) where perturbations excite baroclinic critical
layers, generating "zombie" vortices and turbulence. In this work, we
investigate ZVI with nonuniform vertical stratification; while ZVI is triggered
in the stratified regions away from the midplane, the subsequent turbulence
propagates into and fills the midplane. ZVI turbulence alters the background
Keplerian shear flow, creating a steady-state zonal flow. Intermittency is
observed, where the flow cycles through near-laminar phases of zonal flow
punctuated by chaotic bursts of new vortices. ZVI persists in the presence of
radiative damping, as long as the thermal relaxation timescale is more than a
few orbital periods. We refute the premature claim by Lesur & Latter (2016)
that radiative damping inhibits ZVI for disk radii r>0.3 au. Their conclusions
were based on unrealistically short cooling times using opacities with
virtually no grain growth. We explore different grain growth and vertical
settling scenarios, and find that the gas and dust in off-midplane regions are
not necessarily in local thermodynamic equilibrium (LTE) with each other. In
such cases, thermal relaxation timescales can be orders of magnitude longer
than the optically thin cooling times assuming LTE because of the finite time
for energy to be exchanged between gas and dust grains via collisions. With
minimal amounts of grain growth and dust settling, the off-midplane regions of
disks are susceptible to ZVI and much of the planet-forming regions can be
filled with zombie vortices and turbulence.
