  Radiation pressure dominated accretion discs around compact objects may have
turbulent velocities that greatly exceed the electron thermal velocities within
the disc. Bulk Comptonization by the turbulence may therefore dominate over
thermal Comptonization in determining the emergent spectrum. Bulk
Comptonization by divergenceless turbulence is due to radiation viscous
dissipation only. It can be treated as thermal Comptonization by solving the
Kompaneets equation with an equivalent "wave" temperature, which is a weighted
sum over the power present at each scale in the turbulent cascade. Bulk
Comptonization by turbulence with non-zero divergence is due to both pressure
work and radiation viscous dissipation. Pressure work has negligible effect on
photon spectra in the limit of optically thin turbulence, and in this limit
radiation viscous dissipation alone can be treated as thermal Comptonization
with a temperature equivalent to the full turbulent power. In the limit of
extremely optically thick turbulence, radiation viscous dissipation is
suppressed, and the evolution of local photon spectra can be understood in
terms of compression and expansion of the strongly coupled photon and gas
fluids. We discuss the consequences of these effects for self-consistently
resolving and interpreting turbulent Comptonization in spectral calculations in
radiation MHD simulations of high luminosity accretion flows.
