  Radiation pressure dominated accretion discs may have turbulent velocities
that exceed the electron thermal velocities. Bulk Comptonization by the
turbulence may therefore dominate over thermal Comptonization in determining
the emergent spectrum. We discuss how to self-consistently resolve and
interpret this effect in calculations of spectra of radiation MHD simulations.
In particular, we show that this effect is dominated by radiation viscous
dissipation and can be treated as thermal Comptonization with an equivalent
temperature. We investigate whether bulk Comptonization may provide a physical
basis for warm Comptonization models of the soft X-ray excess in AGN. We
characterize our results with temperatures and optical depths to make contact
with other models of this component. We show that bulk Comptonization shifts
the Wien tail to higher energy and lowers the gas temperature, broadening the
spectrum. More generally, we model the dependence of this effect on a wide
range of fundamental accretion disc parameters, such as mass, luminosity,
radius, spin, inner boundary condition, and alpha. Because our model connects
bulk Comptonization to one dimensional vertical structure temperature profiles
in a physically intuitive way, it will be useful for understanding this effect
in future simulations run in new regimes. We also develop a global Monte Carlo
code to study this effect in global radiation MHD simulations. This code can be
used more broadly to compare global simulations with observed systems, and in
particular to investigate whether magnetically dominated discs can explain why
observed high Eddington accretion discs appear to be thermally stable.
