  We have developed time-dependent models of FU Ori accretion outbursts to
explore the physical properties of protostellar disks. Our two-dimensional,
axisymmetric models incorporate full vertical structure with a new treatment of
the radiative boundary condition for the disk photosphere. We find that FU
Ori-type outbursts can be explained by a slow accumulation of matter due to
gravitational instability. Eventually this triggers the magnetorotational
instability, which leads to rapid accretion. The thermal instability is
triggered in the inner disk but this instability is not necessary for the
outburst. An accurate disk vertical structure, including convection, is
important for understanding the outburst behavior. Large convective eddies
develop during the high state in the inner disk. The models are in agreement
with Spitzer IRS spectra and also with peak accretion rates and decay
timescales of observed outbursts, though some objects show faster rise
timescale. We also propose that convection may account for the observed
mild-supersonic turbulence and the short-timescale variations of FU Orionis
objects.
