  By taking into account the local energy balance per unit volume between the
viscous heating and the advective cooling plus the radiative cooling, we
investigate the vertical structure of radiation pressure-supported accretion
disks in spherical coordinates. Our solutions show that the photosphere of the
disk is close to the polar axis and therefore the disk seems to be extremely
thick. However, the profile of density implies that most of the accreted matter
exists in a moderate range around the equatorial plane. We show that the
well-known polytropic relation between the pressure and the density is
unsuitable for describing the vertical structure of radiation
pressure-supported disks. More importantly, we find that the energy advection
is significant even for slightly sub-Eddington accretion disks. We argue that
the non-negligible advection may help to understand why the standard thin disk
model is likely to be inaccurate above \sim 0.3 Eddington luminosity, which was
found by some works on the black hole spin measurement. Furthermore, the
solutions satisfy the Solberg-Hoiland conditions, which indicates the disk to
be convectively stable. In addition, we discuss the possible link between our
disk model and ultraluminous X-ray sources.
