  It has recently been recognized that the convective velocities achieved in
the current solar convection simulations might be over-estimated. The
newly-revealed effects of the prevailing small-scale magnetic field within the
convection zone may offer possible solutions to this problem. The small-scale
magnetic fields can reduce the convective amplitude of small-scale motions
through the Lorentz-force feedback, which concurrently inhibits the turbulent
mixing of entropy between upflows and downflows. As a result, the effective
Prandtl number may exceed unity inside the solar convection zone. In this
paper, we propose and numerically confirm a possible suppression mechanism of
convective velocity in the effectively high-Prandtl number regime. If the
effective horizontal thermal diffusivity decreases (the Prandtl number
accordingly increases), the subadiabatic layer which is formed near the base of
the convection zone by continuous depositions of low entropy transported by
adiabatically downflowing plumes is enhanced and extended. The global
convective amplitude in the high-Prandtl thermal convection is thus reduced
especially in the lower part of the convection zone via the change in the mean
entropy profile which becomes more subadiabatic near the base and less
superadiabatic in the bulk.
