  We clarify the mechanism of accelerated core formation by large-scale
nonlinear flows in subcritical magnetic clouds by finding a semi-analytical
formula for the core formation time and describing the physical processes that
lead to them. Recent numerical simulations show that nonlinear flows induce
rapid ambipolar diffusion that leads to localized supercritical regions that
can collapse. Here, we employ non-ideal magnetohydrodynamic simulations
including ambipolar diffusion for gravitationally stratified sheets threaded by
vertical magnetic fields. One of the horizontal dimensions is eliminated,
resulting in a simpler two-dimensional simulation that can clarify the basic
process of accelerated core formation. A parameter study of simulations shows
that the core formation time is inversely proportional to the square of the
flow speed when the flow speed is greater than the Alfv\'en speed. We find a
semi-analytical formula that explains this numerical result. The formula also
predicts that the core formation time is about three times shorter than that
with no turbulence, when the turbulent speed is comparable to the Alfv\'en
speed.
