  Under nonlocal convection theory, convection extends without limit therefore
no apparent boundary can be defined clearly as in the local theory. From the
requirement of a similar structure for both local and non-local models having
the same depth of convection zone, and taking into account the driving
mechanism of turbulent convection, we argue that a proper definition of the
boundary of a convective zone should be the place where the convective energy
flux (i.e. the correlation of turbulent velocity and temperature) changes its
sign. Therefore, it is convectively unstable region when the flux is positive,
and it is convective overshooting zone when the flux becomes negative. The
physical picture of the overshooting zone drawn by the usual non-local
mixing-length theory is not correct. In fact, convection is already
sub-adiabatic ($\nabla<\nabla_{ad}$) far before reaching the unstable boundary;
while in the overshooting zone below the convective zone, convection is
sub-adiabatic and super-radiative ($\nabla_{rad}<\nabla<\nabla_{ad}$). The
transition between the adiabatic temperature gradient and the radiative one is
continuous and smooth instead of a sudden switch. In the unstable zone the
temperature gradient is approaching radiative rather than going to adiabatic.
We would like to claim again that, the overshooting distance is different for
different physical quantities......
