  Self-consistent closure theory for passive-scalar turbulence has been
developed on the basis of the Hessian of the scalar field. As a primitive
indicator of spatial structure of the scalar, we employ the Hessian into the
core of the theory to properly characterize the time scale intrinsic to the
scalar field itself. The resultant closure model is now endowed with several
realistic features, i.e. the scale-locality of the interscale interaction, the
detailed conservation, and the memory-fading effect. Applying the current
theory to the inertial-convective range eventually leads to self-consistent
derivation of the Obukhov-Corrsin spectrum with its universal constant
consistent with numerical and experimental data.
