= Density-variance dissipation rate
{title2=$\chi=\kappa_\rho\overline{|\nabla\rho'|^2}$}
Molecular scalar diffusion removes density-fluctuation variance at the positive rate displayed here. With this convention the budget for one half of the variance has dissipation term $-\chi$; the full variance budget has $-2\chi$. In a stationary locally homogeneous shear flow the half-variance budget gives $\overline{\rho'w'}\,\overline\rho_z+\chi=0$. An integral-scale model writes $\chi=\overline{\rho'^2}q/L_\rho$, absorbing any normalization constant into $L_\rho$.
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