Passive scalars in stratified turbulence

Statistics of a passive scalar (or tracer) with a horizontal mean gradient in randomly forced and strongly stratified turbulence are investigated by numerical simulations. We observe that horizontal isotropy of the passive scalar spectrum is satisfied in the inertial range. The spectrum has the form E-theta(k(h)) = C-theta epsilon theta epsilon(-1/3)(K) k(h)(-5/3), where epsilon(theta), epsilon(K) are the dissipation of scalar variance and kinetic energy respectively, and C-theta similar or equal to 0.5 is a constant. This spectrum is consistent with atmospheric measurements in the mesoscale range with wavelengths 1 - 500 km. We also calculate the fourth-order passive scalar structure function and show that intermittency effects are present in stratified turbulence.