  Recently, direct numerical simulations (DNS) of stably stratified turbulence
have shown that as the Prandtl number ($Pr$) is increased from 1 to 7, the mean
turbulent potential energy dissipation rate (TPE-DR) drops dramatically, while
the mean turbulent kinetic energy dissipation rate (TKE-DR) increases
significantly. Through an analysis of the equations governing the fluctuating
velocity and density gradients we provide a mechanistic explanation for this
surprising behavior and test the predictions using DNS. We show that the mean
density gradient gives rise to a mechanism that opposes the production of
fluctuating density gradients, and this is connected to the emergence of
ramp-cliffs. The same term appears in the velocity gradient equation but with
the opposite sign, and is the contribution from buoyancy. This term is
ultimately the reason why the TPE-DR reduces while the TKE-DR increases with
increasing $Pr$. Our analysis also predicts that the effects of buoyancy on the
smallest scales of the flow become stronger as $Pr$ is increased, and this is
confirmed by our DNS data. A consequence of this is that the standard buoyancy
Reynolds number does not correctly estimate the impact of buoyancy at the
smallest scales when $Pr$ deviates from 1, and we derive a suitable alternative
parameter. Finally, an analysis of the filtered gradient equations reveals that
the mean density gradient term changes sign at sufficiently large scales, such
that buoyancy acts as a source for velocity gradients at small scales, but as a
sink at large scales.

Fluid Dynamics;Atmospheric and Oceanic Physics