The organization of turbulent convection

Highly resolved numerical simulations are used to study three-dimensional, compressible convection. The viscous dissipation is sufficiently low that the flow divides itself in depth into two distinct regions: (i) an upper thermal boundary layer containing a smooth flow with a granular appearance, and (ii) a turbulent interior pierced by the strongest downflows from the surface layer. Such downflows span the whole depth of the unstable layer, are temporally coherent, and are thermodynamically well correlated. A remarkable property of such convection, once it becomes turbulent, is that the enthalpy and kinetic fluxes carried by the strong downflows nearly cancel, for they are of opposite sense and nearly equal in amplitude. Thus, although the downflows serve to organize the convection and are the striking feature that emerges from effects of compressibility, it is the small-scale, disorganized turbulent motions (between the coherent downflow structures that serve as the principal carriers of net convected flux.