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Observationally, spectra of brown dwarfs indicate the presence of dust intheir atmospheres while theoretically it is not clear what prevents the dustfrom settling and disappearing from the regions of spectrum formation.Consequently, standard models have to rely on ad hoc assumptions about themechanism that keeps dust grains aloft in the atmosphere. We applyhydrodynamical simulations to develop an improved physical understanding of themixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres,in particular of the influence of the underlying convection zone. We performed2D radiation hydrodynamics simulations including a description of dust grainformation and transport with the CO5BOLD code. The simulations cover the verytop of the convection zone and the photosphere including the dust layers foreffective temperatures between 900K and 2800K, all with logg=5 assuming solarchemical composition. Convective overshoot occurs in the form of exponentiallydeclining velocities with small scale heights, so that it affects only theregion immediately above the almost adiabatic convective layers. From there on,mixing is provided by gravity waves that are strong enough to maintain thindust clouds in the hotter models. With decreasing effective temperature, theamplitudes of the waves become smaller but the clouds become thicker anddevelop internal convective flows that are more efficient in mixing materialthan gravity waves. The presence of clouds leads to a highly structuredappearance of the stellar surface on short temporal and small spatial scales.We identify convectively excited gravity waves as an essential mixing processin M dwarf and brown dwarf atmospheres. Under conditions of strong cloudformation, dust convection is the dominant self-sustaining mixing component.

The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres