  The large scale structure of a brown dwarf atmosphere is determined by an
interplay of convection, radiation, dust formation, and gravitational settling,
which possibly provides an explanation for the observed variability. The result
is an element depletion of the dust forming regions and an element enrichment
of the dust evaporating sites. The formation of dust cloud structures in
substellar atmospheres is demonstrated based on a consistent theoretical
description of dust formation and destruction, gravitational settling, and
element depletion including the effect of convective overshoot.
  Since the viscosity is small in brown dwarf atmospheres, the convection
creates a turbulent environment with fluctuations of all thermodynamic state
variables on a wide range of spatial scales. Hence, the classical turbulent
closure problem needs to be tackled in connection with dust formation in
substellar objects, because a complete three-dimensional and time-dependent
solution of the model equations is simply not possible. Structure formation may
be seeded on the smallest scales, when chemical processes are involved. In
order to understand the interaction of turbulence and dust formation, we have
performed investigations of the smallest scale regimes in 1D and in 2D in order
to identify the governing processes of the unresolved scale regime.
