  Clouds are expected to form in a wide range of conditions in the atmosphere
of exoplanets given the large range of possible condensible species. However
this diversity might lead to very different small-scale dynamics depending on
radiative transfer in various thermal conditions: we aim at providing some
insights into these dynamical regimes. We perform an analytical linear
stability analysis of a compositional discontinuity with a heating source term
that depends on composition. We also perform idealized two-dimensional (2D)
simulations of an opacity discontinuity in a stratified medium with the code
ARK. We use a two-stream grey model for radiative transfer and explore the
brown-dwarf and earth-like regimes. We reveal the existence of a Radiative
Rayleigh-Taylor Instability (RRTI hereafter, a particular case of diabatic
Rayleigh-Taylor instability) when an opacity discontinuity is present in a
stratified medium. This instability is similar in nature to diabatic convection
and relies only on buoyancy with radiative transfer heating and cooling. When
the temperature is decreasing with height in the atmosphere, a lower-opacity
medium on top of a higher-opacity medium is dynamically unstable while a
higher-opacity medium on top of a lower-opacity medium is stable. This
stability/instability behavior is reversed if the temperature is increasing
with height. The existence of the RRTI could have important implications for
the stability of the cloud cover of a wide range of planetary atmospheres. In
our solar system, it could help explain the formation of mammatus cloud in
Earth atmospheres and the existence of Venus cloud deck. Likewise, it suggests
that stable and large scale cloud covers could be ubiquitous in strongly
irradiated exoplanets but might be more patchy in low-irradiated or isolated
objects like brown dwarfs and directly imaged exoplanets.
