Abstract
Aims.Clouds are expected to form in a broad range of conditions in the atmosphere of exoplanets given the variety of possible condensible species. This diversity, however, might lead to very different small-scale dynamics depending on radiative transfer in various thermal conditions. Here, we aim to provide some insight into these dynamical regimes.Methods.We performed an analytical linear stability analysis of a compositional discontinuity with a heating source term that depends on a given composition. We also performed idealized two-dimensional simulations of an opacity discontinuity in a stratified medium, using theARKcode. We used a two-stream gray model for radiative transfer and explored the brown-dwarf and Earth-like regimes.Results.We revealed the existence of a radiative Rayleigh-Taylor instability (RRTI, hereafter, which is 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 shown to be dynamically unstable, whereas 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.Conclusions.The existence of a RRTI could have important implications for the stability of the cloud cover with regard to 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 the 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 such as brown dwarfs and directly imaged exoplanets.
Highlights
With an increasing number of observations of exoplanet atmospheres taking place using transmission spectroscopy (e.g., Sing et al 2016), a growing concern regarding cloud cover has been abound among the exoplanet community
We propose in this paper another complementary approach based on theory with the inclusion of source terms in the Rayleigh-Taylor stability analysis in a similar way to the recent theoretical development of diabatic convection proposed in Tremblin et al (2019)
By using an analytical stability analysis and 2D radiativehydrodynamical simulations, we have shown the existence of an instability of an opacity discontinuity between a lower-opacity and a higher-opacity medium: the Radiative Rayleigh-Taylor instability, namely, a particular case of the general diabatic Rayleigh-Taylor instability
Summary
With an increasing number of observations of exoplanet atmospheres taking place using transmission spectroscopy (e.g., Sing et al 2016), a growing concern regarding cloud cover has been abound among the exoplanet community. Tan & Showman 2021a,b) have identified the key role of radiative instabilities at a global scale – in planetary atmospheres (Venus, Jupiter) and in the atmosphere of exoplanets and brown dwarfs All these approaches use a simplified approach to hydrodynamics, either because they are 1D or because they are global and cannot adequately capture small-scale, nonhydrostatic buoyancy effects. We study in a similar way the dynamical behavior of an opacity discontinuity between a “higher-opacity” and “loweropacity” medium subject to radiative heating and cooling Such a discontinuity could be unstable or stable due to the impact of radiative transfer on buoyancy to the impact of thermal source terms in diabatic convection.
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