Three-dimensional dynamical evolution of cloud particle microphysics in sub-stellar atmospheres I. Description and exploring Y-dwarf atmospheric variability

Understanding of cloud microphysics and the evolution of cloud structures in sub-stellar atmospheres remains a key challenge in the JWST era. The abundance of new JWST data necessitates models that are suitable for coupling with large-scale simulations, such as general circulation models (GCMs), in order to fully understand and assess the complex feedback effects of clouds on the atmosphere, and their influence on observed spectral and variability characteristics. We aim to develop a 2-moment, time-dependent bulk microphysical cloud model that is suitable for GCMs of sub-stellar atmospheres. We derive a set of moment equations for the particle mass distribution and develop a microphysical cloud model employing a 2-moment approach. We include homogeneous nucleation, condensation, and collisional microphysical processes that evolve the moments of a particle size distribution in time. We couple our new 2-moment scheme with the Exo-FMS GCM to simulate the evolution of KCl clouds in a Tint = 400 K, log g = 4.25 Y-dwarf atmosphere and examine the effect of cloud opacity on the atmospheric characteristics. Our results show a generally homogeneous global KCl cloud, with only slight variations occurring in the equatorial regions. The atmosphere is generally sluggish and stagnant, with near-zero vertical velocities throughout most of the atmosphere. Only very small grains ~0.01 um remain lofted in the atmosphere. Our results conform with dynamical theories for this parameter regime, with our model showing minimal (~0.2%) spectral variability at mid-infrared wavelengths. Our study demonstrates that the 2-moment bulk cloud microphysical scheme is a highly suitable method for investigating cloud characteristics and feedback in GCMs and other large scale simulations of sub-stellar atmospheres. Split moment schemes and mixed material grains will be explored in a follow up study.
Comment: Submitted to A&A