Cloud formation in substellar atmospheres

摘要

Clouds seem like an every-day experience. But — do we know how clouds form on browndwarfs and extra-solar planets? How do they look like? Can we see them? What are they composedof? Cloud formation is an old-fashioned but still outstanding problem for the Earth atmosphere,and it has turned into a challenge for the modelling of brown dwarf and exo-planetary atmospheres.Cloud formation imposes strong feedbacks on the atmospheric structure, not only due to the cloudsown opacity, but also due to the depletion of the gas phase, possibly leaving behind a dynamicand still supersaturated atmosphere. I summarise the different approaches taken to model cloudformation in substellar atmospheres and workout their differences. Focusing on the phase-non-equilibrium approach to cloud formation, I demonstrate the inside we gain from detailed micro-physical modelling on for instance the material composition and grain size distribution inside thecloud layer on a Brown Dwarf atmosphere. A comparison study on four different cloud approachesin Brown Dwarf atmosphere simulations demonstrates possible uncertainties in interpretation ofobservational data.