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While small, Neptune-like planets are among the most abundant exoplanets, ourunderstanding of their atmospheric structure and dynamics remains sparse. Inparticular, many unknowns remain on the way moist convection works in theseatmospheres where condensable species are heavier than the non-condensablebackground gas. While it has been predicted that moist convection couldshut-down above some threshold abundance of these condensable species, thisprediction is based on simple linear analysis and relies on strong assumptionson the saturation of the atmosphere. To investigate this issue, we develop a 3Dcloud resolving model for H2 atmospheres with large amounts of condensablespecies and apply this model to a prototypical temperate Neptune-like planet --K2-18b. Our model confirms the shut-down of moist convection and the onset of astably stratified layer in the atmosphere, leading to much hotter deepatmospheres and interiors. Our 3D simulations further provide quantitativeestimates of the turbulent mixing in this stable layer, which is a key driverof the cycling of condensables in the atmosphere. This allows us to build avery simple, yet realistic 1D model that captures the most salient features ofthe structure of Neptune-like atmospheres. Our qualitative findings on thebehavior of moist convection in hydrogen atmospheres go beyond temperateplanets and should also apply to the regions where iron and silicates condensein the deep interior of H2-dominated planets. Finally, we use our model toinvestigate the likelihood of a liquid ocean beneath a H2 dominated atmosphereon K2-18b. We find that the planet would need to have a very high albedo(>0.5-0.6) to sustain a liquid ocean. However, due to the spectral type of thestar, the amount of aerosol scattering that would be needed to provide such ahigh albedo is inconsistent with the latest observational data.

A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18 b