Atmospheric Dynamics of Temperate Sub-Neptunes. I. Dry Dynamics

Abstract Sub-Neptunes (planets with radii between 2 and 4 R ⊕ ) are abundant around M-dwarf stars, yet the atmospheric dynamics of these planets is relatively unexplored. In this paper, we aim to provide a basic underpinning dry general low-mean-molecular-weight, temperate sub-Neptune atmospheres. We use ExoFMS circulation model (GCM) an idealized gray-gas radiation scheme simulate planetary atmospheres different levels instellation rotation rates, using atmosphere K2-18b as our control. find that tidally locked (TL), sub-Neptunes have weak horizontal temperature gradients owing their slow rates hydrogen-dominated composition. The zonal wind structure dominated by high-latitude cyclostrophic jets driven conservation angular momentum. At low pressures observe superrotating equatorial jets, which propose Rossby–Kelvin instability similar type seen in simulations axisymmetric forcing. By viewing flow TL coordinates, predominant overturning be day side night side, derive scaling relations linking stream function vertical velocities instellation. Comparing results only other GCM study K2-18b, significant qualitative differences dynamics, highlighting need for further collaboration investigation into effects dynamical cores physical parameterizations. This paper provides baseline studying sub-Neptunes, will built on part II introduction moist effects.