Atmospheric dynamics of temperate sub-Neptunes. Part I: dry dynamics

TL;DR

This study investigates the dry atmospheric dynamics of temperate sub-Neptunes using GCM simulations, revealing weak temperature gradients, high-latitude jets, and overturning circulation patterns, providing a baseline for future moist atmosphere studies.

Contribution

It offers the first detailed GCM-based analysis of dry atmospheres of temperate sub-Neptunes, highlighting key dynamical features and differences from previous models.

Findings

Weak horizontal temperature gradients due to slow rotation and hydrogen atmosphere.

High-latitude cyclostrophic jets driven by angular momentum conservation.

Superrotating equatorial jets linked to Rossby-Kelvin instability.

Abstract

Sub-Neptunes (planets with radii between 2 and 4 R$_{\oplus}$) 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 of the dry dynamics of general low mean molecular weight, temperate sub-Neptune atmospheres. We use the ExoFMS GCM with an idealised grey gas radiation scheme to simulate planetary atmospheres with different levels of instellation and rotation rates, using the atmosphere of K2-18b as our control. We find that the atmospheres of tidally-locked, temperate sub-Neptunes have weak horizontal temperature gradients owing to their slow rotation rates and hydrogen-dominated composition. The zonal wind structure is dominated by high-latitude cyclostrophic jets driven by the conservation of angular momentum. At low pressures we observe superrotating equatorial jets, which we propose are driven by a Rossby-Kelvin instability similar to the type seen in simulations of idealised atmospheres with axisymmetric forcing. By viewing the flow in tidally-locked coordinates, we find the predominant overturning circulation to be between the dayside and nightside, and we derive scaling relations linking the tidally-locked streamfunction and vertical velocities to instellation. Comparing our results to the only other GCM study of K2-18b, we find significant qualitative differences in dynamics, highlighting the need for further collaboration and investigation into effects of different dynamical cores and physical parameterizations. This paper provides a baseline for studying the dry dynamics of temperate sub-Neptunes, which will be built on in part II with the introduction of moist effects.