GCM simulations of sub-neptunes with a condensable heavy compound

Sub-neptunes are exoplanets that have a thick gaseous envelope dominated by hydrogen and helium, sometimes enriched with water, methane or other volatile compounds while super-Earths could be rocky planets or ocean worlds with a thinner atmosphere. Some of these planets are probably tidally-locked, resulting in a priori important day-night temperature contrast.

However, wind speeds remain poorly constrained and poorly understood. Several global climate models (GCMs) have been developed to determine the atmospheric circulation that might govern such planets.

Recent studies of the atmosphere of K2-18b show broadly consistent results regarding its general circulation, whilst highlighting the influence of several physical parameters. The zonal circulation of K2-18b is predominantly dominated by easterly winds and by the presence of one or more super-rotating equatorial jets in the upper atmosphere, as observed in several simulations. Both [1] and [2] identify this structure. The studies also converge on a global circulation organised between the day side and the night side, typical of a synchronously rotating planet. [3] and [1] describe a dominant day-night overturning circulation in the upper atmosphere. The simulations by [2] also indicate an equatorial updraft and polar subsidence, accompanied by strong vertical mixing. This work also highlights the importance of physical parameters on atmospheric dynamics. Stellar metallicity [3] and spin-orbit resonance significantly alter their atmospheric circulation [3,4].

In the case of sub-neptunes with high metallicity, condensable heavy compounds can have a key dynamic role . It has been shown that condensable heavy compounds can inhibit convection [5], [6]. By adding a convection scheme consistent with the presence of heavy compouds, [4] found a similar overturning circulation between dayside and nightside and pole to equator cells as on [2]. Secondly, the high abundance of these compounds can generate significant molar mass gradients during condensation, thereby altering the zonal and meridional circulation of these planets. However, to date, the effect of a variable molar mass due to the abundance of condensable heavy compounds has not yet been investigated in the primitive and thermodynamic equations.

To reproduce the thermal and dynamic structure of sub-neptunes, numerical simulations with a resolution of 2° were carried out between 1 bar and 10 mbar, using the DYNAMICO dynamic core [7] and coupling it to the Generic PCM. We decided to simulate a K2-18b-like planet using the same abundances and k-distribution model as [6]. The condensable tracer used is water, which is predicted to condense at the pressures and temperatures under consideration. Potential enthalpy was chosen as the conservative variable instead of potential temperature in order to account for the variation in molar mass within the dynamic core. The dry and wet convective adjustments were modified accordingly to allow for these molecular weight effects. The precipitation scheme takes into account condensation, re-evaporation and mass redistribution within the atmospheric layers.

Three configurations were tested: the first, in which the molar mass varies throughout the GCM; the second, in which it varies only in the dry and wet convection schemes, as in [4]; and the third, in which the molar mass is kept constant. In these three simulations, we investigated the zonal circulation and the resulting thermal structure. A dynamic analysis was carried out using the Transformed eulerian formalism to highlight the circulation cells and the transport of H2O within these cells.

 

References

[1] Innes & Pierrehumbert (2022), The Astrophysical Journal, 927(1):38

[2] Liu et al. (2025), Monthly Notices of the Royal Astronomical Society, 538(4):2463–2482, 541(4):2897–2916

[3] Charnay et al. (2021), Astronomy and Astrophysics, 646:A171

[4] Barrier & Madhusudhan (2025), Monthly Notices of the Royal Astronomical Society, 538(4):2463–2482

[5] Leconte et al. (2017), Astronomy and Astrophysics, 598:A98

[6] Leconte et al. (2024), Astronomy and Astrophysics, 686:A131

[7] Dubos et al. (2015), Geoscientific Model Development, 8(10):3131– 3150.