Resolved convection in hydrogen-rich atmospheres

TL;DR

This study uses 3D cloud-resolving models to explore how moist convection behaves in hydrogen-rich atmospheres, revealing a humidity threshold that causes significant structural changes and increased cloudiness, with implications for hot exoplanets.

Contribution

It demonstrates the impact of the Guillot threshold on atmospheric structure and cloud formation in hydrogen-rich atmospheres using detailed 3D simulations, advancing beyond previous one-dimensional theories.

Findings

Identification of a humidity threshold causing a tropospheric structural shift.

Increased cloudiness above the Guillot threshold in hydrogen-rich atmospheres.

Evidence of episodic convection and variability in cloud cover.

Abstract

In hydrogen-rich atmospheres with low mean molecular weight (MMW), an air parcel containing a higher-molecular-weight condensible can be negatively buoyant even if its temperature is higher than the surrounding environment. This should fundamentally alter the dynamics of moist convection, but the low-MMW regime has previously been explored primarily via one-dimensional theories that cannot capture the complexity of moist turbulence. Here, we use a three-dimensional cloud-resolving model to simulate moist convection in atmospheres with a wide range of background MMW, and confirm that a humidity threshold for buoyancy reversal first derived by Guillot (1995) coincides with an abrupt change in tropospheric structure. Crossing the "Guillot threshold" in near-surface humidity causes the dry (subcloud) boundary layer to collapse and be replaced by a very cloudy layer with a temperature lapse rate that exceeds the dry adiabatic rate. Simulations with reduced surface moisture availability in the lower atmosphere feature a deeper dry subcloud layer, which allows the superadiabatic cloud layer to remain aloft. Our simulations support a potentially observable systematic trend toward increased cloudiness for atmospheres with near-surface moisture concentrations above the Guillot threshold. This should apply to \ce{H2O} and potentially to other condensible species on hotter worlds. We also find evidence for episodic convective activity and associated variability in cloud cover in some of our low-MMW simulations, which should be investigated further with global-scale simulations.