# Thermo-compositional diabatic convection in the atmospheres of brown dwarfs and in Earth's atmosphere and oceans

## TL;DR

This paper generalizes convection theory to include diabatic processes, revealing that various convection types across planets and stars are related, and suggests that radiative convection influences brown dwarf atmospheres similarly to moist convection on Earth.

## Contribution

It introduces a unified framework for diabatic convection, generalizes mixing length theory to include source terms, and links brown dwarf atmospheric phenomena to terrestrial and stellar convection processes.

## Key findings

- CO/CH4 radiative convection can reduce brown dwarf temperature gradients.
- Compositional source terms can trigger a bifurcation in convective regimes.
- The L/T transition in brown dwarfs may be a cooling bifurcation analogous to boiling crisis.

## Abstract

By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth oceans, fingering convection in stellar atmospheres, and moist convection in Earth atmosphere are deriving from the same general diabatic convective instability. We show also that "radiative convection" triggered by CO/CH4 transition with radiative transfer in the atmospheres of brown dwarfs is analog to moist and thermohaline convection. We derive a generalization of the mixing length theory to include the effect of source terms in 1D codes. We show that CO/CH4 radiative convection could significantly reduce the temperature gradient in the atmospheres of brown dwarfs similarly to moist convection in Earth atmosphere thus possibly explaining the reddening in brown-dwarf spectra. By using idealized two-dimensional hydrodynamic simulations in the Ledoux unstable regime, we show that compositional source terms can indeed provoke a reduction of the temperature gradient. The L/T transition could be explained by a bifurcation between the adiabatic and diabatic convective transports and could be seen as a giant cooling crisis: an analog of the boiling crisis in liquid/steam-water convective flows. This mechanism with other chemical transitions could be present in many giant and earth-like exoplanets. The study of the impact of different parameters (effective temperature, compositional changes) on CO/CH4 radiative convection and the analogy with Earth moist and thermohaline convection is opening the possibility to use brown dwarfs to better understand some aspects of the physics at play in the climate of our own planet.