Convective dynamics and disequilibrium chemistry in the atmospheres of giant planets and brown dwarfs

TL;DR

This study uses numerical simulations to analyze convective dynamics in giant planet and brown dwarf atmospheres, revealing that chemical quenching occurs deeper than simple models predict, and introduces a new length scale for better predictions.

Contribution

It introduces a new length scale based on chemical scale height for improved modeling of disequilibrium chemistry in planetary atmospheres.

Findings

Chemical species quench deeper than simple mixing length models predict.

A new length scale based on chemical equilibrium profile accurately predicts quenching depths.

Simulations demonstrate the importance of dynamical regimes in disequilibrium chemistry modeling.

Abstract

Disequilibrium chemical processes have a large effect upon the spectra of substellar objects. To study these effects, dynamical disequilibrium has been parameterized using the quench and eddy diffusion approximations, but little work has been done to explore how these approximations perform under realistic planetary conditions in different dynamical regimes. As a first step in addressing this problem, we study the localized, small scale convective dynamics of planetary atmospheres by direct numerical simulation of fully compressible hydrodynamics with reactive tracers using the Dedalus code. Using polytropically-stratified, plane parallel atmospheres in 2- and 3-D, we explore the quenching behavior of different abstract chemical species as a function of the dynamical conditions of the atmosphere as parameterized by the Rayleigh number. We find that in both 2- and 3-D, chemical species quench deeper than would be predicted based on simple mixing length arguments. Instead, it is necessary to employ length scales based on the chemical equilibrium profile of the reacting species in order to predict quench points and perform chemical kinetics modeling in 1-D. Based on the results of our simulations, we provide a new length scale, derived from the chemical scale height, which can be used to perform these calculations. This length scale is simple to calculate from known chemical data and makes reasonable predictions for our dynamical simulations.