Longitudinal Filtering, Sponge Layers, and Equatorial Jet Formation in a General Circulation Model of Gaseous Exoplanets

TL;DR

This study examines how artificial dissipation in a general circulation model affects the simulation of exoplanet atmospheres, revealing that excessive dissipation causes inaccuracies, but proper tuning yields stable, realistic jet formations and thermal structures.

Contribution

It demonstrates the importance of carefully tuning dissipation parameters in GCMs to accurately simulate exoplanet atmospheric dynamics and jet formation.

Findings

Excessive dissipation causes counter-rotating jets and angular momentum loss.

Reducing dissipation allows super-rotating jets to form, with stable thermal structures.

Vertical velocities are sensitive to dissipation, affecting vertical transport processes.

Abstract

General circulation models are a useful tool in understanding the three dimensional structure of hot Jupiter and sub-Neptune atmospheres; however, understanding the validity of the results from these simulations requires an understanding the artificial dissipation required for numerical stability. In this paper, we investigate the impact of the longitudinal filter and vertical ``sponge'' used in the Met Office's {\sc Unified Model} when simulating gaseous exoplanets. We demonstrate that excessive dissipation can result in counter-rotating jets and a catastrophic failure to conserve angular momentum. Once the dissipation is reduced to a level where a super-rotating jet forms, however, the jet and thermal structure are relatively insensitive to the dissipation, except in the nightside gyres where temperatures can vary by $\sim 100\,\mathrm{K}$. We do find, however, that flattening the latitudinal profile of the longitudinal filtering alters the results more than a reduction in the strength of the filtering itself. We also show that even in situations where the temperatures are relatively insensitive to the dissipation, the vertical velocities can still vary with the dissipation, potentially impacting physical processes that depend on the local vertical transport.