Hot Jupiter Atmospheric Flows at High Resolution

TL;DR

High-resolution global models of Hot Jupiter atmospheres show increased variability but do not significantly alter observable infrared flux, indicating that ultra-high resolutions may not be necessary for accurate modeling.

Contribution

This study adapts a pseudo-spectral Earth climate model for Hot Jupiter atmospheres and evaluates the impact of high resolution on flow dynamics and observables.

Findings

High resolution models do not reproduce equatorial jet instabilities seen in local simulations.

High resolution increases long-term flow variability and breaks north-south symmetry.

Infrared flux variability remains similar across different resolutions.

Abstract

Global Circulation Models (GCMs) of atmospheric flows are now routinely used to interpret observational data on Hot Jupiters. Localized "equatorial $\beta$-plane" simulations by Fromang et al. (2016) have revealed that a barotropic (horizontal shear) instability of the equatorial jet appears at horizontal resolutions beyond those typically achieved in global models; this instability could limit wind speeds and lead to increased atmospheric variability. To address this possibility, we adapt the computationally efficient, pseudo-spectral PlaSim GCM, originally designed for Earth studies, to model Hot Jupiter atmospheric flows and validate it on the Heng et al. (2011) reference benchmark. We then present high resolution global models of HD209458b, with horizontal resolutions of T85 (128x256) and T127 (192x384). The barotropic instability phenomenology found in $\beta$-plane simulations is not reproduced in these global models, despite comparably high resolutions. Nevertheless, high resolution models do exhibit additional flow variability on long timescales (of order 100 planet days or more), which is absent from the lower resolution models. It manifests as a breakdown of north-south symmetry of the equatorial wind. From post-processing the atmospheric flows at various resolutions (assuming a cloud-free situation), we show that the stronger flow variability achieved at high resolution does not translate into noticeably stronger dayside infrared flux variability. More generally, our results suggest that high horizontal resolutions are not required to capture the key features of hot Jupiter atmospheric flows.