Utility of the Weak Temperature Gradient Approximation for Earth-Like Tidally Locked Exoplanets

TL;DR

This study evaluates the weak temperature gradient (WTG) approximation's effectiveness in modeling the climate of tidally locked Earth-like exoplanets, showing it closely matches complex GCM results and is observationally indistinguishable with JWST.

Contribution

The paper demonstrates that a WTG model can accurately approximate atmospheric dynamics of tidally locked exoplanets, offering a computationally efficient alternative to GCMs.

Findings

WTG model closely matches GCM outputs for phase curves.

Differences between WTG and GCM are undetectable with JWST.

WTG approximation is useful for planetary climate modeling hierarchy.

Abstract

Planets in M dwarf stars' habitable zones are likely to be tidally locked with orbital periods of order tens of days. This means that the effects of rotation on atmospheric dynamics will be relatively weak, which requires small horizontal temperature gradients above the boundary layer of terrestrial atmospheres. An analytically solvable and dynamically consistent model for planetary climate with only three free parameters can be constructed by making the weak temperature gradient (WTG) approximation, which assumes temperatures are horizontally uniform aloft. The extreme numerical efficiency of a WTG model compared to a 3D general circulation model (GCM) makes it an optimal tool for Monte Carlo fits to observables over parameter space. Additionally, such low-order models are critical for developing physical intuition and coupling atmospheric dynamics to models of other components of planetary climate. The objective of this paper is to determine whether a WTG model provides an adequate approximation of the effect of atmospheric dynamics on quantities likely to be observed over the next decade. To do this we first tune a WTG model to GCM output for an Earth-like tidally locked planet with a dry, 1 bar atmosphere, then generate and compare the expected phase curves of both models. We find that differences between the two models would be extremely difficult to detect from phase curves using JWST. This result demonstrates the usefulness of the WTG approximation when used in conjunction with GCMs as part of a modeling hierarchy to understand the climate of remote planets.