Atmospheric circulation of tidally locked exoplanets: a suite of benchmark tests for dynamical solvers

TL;DR

This study develops and applies benchmark tests for dynamical solvers to model atmospheric circulation on tidally-locked exoplanets, revealing both agreements and limitations in current modeling approaches.

Contribution

It introduces a suite of benchmark tests for dynamical solvers tailored to tidally-locked exoplanets, extending the Held-Suarez test framework.

Findings

Qualitative and quantitative agreement between solvers for Earth and hot Jupiter models.

Significant discrepancies for the deep HD 209458b model.

Horizontal dissipation parameters influence solver agreement.

Abstract

The complexity of atmospheric modelling and its inherent non-linearity, together with the limited amount of data of exoplanets available, motivate model intercomparisons and benchmark tests. In the geophysical community, the Held-Suarez test is a standard benchmark for comparing dynamical core simulations of the Earth's atmosphere with different solvers, based on statistically-averaged flow quantities. In the present study, we perform analogues of the Held-Suarez test for tidally-locked exoplanets with the GFDL-Princeton Flexible Modeling System (FMS) by subjecting both the spectral and finite difference dynamical cores to a suite of tests, including the standard benchmark for Earth, a hypothetical tidally-locked Earth, a "shallow" hot Jupiter model and a "deep" model of HD 209458b. We find qualitative and quantitative agreement between the solvers for the Earth, tidally-locked Earth and shallow hot Jupiter benchmarks, but the agreement is less than satisfactory for the deep model of HD 209458b. Further investigation reveals that closer agreement may be attained by arbitrarily adjusting the values of the horizontal dissipation parameters in the two solvers, but it remains the case that the magnitude of the horizontal dissipation is not easily specified from first principles. Irrespective of radiative transfer or chemical composition considerations, our study points to limitations in our ability to accurately model hot Jupiter atmospheres with meteorological solvers at the level of ten percent for the temperature field and several tens of percent for the velocity field. Direct wind measurements should thus be particularly constraining for the models. Our suite of benchmark tests also provides a reference point for researchers wishing to adapt their codes to study the atmospheric circulation regimes of tidally-locked Earths/Neptunes/Jupiters.