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

Abstract

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