The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). I. Dry Cases—The Fellowship of the GCMs

Abstract

With the commissioning of powerful, new-generation telescopes such as the James Webb Space Telescope (JWST) and the ground-based Extremely Large Telescopes, the first characterization of a high molecular weight atmosphere around a temperate rocky exoplanet is imminent. Atmospheric simulations and synthetic observables of target exoplanets are essential to prepare and interpret these observations. Here we report the results of the first part of the TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project, which compares 3D numerical simulations performed with four state-of-the-art global climate models (ExoCAM, LMD-Generic, ROCKE-3D, Unified Model) for the potentially habitable target TRAPPIST-1e. In this first part, we present the results of dry atmospheric simulations. These simulations serve as a benchmark to test how radiative transfer, subgrid-scale mixing (dry turbulence and convection), and large-scale dynamics impact the climate of TRAPPIST-1e and consequently the transit spectroscopy signature as seen by JWST. To first order, the four models give results in good agreement. The intermodel spread in the global mean surface temperature amounts to 7 K (6 K) for the N 2 -dominated (CO 2 -dominated) atmosphere. The radiative fluxes are also remarkably similar (intermodel variations less than 5%), from the surface (1 bar) up to atmospheric pressures ∼5 mbar. Moderate differences between the models appear in the atmospheric circulation pattern (winds) and the (stratospheric) thermal structure. These differences arise between the models from (1) large-scale dynamics, because TRAPPIST-1e lies at the tipping point between two different circulation regimes (fast and Rhines rotators) in which the models can be alternatively trapped, and (2) parameterizations used in the upper atmosphere such as numerical damping.Unified Astronomy Thesaurus concepts: Planetary climates (2184); Exoplanet atmospheres (487); Habitable planets (695); Atmospheric circulation (112); Extrasolar rocky planets (511); Hydrodynamical simulations (767); M dwarf stars (982); Radiative transfer (1335)

Bistability of the Atmospheric Circulation on TRAPPIST-1e

“…The divergent component is notably weaker in the SJ regime (Figure 8(e)) and stronger in the DJ regime (Figure 8(f)). The differences in the rotational and divergent components between our simulations are analogous to those found in the THAI results Turbet et al 2022), confirming that it is one of the characteristic features of the two regimes.…”

“…Transitions of the circulation regime for tidally locked rocky exoplanets were reported in several modeling studies, all based on 3D GCMs (e.g., Edson et al 2011;Carone et al 2014;Noda et al 2017). It was also noted in recent studies of the TRAPPIST-1e climate (Eager et al 2020;Sergeev et al 2020Turbet et al 2022), in which the dominant climatic feature was either a strong superrotating jet at the equator or two eastward jets in mid-latitudes with a weaker equatorial superrotation. Throughout this paper, we refer to the former as the "single jet" (SJ) regime, and the latter as the "double jet" (DJ) regime.…”

“…This study is motivated by the need to understand the atmospheric circulation on tidally locked exoplanets in order to make the best use of observational data by enabling the community to both refine target selection for observational campaigns and improve confidence in the interpretation of the observations. Recent simulations of possible climates on TRAPPIST-1e, a rocky planet orbiting an ultracool M-dwarf star, allude to a potential bistability of the atmospheric circulation for this planet (e.g., Eager et al 2020;Sergeev et al 2020Turbet et al 2022). Here we study the emergence and maintenance of two different circulation regimes of TRAPPIST-1e, a primary observational target, using a 3D general circulation model (GCM).…”

The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). III. Simulated Observables—the Return of the Spectrum

“…In this paper, Part III of a trilogy of THAI papers, we use the same GCM simulations that have been extensively analyzed in Part I (Turbet et al 2022) and Part II (Sergeev et al 2022), namely, the Ben 1 and Ben 2 and Hab 1 and Hab 2 cases, respectively. Briefly, the Ben 1 and Ben 2 cases are dry land planet simulations, while the Hab 1 and Hab 2 cases assume that the surface is fully covered by a global (static) ocean and that there are water vapor and clouds in the atmosphere.…”

“…Each of these simulations has been performed by the four THAI GCMs: ExoCAM (Wolf et al 2022), the exoplanet branch of the Community Earth System Model (CESM; http:// www.cesm.ucar.edu/models/cesm1.2/) version 1.2.1; the LMD-Generic model (LMD-G; see, e.g., Wordsworth et al 2011;Turbet et al 2018); the Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D; Way et al 2017); and the Met Office Unified Model (UM; see, e.g., Mayne et al 2014;Boutle et al 2017). More details on these four GCMs are also provided in the THAI protocol (Sergeev et al 2022;Turbet et al 2022) and the THAI workshop report (Fauchez et al 2021).…”