Planetary Climate Dynamics

Abstract:

We present a 5 Å–100 μm spectral energy distribution (SED) of the ultracool dwarf star TRAPPIST-1, obtained as part of the Mega-MUSCLES Treasury Survey. The SED combines ultraviolet and blue-optical spectroscopy obtained with the Hubble Space Telescope, X-ray spectroscopy obtained with XMM-Newton, and models of the stellar photosphere, chromosphere, transition region, and corona. A new differential emission measure model of the unobserved extreme-ultraviolet spectrum is provided, improving on the Lyα–EUV relations often used to estimate the 100–911 Å flux from low-mass stars. We describe the observations and models used, as well as the recipe for combining them into an SED. We also provide a semiempirical, noise-free model of the stellar ultraviolet spectrum based on our observations for use in atmospheric modeling of the TRAPPIST-1 planets.

Abstract:

<jats:title>Abstract</jats:title><jats:p>The global superrotation index <jats:italic>S</jats:italic> compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body corotation with the underlying planet. The index <jats:italic>S</jats:italic> is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of <jats:italic>S</jats:italic> for characterizing regimes of atmospheric circulation by running idealized Earthlike general circulation model experiments over a wide range of rotation rates Ω, 8Ω<jats:sub><jats:italic>E</jats:italic></jats:sub> to Ω<jats:sub><jats:italic>E</jats:italic></jats:sub>/512, where Ω<jats:sub><jats:italic>E</jats:italic></jats:sub> is Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute <jats:italic>S</jats:italic> for each simulated circulation, and study the dependence of <jats:italic>S</jats:italic> on Ω. For all rotation rates considered, <jats:italic>S</jats:italic> is on the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, <jats:italic>S</jats:italic> ≪ 1 and <jats:italic>S</jats:italic> ∝ Ω<jats:sup>−2</jats:sup>, while at low rotation rates <jats:italic>S</jats:italic> ≈ 1/2 = constant. By considering the limiting behavior of theoretical models for <jats:italic>S</jats:italic>, we show how the value of <jats:italic>S</jats:italic> and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. Indices of <jats:italic>S</jats:italic> ≪ 1 and <jats:italic>S</jats:italic> ∝ Ω<jats:sup>−2</jats:sup> define a regime dominated by geostrophic thermal wind balance, and <jats:italic>S</jats:italic> ≈ 1/2 = constant defines a regime where the dynamics are characterized by conservation of angular momentum within a planetary-scale Hadley circulation. Indices of <jats:italic>S</jats:italic> ≫ 1 and <jats:italic>S</jats:italic> ∝ Ω<jats:sup>−2</jats:sup> define an additional regime dominated by cyclostrophic balance and strong equatorial superrotation that is not realized in our simulations.</jats:p>

Abstract:

The global superrotation index S compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body corotation with the underlying planet. The index S is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of S for characterizing regimes of atmospheric circulation by running idealized Earthlike general circulation model experiments over a wide range of rotation rates Ω, 8ΩE to ΩE/512, where ΩE is Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute S for each simulated circulation, and study the dependence of S on Ω. For all rotation rates considered, S is on the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, S ≪ 1 and S ∝ Ω−2, while at low rotation rates S ≈ 1/2 = constant. By considering the limiting behavior of theoretical models for S, we show how the value of S and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. Indices of S ≪ 1 and S ∝ Ω−2 define a regime dominated by geostrophic thermal wind balance, and S ≈ 1/2 = constant defines a regime where the dynamics are characterized by conservation of angular momentum within a planetary-scale Hadley circulation. Indices of S ≫ 1 and S ∝ Ω−2 define an additional regime dominated by cyclostrophic balance and strong equatorial superrotation that is not realized in our simulations.

Abstract:

<jats:p>Tidally locked exoplanets likely host global atmospheric circulations with a superrotating equatorial jet, planetary-scale stationary waves, and thermally driven overturning circulation. In this work, we show that each of these features can be separated from the total circulation by using a Helmholtz decomposition, which splits the circulation into rotational (divergence-free) and divergent (vorticity-free) components. This technique is applied to the simulated circulation of a terrestrial planet and a gaseous hot Jupiter. For both planets, the rotational component comprises the equatorial jet and stationary waves, and the divergent component contains the overturning circulation. Separating out each component allows us to evaluate their spatial structure and relative contribution to the total flow. In contrast with previous work, we show that divergent velocities are not negligible when compared with rotational velocities and that divergent, overturning circulation takes the form of a single, roughly isotropic cell that ascends on the day side and descends on the night side. These conclusions are drawn for both the terrestrial case and the hot Jupiter. To illustrate the utility of the Helmholtz decomposition for studying atmospheric processes, we compute the contribution of each of the circulation components to heat transport from day side to night side. Surprisingly, we find that the divergent circulation dominates day–night heat transport in the terrestrial case and accounts for around half of the heat transport for the hot Jupiter. The relative contributions of the rotational and divergent components to day–night heat transport are likely sensitive to multiple planetary parameters and atmospheric processes and merit further study.</jats:p>