Planetary Climate Dynamics

Abstract:

ABSTRACT Photochemical hazes have been suggested as candidate for the high-altitude aerosols observed in the transmission spectra of many hot Jupiters. We present 3D simulations of the hot Jupiter HD 189733b to study how photochemical hazes are transported by atmospheric circulation. The model includes spherical, constant-size haze particles that gravitationally settle and are transported by the winds as passive tracers, with particle radii ranging from 1 nm to 1 $\mu$m. We identify two general types of haze distribution based on particle size: In the small-particle regime (<30 nm), gravitational settling is unimportant, and hazes accumulate in two large mid-latitude vortices centred on the nightside that extend across the morning terminator. Therefore, small hazes are more concentrated at the morning terminator than at the evening terminator. In the large-particle regime (>30 nm), hazes settle out quickly on the nightside, resulting in more hazes at the evening terminator. For small particles, terminator differences in haze mass mixing ratio and temperature considered individually can result in significant differences in the transit spectra of the terminators. When combining both effects for HD 189733b, however, they largely cancel out each other, resulting in very small terminator differences in the spectra. Transit spectra based on the GCM-derived haze distribution fail to reproduce the steep spectral slope at short wavelengths in the current transit observations of HD 189733b. Enhanced sub-grid scale mixing and/or optical properties of hazes differing from soot can explain the mismatch between the model and observations, although uncertainties in temperature and star spots may also contribute to the spectral slope.

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>