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Abstract:

The circulation of the Earth’s atmosphere and those of many other planets is dominated by turbulent interactions in a baroclinically unstable, rotating, stratified flow. Even for the Earth, which has been well observed for many years, the energy spectrum and complex properties of the anisotropic and inhomogeneous turbulent cascades of energy and enstrophy remain poorly understood and difficult to model accurately. Here we measure geostrophic turbulence energised by baroclinic instability in a rotating, differentially heated fluid annulus in the laboratory, which is bounded by convectively-driven warm and cold flows at the outer and inner boundaries, respectively (see Fig. 1a). Horizontal velocity fields (Fig. 1b-c) are obtained via particle image velocimetry of neutrally buoyant particles suspended in the flow, while the temperature structure is sampled using a vertical array of thermocouples located in the middle of the channel. The horizontal kinetic energy spectra exhibit a wavenumber range at relatively large length scales which scales as k−3, where k denotes the horizontal wavenumber (see Fig. 1d-e). Moreover, the spectral amplitude is found to correlate with the square of the Brunt–Vaisala frequency N at the same heights as the velocity measurements. The observed turbulent state exhibits a net forward enstrophy cascade across all scales, along with bidirectional kinetic energy transfer, which is indicated by a reversal in the sign of the spectral energy flux. The change of sign of the kinetic energy cascade occurs at a scale proportional to the internal Rossby radius of deformation Ld. These findings highlight the role of baroclinic instability in shaping the distribution of energy across scales with implications for synoptic- and meso-scale turbulent flows in the atmospheres of the Earth and other terrestrial planet atmospheres and oceans.FIG. 1. (a) Schematic plot of the convective tank. Snapshots of vorticity ζ for thermal Rossby number RoT = 5.41 (b) and RoT = 0.03 (c). On the scale bar, Lid = 2.4 cm and Liid = 22.6 cm are the Rossby radius of deformation for (c) and (b), respectively. (d) Kinetic energy spectra, E(k), for various values of RoT. The arrow indicates the wave number kp corresponding to the peak of E(k) when RoT = 0.03. Inset: radial profiles of temporal- and zonal-averaged azimuthal velocity, Uθ. (e) Kinetic energy spectra compensated by k−3 and normalised by N2 versus LRk. The dashed line indicates the plateau segment for LRk ∈ [2, 10] and has a magnitude of ∼ 0.5. Data are for height h = 0.18 m.

Abstract:

The midlatitude atmospheres of gas giant planets are characteristic of strong and persistent zonal jets; however, the processes governing their formation and the associated energy pathways remain less understood. To investigate these mechanisms, we conducted a laboratory study of zonal jets driven by thermal forcing in an annular cylindrical tank partially filled with distilled water as the working fluid. Heating is applied at the outer boundary, cooling at the inner boundary, the bottom is thermally insulated, and the top is a free surface. An array of laser diodes embedded in the inner cylinder generates an annular laser sheet, enabling the measurement of velocity fields at a fixed height using particle image velocimetry. By systematically varying the rotation rate and the imposed temperature contrast, we adjusted the steepness of the free surface, thus the topographic β effect, and the thermal forcing strength, respectively. The non-dimensional controlling parameter, thermal Rossby number, RoT, ranges from 0.0012 to 0.01 and Taylor number, Ta, from 2.3 × 1010 to1.7 × 1011. We discerned the emergence of robust zonal jets, of which the zonal-mean kinetic energy accounts for up to 70% of the total kinetic energy, corresponding to a zonostrophic index of 2.7. In this regime, two coherent and persistent prograde jets form near the inner and outer boundaries. The radial profile of the potential vorticity develops toward a pronounced staircase-like structure, consistent with previous numerical studies (Scott and Dritschel, J. Fluid Mech., 2012). Analysis of the inter-scale energy transfer reveals a dominant interaction between the zonal-mean flow and eddies, while the kinetic energy spectrum of the zonal-mean component exhibits k−5 (where k denotes the wavenumber), in agreement with the theory of zonostrophic turbulence (Sukoriansky and Galperin, PRL, 2002).                                     Figure 1: A snapshot of azimuthal velocity contour for RoT = 7.1 × 10−3, Ta = 1.44 × 1011 and β =49.7 m−1 s−1.

Abstract:

Whether a planet's volcanic gas is oxidising or reducing is inherited from redox conditions in the planet's mantle. It is often presumed that reactions between iron species control mantle oxygen fugacity. However, iron alone need not be the sole dictator of how oxidising the interior of a planet is. Carbon is a powerful redox element, with great potential to feed back upon the mantle redox state as it melts. Despite Earth being carbon-poor, it has been proposed that the oxygen fugacity of Earth's upper mantle is in part controlled by carbon (Holloway et al., 1992; Stagno et al., 2013); a slightly-higher volatile endowment could make carbon-powered geochemistry inescapable. Indeed, a number of known rocky exoplanets are predicted to have formed with carbon contents greater than Earth (Bergin et al., 2023). We offer a framework for how carbon is transported from solid planetary interior to atmosphere, tracking redox couplings between carbon and iron. We also incorporate a coupled 1D energy- and mass-balance model to provide first-order predictions of the rate of volcanism. We show that carbon-iron redox coupling would maintain interior oxygen fugacity in a narrow range: more reducing than Earth magma, but not reducing enough to prevent CO2 outgassing entirely.Bergin, E. A., Kempton, E. M.-R., Hirschmann, M., Bastelberger, S. T., Teal, D. J., Blake, G. A., Ciesla, F. J., & Li, J. (2023). Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation. The Astrophysical Journal, 949, L17. Holloway, J. R., Pan, V., & Gudmundsson, G. (1992). High-pressure fluid-absent melting experiments in the presence of graphite: Oxygen fugacity, ferric/ferrous ratio and dissolved CO2. European Journal of Mineralogy, 4(1), 105–114. Stagno, V., Ojwang, D. O., McCammon, C. A., & Frost, D. J. (2013). The oxidation state of the mantle and the extraction of carbon from Earth’s interior. Nature, 493(7430).

Abstract:

Context. Close-in warm Jupiters orbiting M dwarf stars are expected to exhibit diverse atmospheric chemistry, with clouds playing a key role in shaping their albedo, heat distribution, and spectral properties. Aims. We study WASP-80b, a warm Jupiter orbiting an M dwarf star, using the latest JWST panchromatic emission and transmission spectra to comprehensively characterise its atmosphere, including cloud coverage, chemical composition, and particle sizes, and compare the observations with predictions from the general circulation models (GCMs). Methods. We used a GCM, ADAM (ADvanced Atmospheric MITgcm, formerly known as SPARC/MITgcm), combined with the latest JWST data to study the atmosphere of WASP-80b. A cloud module with radiatively active, tracer-based clouds was integrated with the GCM to study the effects on the atmosphere and the spectrum. Results. We find that the emission and transmission spectra of WASP-80b are only compatible with cloudless atmospheres or with clouds composed of sufficiently large particles, namely Na2S (≥10 μm), KCl (≥1 μm), and MgSiO3 (≥5 μm). For these large-particle cloud cases, efficient gravitational settling confines the clouds to deeper atmospheric layers, resulting in weak spectral signatures. Smaller particles are ruled out due to their strong radiative feedback on the atmospheric structure. Conclusions. Overall, our results suggest that WASP-80b’s atmosphere is either effectively cloud-free or contains clouds composed of large, settled particles whose opacity has little impact on the observable atmosphere. This underscores the importance of particle size and vertical cloud distribution in interpreting exoplanet spectra. Future observations at shorter wavelengths may help distinguish between large-particle cloud scenarios and a truly cloudless atmosphere.

Abstract:

Plain Language Summary: In hot and wet “hothouse” climate conditions, rainfall transitions from a pattern that fluctuates from about a mean of 3 mm day − 1 ${\text{day}}^{-1}$ to more intense outbursts that are separated by multi‐day dry spells. Previous studies on hothouse climates did not consider the role of the diurnal cycle even though it strongly controls precipitation in Earth's current climate. This study uses radiative‐convective equilibrium simulations to investigate the impact of rising temperatures on the transition to hothouse conditions, incorporating the diurnal cycle with both swamp‐like and open ocean surface conditions. We find that episodic precipitation occurs at surface temperatures above 322 K even when accounting for the diurnal cycle. However, the diurnal cycle significantly influences the timing of convection and rainfall at high temperatures with precipitation primarily starting late at night or in the early morning.