Planetary ageostrophic instability leads to superrotation

Wang, Peng; Mitchell, Jonathan L.

doi:10.1002/2014gl060345

Cited by 42 publications

(80 citation statements)

References 34 publications

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“…The results of models that generate superrotating winds [see, e.g., Hourdin et al , ; Del Genio and Zhou , ; Newman et al , ; Lebonnois et al , ] seem to support the idea that the mean meridional circulation transports excess angular momentum upward and poleward and that barotropic waves generated by instabilities on the edges of the high‐latitude jets transport momentum toward the equator [ Lebonnois et al , ], which is known as the GRW mechanism [ Gierasch , ; Rossow and Williams , ]. For example, some models find that superrotation requires coupling between equatorial and high‐latitude waves but that generation and maintenance require equatorial Kelvin‐like waves and Rossby‐like waves, respectively [ Dias Pinto and Mitchell , ; Wang and Mitchell , ]. Though waves play a key role in the atmospheric dynamics, they are difficult to constrain; observations of clouds may play a key role in revealing the characteristics of waves in Titan's atmosphere [ Mitchell et al , ].…”

Section: Dynamicsmentioning

confidence: 99%

Titan's atmosphere and climate

2017

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Titan is the only moon with a substantial atmosphere, the only other thick N2 atmosphere besides Earth's, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body with stable liquid currently on its surface. The connection between Titan's surface and atmosphere is also unique in our solar system; atmospheric chemistry produces materials that are deposited on the surface and subsequently altered by surface‐atmosphere interactions such as aeolian and fluvial processes resulting in the formation of extensive dune fields and expansive lakes and seas. Titan's atmosphere is favorable for organic haze formation, which combined with the presence of some oxygen‐bearing molecules indicates that Titan's atmosphere may produce molecules of prebiotic interest. The combination of organics and liquid, in the form of water in a subsurface ocean and methane/ethane in the surface lakes and seas, means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the universe. The Cassini‐Huygens mission to the Saturn system has provided a wealth of new information allowing for study of Titan as a complex system. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth‐based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with some of our remaining unanswered questions as the incredible era of exploration with Cassini‐Huygens comes to an end.

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Section: Dynamicsmentioning

confidence: 99%

Titan's atmosphere and climate

2017

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“…For a planet without deviations from axisymmetry in boundary conditions, it is less obvious why waves should be preferentially generated near the equator. Wang and Mitchell (2014) and Pinto and Mitchell (2014) find that a Rossby-Kelvin instability produces angular momentum flux convergence at the equator that is responsible for the generation of superrotation in statically stable atmospheres. In convecting atmospheres, the variation of the Rossby number with latitude provides an alternative mechanism: Near the equator, where the Rossby number can be O(1), horizontal and temporal temperature variations are small when the Froude number is small.…”

Section: Introductionmentioning

confidence: 99%

Superrotation in Terrestrial Atmospheres

Laraia

Schneider

2015

Journal of the Atmospheric Sciences

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Atmospheric superrotation with prograde equatorial winds and an equatorial angular momentum maximum is ubiquitous in planetary atmospheres. It is clear that eddy fluxes of angular momentum toward the equator are necessary to generate it. But under what conditions superrotation arises has remained unclear. This paper presents simulations and a scaling theory that establish conditions under which superrotation occurs in terrestrial atmospheres. Whether superrotation arises depends on the relative importance of factors that favor or disfavor superrotation. Convection preferentially generates Rossby waves near the equator, where the Rossby number is O(1). Since the Rossby waves transport angular momentum toward their source regions, this favors superrotation. Meridional temperature gradients preferentially lead to baroclinic instability and wave generation away from the equator. Eddy transport of angular momentum toward the baroclinic source region implies transport out of low latitudes, which disfavors superrotation. Simulations with an idealized GCM show that superrotation tends to arise when the equatorial convective generation of wave activity and its associated eddy angular momentum flux convergence exceed the baroclinic eddy angular momentum flux divergence. Convective and baroclinic wave activity generation is related through scaling arguments to mean-flow properties, such as planetary rotation rates and meridional temperature gradients. The scaling arguments show, for example, that superrotation is favored when the off-equatorial baroclinicity and planetary rotation rates are low, as they are, for example, on Venus. Similarly, superrotation is favored when the convective heating strengthens, which may account for the superrotation seen in extreme global warming simulations.

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“…The next benchmark case is an initialstate which includes an initial velocity field. Here we follow Wang & Mitchell (2014) who identified an exponentially growing linear mode, bringing eastward momentum to the equator, under axisymmetric forcing. This study essentially identifies unstable modes in an atmosphere similar to Thuburn et al (2002) but including a midlatitude unstable jet.…”

Section: Unstable Jetmentioning

confidence: 91%

“…The value of α is not given in Wang & Mitchell (2014), here we chose α = 50 which mimics the shape of their initial velocity field in their Figure 1. Wang & Mitchell (2014) identify two instabilities, firstly a well-known baroclinic instability (such as studied in Section 3.3), and secondly a new instability not captured by analytical treatments under the β-plane approximation. This new mode results in the convergence of eastward momentum at the equator, and is related to the Rossby and Froude numbers.…”

Section: Unstable Jetmentioning

confidence: 99%

“…Following Wang & Mitchell (2014) we explore the effect on the most unstable mode of varying the planetary parameters. Figure 6 shows results for different φ 0 , a characteristic latitude of the initial flow (see Wang & Mitchell 2014, for definitions) and the Burger number Bu = ((N i H)/(2Ωa)) 2 where H is a characteristic height, revealing a change in the growth rate as H is altered (see Wang & Mitchell 2014, for more details). Figure 6a and 6b therefore represent the most unstable mode for a broad inital jet in latitude, up to 50 • , which growth rate is about Ω/5 and characteristic height a third of the total height.…”

Section: Unstable Jetmentioning

confidence: 99%

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Eigenvectors, Circulation, and Linear Instabilities for Planetary Science in 3 Dimensions (ECLIPS3D)

Debras

Mayne

Baraffe

et al. 2019

A&A

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Context. The study of linear waves and instabilities is necessary to understand the physical evolution of an atmosphere, and can provide physical interpretation of the complex flows found in simulations performed using Global Circulation Models (GCM). In particular, the acceleration of superrotating flow at the equator of hot Jupiters has mostly been studied under several simplifying assumptions, the relaxing of which may impact final results. Aims. We develop and benchmark a publicly available algorithm to identify the eigenmodes of an atmosphere around any initial steady state. We also solve for linear steady states indicated to be essential in existing theories of the acceleration of hot Jupiter superrotation. Methods. We linearise the hydrodynamical equations of a planetary atmosphere in a steady state with arbitrary velocities and thermal profile. We then discretise the linearised equations on an appropriate staggered grid, and solve for eigenvectors and linear steady solutions with the use of a parallel library for linear algebra: ScaLAPACK. We also implement a posteriori calculation of an energy equation in order to obtain more information on the underlying physics of the mode. Results. Our code is benchmarked against classical wave and instability test cases in multiple geometries (2D, 3D, two layer equivalent depth). The steady linear circulation calculations also reproduce expected results for the atmosphere of hot Jupiters. We finally show the robustness of our energy equation, and its power to obtain physical insight into the modes. Conclusions. We have developed and benchmarked a code for the study of linear processes in planetary atmospheres, with an arbitrary steady state. The calculation of an a posteriori energy equation provides both increased robustness and physical meaning to the obtained eigenmodes. This code can be applied to various problems, and notably to further study the initial spin up of superrotation of GCM simulations of hot Jupiter.

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Planetary ageostrophic instability leads to superrotation

Cited by 42 publications

References 34 publications

Titan's atmosphere and climate

Titan's atmosphere and climate

Superrotation in Terrestrial Atmospheres

Eigenvectors, Circulation, and Linear Instabilities for Planetary Science in 3 Dimensions (ECLIPS3D)

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