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Lorenzo M. Polvani - One of the best experts on this subject based on the ideXlab platform.

  • Equatorial Superrotation on tidally locked exoplanets
    The Astrophysical Journal, 2011
    Co-Authors: Adam P. Showman, Lorenzo M. Polvani
    Abstract:

    The increasing richness of exoplanet observations has motivated a variety of three-dimensional (3D) atmospheric circulation models of these planets. Under strongly irradiated conditions, models of tidally locked, short-period planets (both hot Jupiters and terrestrial planets) tend to exhibit a circulation dominated by a fast eastward, or 'superrotating', jet stream at the equator. When the radiative and advection timescales are comparable, this phenomenon can cause the hottest regions to be displaced eastward from the substellar point by tens of degrees longitude. Such an offset has been subsequently observed on HD 189733b, supporting the possibility of equatorial jets on short-period exoplanets. Despite its relevance, however, the dynamical mechanisms responsible for generating the equatorial Superrotation in such models have not been identified. Here, we show that the equatorial jet results from the interaction of the mean flow with standing Rossby waves induced by the day-night thermal forcing. The strong longitudinal variations in radiative heating-namely intense dayside heating and nightside cooling-trigger the formation of standing, planetary-scale equatorial Rossby and Kelvin waves. The Rossby waves develop phase tilts that pump eastward momentum from high latitudes to the equator, thereby inducing equatorial Superrotation. We present an analytic theory demonstrating this mechanism and explore its properties inmore » a hierarchy of one-layer (shallow-water) calculations and fully 3D models. The wave-mean-flow interaction produces an equatorial jet whose latitudinal width is comparable to that of the Rossby waves, namely the equatorial Rossby deformation radius modified by radiative and frictional effects. For conditions typical of synchronously rotating hot Jupiters, this length is comparable to a planetary radius, explaining the broad scale of the equatorial jet obtained in most hot-Jupiter models. Our theory illuminates the dependence of the equatorial jet speed on forcing amplitude, strength of friction, and other parameters, as well as the conditions under which jets can form at all.« less

  • The Matsuno‐Gill Model and Equatorial Superrotation
    Geophysical Research Letters, 2010
    Co-Authors: Adam P. Showman, Lorenzo M. Polvani
    Abstract:

    [1] Equatorial Superrotation can be generated in global general circulation models (GCMs) when forced with longitudinally varying heating, similar to that postulated in the Matsuno-Gill model. However, the implications of the classical Matsuno-Gill theory for equatorial Superrotation have not, to date, been addressed. Here, we show that the classic, shallow-water Matsuno-Gill solutions do not exhibit equatorial Superrotation: although the flow converges westerly momentum from high latitudes to the equator—promoting Superrotation—they also contain an artificial source of easterly momentum at the equator that cancels the latitudinal momentum convergence and prevents Superrotation from emerging. This artificial momentum source results from a physically inconsistent representation of vertical momentum transport in the model. We show that if the Matsuno-Gill model is modified to properly account for momentum exchange with an underlying quiescent layer, the solutions naturally exhibit equatorial Superrotation, at any forcing amplitude.

  • the matsuno gill model and equatorial Superrotation
    Geophysical Research Letters, 2010
    Co-Authors: Adam P. Showman, Lorenzo M. Polvani
    Abstract:

    [1] Equatorial Superrotation can be generated in global general circulation models (GCMs) when forced with longitudinally varying heating, similar to that postulated in the Matsuno-Gill model. However, the implications of the classical Matsuno-Gill theory for equatorial Superrotation have not, to date, been addressed. Here, we show that the classic, shallow-water Matsuno-Gill solutions do not exhibit equatorial Superrotation: although the flow converges westerly momentum from high latitudes to the equator—promoting Superrotation—they also contain an artificial source of easterly momentum at the equator that cancels the latitudinal momentum convergence and prevents Superrotation from emerging. This artificial momentum source results from a physically inconsistent representation of vertical momentum transport in the model. We show that if the Matsuno-Gill model is modified to properly account for momentum exchange with an underlying quiescent layer, the solutions naturally exhibit equatorial Superrotation, at any forcing amplitude.

  • Equatorial Superrotation in shallow atmospheres
    Geophysical Research Letters, 2008
    Co-Authors: Richard K. Scott, Lorenzo M. Polvani
    Abstract:

    [1] Simple, shallow-water models have been successful in reproducing two key observables in the atmospheres of the giant planets: the formation of robust, and fully turbulent, latitudinal jets and the decrease of the zonal wind amplitude with latitude. However, they have to date consistently failed in reproducing the strong prograde (superrotating) equatorial winds that are often observed on such planets. In this paper we show that shallow water models not only can give rise to superrotating winds, but can do so very robustly, provided that the physical process of large-scale energy dissipation by radiative relaxation is taken into account. When energy is removed by linear friction, equatorial Superrotation does not develop; when energy is removed by radiative relaxation, Superrotation develops at apparently any deformation radius.

Adam P. Showman - One of the best experts on this subject based on the ideXlab platform.

  • Superrotation in Planetary Atmospheres
    Space Science Reviews, 2020
    Co-Authors: Takeshi Imamura, Adam P. Showman, Jonathan Mitchell, Sebastien Lebonnois, Yohai Kaspi, Oleg Korablev
    Abstract:

    Superrotation is a dynamical regime where the atmosphere circulates around the planet in the direction of planetary rotation with excess angular momentum in the equatorial region. Superrotation is known to exist in the atmospheres of Venus, Titan, Jupiter, and Saturn in the solar system. Some of the exoplanets also exhibit Superrotation. Our understanding of Superrotation in a framework of circulation regimes of the atmospheres of terrestrial planets is in progress thanks to the development of numerical models; a global instability involving planetary-scale waves seems to play a key role, and the dynamical state depends on the Rossby number, a measure of the relative importance of the inertial and Coriolis forces, and the thermal inertia of the atmosphere. Recent general circulation models of Venus’s and Titan’s atmospheres demonstrated the importance of horizontal waves in the angular momentum transport in these atmospheres and also an additional contribution of thermal tides in Venus’s atmosphere. The atmospheres of Jupiter and Saturn also exhibit strong Superrotation. Recent gravity data suggests that these Superrotational flows extend deep into the planet, yet currently no single mechanism has been identified as driving this Superrotation. Moreover, atmospheric circulation models of tidally locked, strongly irradiated exoplanets have long predicted the existence of equatorial Superrotation in their atmospheres, which has been attributed to the result of the strong day-night thermal forcing. As predicted, recent Doppler observations and infrared phase curves of hot Jupiters appear to confirm the presence of Superrotation on these objects.

  • Equatorial Superrotation on tidally locked exoplanets
    The Astrophysical Journal, 2011
    Co-Authors: Adam P. Showman, Lorenzo M. Polvani
    Abstract:

    The increasing richness of exoplanet observations has motivated a variety of three-dimensional (3D) atmospheric circulation models of these planets. Under strongly irradiated conditions, models of tidally locked, short-period planets (both hot Jupiters and terrestrial planets) tend to exhibit a circulation dominated by a fast eastward, or 'superrotating', jet stream at the equator. When the radiative and advection timescales are comparable, this phenomenon can cause the hottest regions to be displaced eastward from the substellar point by tens of degrees longitude. Such an offset has been subsequently observed on HD 189733b, supporting the possibility of equatorial jets on short-period exoplanets. Despite its relevance, however, the dynamical mechanisms responsible for generating the equatorial Superrotation in such models have not been identified. Here, we show that the equatorial jet results from the interaction of the mean flow with standing Rossby waves induced by the day-night thermal forcing. The strong longitudinal variations in radiative heating-namely intense dayside heating and nightside cooling-trigger the formation of standing, planetary-scale equatorial Rossby and Kelvin waves. The Rossby waves develop phase tilts that pump eastward momentum from high latitudes to the equator, thereby inducing equatorial Superrotation. We present an analytic theory demonstrating this mechanism and explore its properties inmore » a hierarchy of one-layer (shallow-water) calculations and fully 3D models. The wave-mean-flow interaction produces an equatorial jet whose latitudinal width is comparable to that of the Rossby waves, namely the equatorial Rossby deformation radius modified by radiative and frictional effects. For conditions typical of synchronously rotating hot Jupiters, this length is comparable to a planetary radius, explaining the broad scale of the equatorial jet obtained in most hot-Jupiter models. Our theory illuminates the dependence of the equatorial jet speed on forcing amplitude, strength of friction, and other parameters, as well as the conditions under which jets can form at all.« less

  • The Matsuno‐Gill Model and Equatorial Superrotation
    Geophysical Research Letters, 2010
    Co-Authors: Adam P. Showman, Lorenzo M. Polvani
    Abstract:

    [1] Equatorial Superrotation can be generated in global general circulation models (GCMs) when forced with longitudinally varying heating, similar to that postulated in the Matsuno-Gill model. However, the implications of the classical Matsuno-Gill theory for equatorial Superrotation have not, to date, been addressed. Here, we show that the classic, shallow-water Matsuno-Gill solutions do not exhibit equatorial Superrotation: although the flow converges westerly momentum from high latitudes to the equator—promoting Superrotation—they also contain an artificial source of easterly momentum at the equator that cancels the latitudinal momentum convergence and prevents Superrotation from emerging. This artificial momentum source results from a physically inconsistent representation of vertical momentum transport in the model. We show that if the Matsuno-Gill model is modified to properly account for momentum exchange with an underlying quiescent layer, the solutions naturally exhibit equatorial Superrotation, at any forcing amplitude.

  • the matsuno gill model and equatorial Superrotation
    Geophysical Research Letters, 2010
    Co-Authors: Adam P. Showman, Lorenzo M. Polvani
    Abstract:

    [1] Equatorial Superrotation can be generated in global general circulation models (GCMs) when forced with longitudinally varying heating, similar to that postulated in the Matsuno-Gill model. However, the implications of the classical Matsuno-Gill theory for equatorial Superrotation have not, to date, been addressed. Here, we show that the classic, shallow-water Matsuno-Gill solutions do not exhibit equatorial Superrotation: although the flow converges westerly momentum from high latitudes to the equator—promoting Superrotation—they also contain an artificial source of easterly momentum at the equator that cancels the latitudinal momentum convergence and prevents Superrotation from emerging. This artificial momentum source results from a physically inconsistent representation of vertical momentum transport in the model. We show that if the Matsuno-Gill model is modified to properly account for momentum exchange with an underlying quiescent layer, the solutions naturally exhibit equatorial Superrotation, at any forcing amplitude.

  • Generation of equatorial jets by large-scale latent heating on the giant planets
    Icarus, 2010
    Co-Authors: Yuan Lian, Adam P. Showman
    Abstract:

    Abstract Three-dimensional numerical simulations show that large-scale latent heating resulting from condensation of water vapor can produce multiple zonal jets similar to those on the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). For plausible water abundances (3–5 times solar on Jupiter/Saturn and 30 times solar on Uranus/Neptune), our simulations produce ∼ 20 zonal jets for Jupiter and Saturn and 3 zonal jets on Uranus and Neptune, similar to the number of jets observed on these planets. Moreover, these Jupiter/Saturn cases produce equatorial Superrotation whereas the Uranus/Neptune cases produce equatorial subrotation, consistent with the observed equatorial-jet direction on these planets. Sensitivity tests show that water abundance, planetary rotation rate, and planetary radius are all controlling factors, with water playing the most important role; modest water abundances, large planetary radii, and fast rotation rates favor equatorial Superrotation, whereas large water abundances favor equatorial subrotation regardless of the planetary radius and rotation rate. Given the larger radii, faster rotation rates, and probable lower water abundances of Jupiter and Saturn relative to Uranus and Neptune, our simulations therefore provide a possible mechanism for the existence of equatorial Superrotation on Jupiter and Saturn and the lack of Superrotation on Uranus and Neptune. Nevertheless, Saturn poses a possible difficulty, as our simulations were unable to explain the unusually high speed (∼ 400 m s - 1 ) of that planet’s superrotating jet. The zonal jets in our simulations exhibit modest violations of the barotropic and Charney–Stern stability criteria. Overall, our simulations, while idealized, support the idea that latent heating plays an important role in generating the jets on the giant planets.

Masaaki Takahashi - One of the best experts on this subject based on the ideXlab platform.

  • Venusian middle-atmospheric dynamics in the presence of a strong planetary-scale 5.5-day wave
    Icarus, 2012
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    Abstract The middle atmospheric dynamics on Venus are investigated using a middle atmosphere general circulation model. The magnitude of the Superrotation is sensitive to the amplitude of the planetary-scale waves. In particular, the critical level absorptions of the forced planetary-scale waves might contribute to the maintenance of the Superrotation near the cloud base. In the case of strong 5.5-day wave forcing, the Superrotation with zonal wind speed higher than 100 m s −1 is maintained by the forced wave. Four-day and 5.5-day waves are found near the equatorial cloud top and base, respectively. The planetary-scale waves have a Y-shaped pattern maintained by the amplitude modulation in the presence of strong thermal tides. The polar hot dipole is unstable and its dynamical behavior is complex near the cloud top in this model. The dipole merges into a monopole or breaks up into a tripole when the divergent eddies with high zonal wavenumbers are predominant in the hot dipole region. A cold collar is partly enhanced by a cold phase of slowly propagating waves with zonal wavenumber 1. Although such a complex dipole behavior has not been observed yet, it is likely to occur under a dynamical condition similar to the present simulation. Thus, the dynamical approach using a general circulation model might be useful for analyzing Venus Express and ground-based observation data.

  • Dynamical effects of solar heating below the cloud layer in a Venus-like atmosphere
    Journal of Geophysical Research, 2009
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    [1] The dynamics of Venus' lower-atmospheric Superrotation remain an open issue. In the present study, we investigate the sensitivity of the Superrotation to diabatic heating rate below the cloud layer using a simplified GCM. A fully developed Superrotation fails to develop in the lower atmosphere under the condition with a fairly low diabatic heating below the cloud layer, as is thought to be realistic. Additional radiative forcing in the lower atmosphere and/or eddy momentum source should be considered to produce lower-atmospheric Superrotation. The diabatic heating rate below the cloud layer controls the formation of the multiple Superrotation states (slow, fast, and extremely fast Superrotations). Small changes in the weak lower-atmospheric heating lead to drastic changes of the middle- and lower-atmospheric Superrotation.

  • Influences of Venus’ topography on fully developed Superrotation and near-surface flow
    Earth Planets and Space, 2009
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    We investigate the influence of topography on Venus’ atmospheric general circulation. Based on comparative simulations with and without the Venusian topography, we elucidate the role of the topography in the fully developed Superrotation. Orographically forced stationary waves are predominant over Mt. Maxwell: slightly weakening the Superrotation near the cloud top. Differently from previous GCM results, the orographically forced waves do not produce significant asymmetry between the northern and southern hemispheric Superrotations in the present model. Weak surface flows from mountains to lowlands are caused by the pressure dependence of the Newtonian cooling. The pattern and magnitude of the near-surface flow are largely different from those simulated in the Herrnstein and Dowling (2007) model. This implies that the parameterizations of physical processes (such as Newtonian cooling, turbulence, diffusion, and surface drag) and the model resolution could significantly influence the pattern and magnitude of the near-surface flow and the orographical forcing of planetary-scale stationary waves.

  • simulations of Superrotation using a gcm for venus middle atmosphere
    Earth Planets and Space, 2007
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    A Superrotation is simulated in a T10L100 general circulation model for Venus’ middle atmosphere (VMAGCM), in which the radiative effects of aerosols are calculated. The simulation in a domain of 30–100 km is conducted under the condition of a bottom zonal flow with a velocity of 50 m s−1 at the equator. Thermal tides contribute to the maintenance of the cloud-top Superrotation together with meridional circulation and vertically propagating gravity waves. The meridional circulation and wave activity are sensitive to the vertical eddy diffusion. Although the equatorial zonal flow has a velocity of about 70 m s−1 when the vertical eddy diffusion coefficient (KV) is set at 5.0 m s−2, it has a velocity of <100 m s−1 when KV = 2.5 m s−2. The fully developed equatorial jet for the small KV case is enhanced at 65 km by small-scale gravity waves emitted from the cloud top.

  • A parametric study of atmospheric Superrotation on Venus-like planets: Effects of oblique angle of planetary rotation axis
    Geophysical Research Letters, 2007
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    [1] Parametric experiments of Superrotation on slowly rotating planets, such as Venus, were conducted. The oblique angle of the planetary rotation axis is one of the most important parameters in the dynamics of the Superrotation. New relationships are found for the maximum mean zonal flow, meridional flow, and equator-pole temperature difference. For low obliquity, the dynamic state alternates between weak and strong Superrotations with a period of about 10000 days in the phase space, and small fluctuations of about 1000 days are also seen. Fully-developed Superrotation is maintained by meridional circulation and waves in the case of a slightly oblique angle. However, it is not formed in the case of a highly oblique angle, since the vertical transport of angular momentum due to meridional circulation is ineffective at the solstices. The Superrotational wind becomes larger with decreasing obliquity.

Inna Polichtchouk - One of the best experts on this subject based on the ideXlab platform.

  • equatorial Superrotation in held and suarez like flows with weak equator to pole surface temperature gradient
    Quarterly Journal of the Royal Meteorological Society, 2016
    Co-Authors: Inna Polichtchouk
    Abstract:

    Equatorial Superrotation under zonally symmetric thermal forcing is investigated in a set-up close to that of the classic Held and Suarez set-up. In contrast to the behaviour in the classic set-up, a transition to equatorial Superrotation occurs when the equator-to-pole surface equilibrium entropy gradient is weakened. Two factors contribute to this transition: (i) the reduction of breaking Rossby waves from the midlatitudes that decelerate the equatorial flow and (ii) the presence of barotropic instability in the equatorial region, providing stirring to accelerate the equatorial flow. In the latter, Kelvin waves excited by instability near the Equator generate and maintain the Superrotation. However, the Superrotation is unphysically enhanced if simulations are underresolved and/or overdissipated.

  • equatorial Superrotation in held suarez like flows with weak equator to pole surface temperature gradient
    arXiv: Atmospheric and Oceanic Physics, 2016
    Co-Authors: Inna Polichtchouk, James Y K Cho
    Abstract:

    Equatorial Superrotation under zonally-symmetric thermal forcing is investigated in a setup close to that of the classic Held & Suarez (1994) setup. In contrast to the behaviour in the classic setup, a transition to equatorial Superrotation occurs when the equator-to-pole surface equilibrium entropy gradient is weakened. Two factors contribute to this transition: 1) the reduction of breaking Rossby waves from the mid-latitude that decelerate the equatorial flow and 2) the presence of barotropic instability in the equatorial region, providing stirring to accelerate the equatorial flow. In the latter, Kelvin waves excited by instability near the equator generate and maintain the Superrotation. However, the Superrotation is unphysically enhanced if simulations are under-resolved and/or over-dissipated.

Masaru Yamamoto - One of the best experts on this subject based on the ideXlab platform.

  • Venusian middle-atmospheric dynamics in the presence of a strong planetary-scale 5.5-day wave
    Icarus, 2012
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    Abstract The middle atmospheric dynamics on Venus are investigated using a middle atmosphere general circulation model. The magnitude of the Superrotation is sensitive to the amplitude of the planetary-scale waves. In particular, the critical level absorptions of the forced planetary-scale waves might contribute to the maintenance of the Superrotation near the cloud base. In the case of strong 5.5-day wave forcing, the Superrotation with zonal wind speed higher than 100 m s −1 is maintained by the forced wave. Four-day and 5.5-day waves are found near the equatorial cloud top and base, respectively. The planetary-scale waves have a Y-shaped pattern maintained by the amplitude modulation in the presence of strong thermal tides. The polar hot dipole is unstable and its dynamical behavior is complex near the cloud top in this model. The dipole merges into a monopole or breaks up into a tripole when the divergent eddies with high zonal wavenumbers are predominant in the hot dipole region. A cold collar is partly enhanced by a cold phase of slowly propagating waves with zonal wavenumber 1. Although such a complex dipole behavior has not been observed yet, it is likely to occur under a dynamical condition similar to the present simulation. Thus, the dynamical approach using a general circulation model might be useful for analyzing Venus Express and ground-based observation data.

  • Dynamical effects of solar heating below the cloud layer in a Venus-like atmosphere
    Journal of Geophysical Research, 2009
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    [1] The dynamics of Venus' lower-atmospheric Superrotation remain an open issue. In the present study, we investigate the sensitivity of the Superrotation to diabatic heating rate below the cloud layer using a simplified GCM. A fully developed Superrotation fails to develop in the lower atmosphere under the condition with a fairly low diabatic heating below the cloud layer, as is thought to be realistic. Additional radiative forcing in the lower atmosphere and/or eddy momentum source should be considered to produce lower-atmospheric Superrotation. The diabatic heating rate below the cloud layer controls the formation of the multiple Superrotation states (slow, fast, and extremely fast Superrotations). Small changes in the weak lower-atmospheric heating lead to drastic changes of the middle- and lower-atmospheric Superrotation.

  • Influences of Venus’ topography on fully developed Superrotation and near-surface flow
    Earth Planets and Space, 2009
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    We investigate the influence of topography on Venus’ atmospheric general circulation. Based on comparative simulations with and without the Venusian topography, we elucidate the role of the topography in the fully developed Superrotation. Orographically forced stationary waves are predominant over Mt. Maxwell: slightly weakening the Superrotation near the cloud top. Differently from previous GCM results, the orographically forced waves do not produce significant asymmetry between the northern and southern hemispheric Superrotations in the present model. Weak surface flows from mountains to lowlands are caused by the pressure dependence of the Newtonian cooling. The pattern and magnitude of the near-surface flow are largely different from those simulated in the Herrnstein and Dowling (2007) model. This implies that the parameterizations of physical processes (such as Newtonian cooling, turbulence, diffusion, and surface drag) and the model resolution could significantly influence the pattern and magnitude of the near-surface flow and the orographical forcing of planetary-scale stationary waves.

  • simulations of Superrotation using a gcm for venus middle atmosphere
    Earth Planets and Space, 2007
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    A Superrotation is simulated in a T10L100 general circulation model for Venus’ middle atmosphere (VMAGCM), in which the radiative effects of aerosols are calculated. The simulation in a domain of 30–100 km is conducted under the condition of a bottom zonal flow with a velocity of 50 m s−1 at the equator. Thermal tides contribute to the maintenance of the cloud-top Superrotation together with meridional circulation and vertically propagating gravity waves. The meridional circulation and wave activity are sensitive to the vertical eddy diffusion. Although the equatorial zonal flow has a velocity of about 70 m s−1 when the vertical eddy diffusion coefficient (KV) is set at 5.0 m s−2, it has a velocity of <100 m s−1 when KV = 2.5 m s−2. The fully developed equatorial jet for the small KV case is enhanced at 65 km by small-scale gravity waves emitted from the cloud top.

  • A parametric study of atmospheric Superrotation on Venus-like planets: Effects of oblique angle of planetary rotation axis
    Geophysical Research Letters, 2007
    Co-Authors: Masaru Yamamoto, Masaaki Takahashi
    Abstract:

    [1] Parametric experiments of Superrotation on slowly rotating planets, such as Venus, were conducted. The oblique angle of the planetary rotation axis is one of the most important parameters in the dynamics of the Superrotation. New relationships are found for the maximum mean zonal flow, meridional flow, and equator-pole temperature difference. For low obliquity, the dynamic state alternates between weak and strong Superrotations with a period of about 10000 days in the phase space, and small fluctuations of about 1000 days are also seen. Fully-developed Superrotation is maintained by meridional circulation and waves in the case of a slightly oblique angle. However, it is not formed in the case of a highly oblique angle, since the vertical transport of angular momentum due to meridional circulation is ineffective at the solstices. The Superrotational wind becomes larger with decreasing obliquity.