Jupiter's turbulent weather layer contains phenomena of many different sizes, from local storms up to the Great Red Spot and banded jets. The global circulation is driven by complex interactions with ...(as yet uncertain) small-scale processes. We have calculated structure functions and kinetic energy spectral fluxes from Cassini observations over a wide range of length scales in Jupiter's atmosphere. We found evidence for an inverse cascade of kinetic energy from length scales comparable to the first baroclinic Rossby deformation radius up to the global jet scale, but also a forward cascade of kinetic energy from the deformation radius to smaller scales. This second result disagrees with the traditional picture of Jupiter's atmospheric dynamics, but has some similarities with mesoscale phenomena in the Earth's atmosphere and oceans. We conclude that the inverse cascade driving Jupiter's jets may have a dominant energy source at scales close to the deformation radius, such as baroclinic instability.
The Atmospheric Dynamics of Venus Sánchez-Lavega, Agustín; Lebonnois, Sebastien; Imamura, Takeshi ...
Space science reviews,
11/2017, Letnik:
212, Številka:
3-4
Journal Article
Recenzirano
Odprti dostop
We review our current knowledge of the atmospheric dynamics of Venus prior to the Akatsuki mission, in the altitude range from the surface to approximately the cloud tops located at about 100 km ...altitude. The three-dimensional structure of the wind field in this region has been determined with a variety of techniques over a broad range of spatial and temporal scales (from the mesoscale to planetary, from days to years, in daytime and nighttime), spanning a period of about 50 years (from the 1960s to the present). The global panorama is that the mean atmospheric motions are essentially zonal, dominated by the so-called super-rotation (an atmospheric rotation that is 60 to 80 times faster than that of the planetary body). The zonal winds blow westward (in the same direction as the planet rotation) with a nearly constant speed of
∼
100
m
s
−
1
at the cloud tops (65–70 km altitude) from latitude 50°N to 50°S, then decreasing their speeds monotonically from these latitudes toward the poles. Vertically, the zonal winds decrease with decreasing altitude towards velocities
∼
1
–
3
m
s
−
1
in a layer of thickness
∼
10
km
close to the surface. Meridional motions with peak speeds of
∼
15
m
s
−
1
occur within the upper cloud at 65 km altitude and are related to a Hadley cell circulation and to the solar thermal tide. Vertical motions with speeds
∼
1
–
3
m
s
−
1
occur in the statically unstable layer between altitudes of
∼
50
–
55
km
. All these motions are permanent with speed variations of the order of
∼
10
%
. Various types of wave, from mesoscale gravity waves to Rossby-Kelvin planetary scale waves, have been detected at and above cloud heights, and are considered to be candidates as agents for carrying momentum that drives the super-rotation, although numerical models do not fully reproduce all the observed features. Momentum transport by atmospheric waves and the solar tide is thought to be an indispensable component of the general circulation of the Venus atmosphere. Another conspicuous feature of the atmospheric circulation is the presence of polar vortices. These are present in both hemispheres and are regions of warmer and lower clouds, seen prominently at infrared wavelengths, showing a highly variable morphology and motions. The vortices spin with a period of 2–3 days. The South polar vortex rotates around a geographical point which is itself displaced from the true pole of rotation by
∼
3
degrees. The polar vortex is surrounded and constrained by the cold collar, an infrared-dark region of lower temperatures. We still lack detailed models of the mechanisms underlying the dynamics of these features and how they couple (or not) to the super-rotation. The nature of the super-rotation relates to the angular momentum stored in the atmosphere and how it is transported between the tropics and higher latitudes, and between the deep atmosphere and upper levels. The role of eddy processes is crucial, but likely involves the complex interaction of a variety of different types of eddy, either forced directly by radiative heating and mechanical interactions with the surface or through various forms of instability. Numerical models have achieved some significant recent success in capturing some aspects of the observed super-rotation, consistent with the scenario discussed by Gierasch (J. Atmos. Sci. 32:1038–1044,
1975
) and Rossow and Williams (J. Atmos. Sci. 36:377–389,
1979
), but many uncertainties remain, especially in the deep atmosphere. The theoretical framework developed to explain the circulation in Venus’s atmosphere is reviewed, as well as the numerical models that have been built to elucidate the super-rotation mechanism. These tools are used to analyze the respective roles of the different waves in the processes driving the observed motions. Their limitations and suggested directions for improvements are discussed.
The regimes of possible global atmospheric circulation patterns in an Earth‐like atmosphere are explored using a simplified Global Circulation Model (GCM) based on the University of Hamburg's ...Portable University Model for the Atmosphere (PUMA)—with simplified (linear) boundary‐layer friction, a Newtonian cooling scheme, and dry convective adjustment (designated here as PUMA‐S). A series of controlled experiments is conducted by varying planetary rotation rate and imposed equator‐to‐pole temperature difference. These defining parameters are combined further with each other into dimensionless forms to establish a parameter space in which the occurrences of different circulation regimes are mapped and classified. Clear, coherent trends are found when varying planetary rotation rate (thermal Rossby number) and frictional and thermal relaxation time‐scales. The sequence of circulation regimes as a function of parameters, such as the planetary rotation rate, strongly resembles that obtained in laboratory experiments on rotating, stratified flows, especially if a topographic β‐effect is included in those experiments to emulate the planetary vorticity gradients in an atmosphere induced by the spherical curvature of the planet. A regular baroclinic wave regime is also obtained at intermediate values of thermal Rossby number and its characteristics and dominant zonal wavenumber depend strongly on the strength of radiative and frictional damping. These regular waves exhibit some strong similarities to baroclinic storms observed on Mars under some conditions. Multiple jets are found at the highest rotation rates, when the Rossby deformation radius and other eddy‐related length‐scales are much smaller than the radius of the planet. These exhibit some similarity to the multiple zonal jets observed on gas giant planets. Jets form on a scale comparable to the most energetic eddies and the Rhines scale poleward of the supercritical latitude. The balance of heat transport varies strongly with Ω∗ between eddies and zonally symmetric flows, becoming weak with fast rotation.
The regimes of global atmospheric circulation patterns in an Earth‐like atmosphere are explored as a function of rotation rate Ω, thermal contrast, and other parameters expressed in dimensionless form, using a simplified GCM based on the University of Hamburg's Portable University Model for the Atmosphere. Several distinct flow regimes are identified, including barotropically unstable cyclostrophic flow or a regular baroclinic wave regime at low Ω and more Earth‐like chaotic flows and circulations with multiple eddy‐driven zonal jets at larger Ω.
•We diagnose martian planetary waves by assimilation of spacecraft data.•Transient waves are largest from autumn to spring equinox.•There is a strong minimum in near-surface amplitude centred on the ...winter solstice.•The solsticial pause, occurs in every year of the analysis and in both hemispheres.•The solsticial pause is under-represented in many independent model experiments.
Large-scale planetary waves are diagnosed from an analysis of profiles retrieved from the Thermal Emission Spectrometer aboard the Mars Global Surveyor spacecraft during its scientific mapping phase. The analysis is conducted by assimilating thermal profiles and total dust opacity retrievals into a Mars global circulation model. Transient waves are largest throughout the northern hemisphere autumn, winter and spring period and almost absent during the summer. The southern hemisphere exhibits generally weaker transient wave behaviour. A striking feature of the low-altitude transient waves in the analysis is that they show a broad subsidiary minimum in amplitude centred on the winter solstice, a period when the thermal contrast between the summer hemisphere and the winter pole is strongest and baroclinic wave activity might be expected to be strong. This behaviour, here called the ‘solsticial pause,’ is present in every year of the analysis. This strong pause is under-represented in many independent model experiments, which tend to produce relatively uniform baroclinic wave activity throughout the winter. This paper documents and diagnoses the transient wave solsticial pause found in the analysis; a companion paper investigates the origin of the phenomenon in a series of model experiments.
We present an experimental study of flows in a cylindrical rotating annulus convectively forced by local heating in an annular ring at the bottom near the external wall and via a cooled circular disk ...near the axis at the top surface of the annulus. This new configuration is distinct from the classical thermally driven annulus analogue of the atmosphere circulation, in which thermal forcing is applied uniformly on the sidewalls, but with a similar aim to investigate the baroclinic instability of a rotating, stratified flow subjected to zonally symmetric forcing. Two vertically and horizontally displaced heat sources/sinks are arranged, so that in the absence of background rotation, statically unstable Rayleigh–Bénard convection would be induced above the source and beneath the sink, thereby relaxing strong constraints placed on background temperature gradients in previous experimental configurations based on the conventional rotating annulus. This better emulates local vigorous convection in the tropics and polar regions of the atmosphere while also allowing stably-stratified baroclinic motion in the central zone of the annulus, as in mid-latitude regions in the Earth’s atmosphere. Regimes of flow are identified, depending mainly upon control parameters that in turn depend on rotation rate and the strength of differential heating. Several regimes exhibit baroclinically unstable flows which are qualitatively similar to those previously observed in the classical thermally driven annulus. However, in contrast to the classical configuration, they typically exhibit more spatio-temporal complexity. Thus, several regimes of flow demonstrate the equilibrated co-existence of, and interaction between, free convection and baroclinic wave modes. These new features were not previously observed in the classical annulus and validate the new setup as a tool for exploring fundamental atmosphere-like dynamics in a more realistic framework. Thermal structure in the fluid is investigated and found to be qualitatively consistent with previous numerical results, with nearly isothermal conditions, respectively, above and below the heat source and sink, and stably-stratified, sloping isotherms in the near-adiabatic interior.
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, 8Ω
E
to Ω
E
/512, where Ω
E
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
The response of the quasi-biennial oscillation (QBO) to an imposed mean upwelling with a periodic modulation is studied, by modeling the dynamics of the zero wind line at the equator using a ...class of equations known as descent rate models. These are simple mathematical models that capture the essence of QBO synchronization by focusing on the dynamics of the height of the zero wind line. A heuristic descent rate model for the zero wind line is described and is shown to capture many of the synchronization features seen in previous studies of the QBO. It is then demonstrated using a simple transformation that the standard Holton–Lindzen model of the QBO can itself be put into the form of a descent rate model if a quadratic velocity profile is assumed below the zero wind line. The resulting nonautonomous ordinary differential equation captures much of the synchronization behavior observed in the full Holton–Lindzen partial differential equation. The new class of models provides a novel framework within which to understand synchronization of the QBO, and we demonstrate a close relationship between these models and the circle map well known in the mathematics literature. Finally, we analyze reanalysis datasets to validate some of the predictions of our descent rate models and find statistically significant evidence for synchronization of the QBO that is consistent with model behavior.
•The martian solsticial pause has been studied using a set of model simulations.•The pause occurs via a tilt of the zonal jet and reduction in vertical wind shear.•The radiative effects of water ice ...clouds and dust both act to deepen the pause.•Surface topography is the fundamental reason for the development of the pause.•Zonal topographic heterogeneity weakens southern hemisphere eddy activity in general.
The martian solsticial pause, presented in a companion paper (Lewis et al., 2016), was investigated further through a series of model runs using the UK version of the LMD/UK Mars Global Climate Model. It was found that the pause could not be adequately reproduced if radiatively active water ice clouds were omitted from the model. When clouds were used, along with a realistic time-dependent dust opacity distribution, a substantial minimum in near-surface transient eddy activity formed around solstice in both hemispheres. The net effect of the clouds in the model is, by altering the thermal structure of the atmosphere, to decrease the vertical shear of the westerly jet near the surface around solstice, and thus reduce baroclinic growth rates. A similar effect was seen under conditions of large dust loading, implying that northern midlatitude eddy activity will tend to become suppressed after a period of intense flushing storm formation around the northern cap edge. Suppression of baroclinic eddy generation by the barotropic component of the flow and via diabatic eddy dissipation were also investigated as possible mechanisms leading to the formation of the solsticial pause but were found not to make major contributions. Zonal variations in topography were found to be important, as their presence results in weakened transient eddies around winter solstice in both hemispheres, through modification of the near-surface flow. The zonal topographic asymmetry appears to be the primary reason for the weakness of eddy activity in the southern hemisphere relative to the northern hemisphere, and the ultimate cause of the solsticial pause in both hemispheres. The meridional topographic gradient was found to exert a much weaker influence on near-surface transient eddies.
PDF
