Publications by Peter Read

Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities

Journal of Geophysical Research: Planets American Geophysical Union (AGU) (2019)

A Antuñano, T del Río-Gaztelurrutia, A Sánchez-Lavega, PL Read, LN Fletcher

Simulating Jupiter’s weather layer. Part I: Jet spin-up in a dry atmosphere

Icarus Elsevier 326 (2018) 225-252

R Young, P Read, Y Wang

We investigate the dynamics of Jupiter's upper troposphere and lower stratosphere using a General Circulation Model that includes two-stream radiation and optional heating from below. Based on the MITgcm dynamical core, this is a new generation of the Oxford Jupiter model [Zuchowski, L.C. et al., 2009. Plan. Space Sci., 57, 1525--1537, doi:10.1016/j.pss.2009.05.008]. We simulate Jupiter's atmosphere at up to 0.7 degree horizontal resolution with 33 vertical levels down to a pressure of 18 bar, in configurations with and without a 5.7 W/m2 interior heat flux. Simulations ran for 130000-150000 days to allow the deep atmosphere to come into radiative equilibrium. Baroclinic instability generates alternating, eddy-driven, midlatitude jets in both cases. With interior heating the zonal jets migrate towards the equator and become barotropically unstable. This generates Rossby waves that radiate away from the equator, depositing westerly momentum there via eddy angular momentum flux convergence and spinning up a super-rotating 20 m/s equatorial jet throughout the troposphere. There are 30-35 zonal jets with latitudinal separation comparable with the real planet, and there is strong eddy activity throughout. Without interior heating the jets do not migrate and a divergent eddy angular momentum flux at the equator spins up a broad, 50 m/s sub-rotating equatorial jet with weak eddy activity at low latitudes.

Simulating Jupiter's weather layer. Part II: Passive ammonia and water cycles

Icarus Elsevier 326 (2018) 253-268

R Young, P Read, Y Wang

We examine the ammonia and water cycles in Jupiter's upper troposphere and lower stratosphere during spin-up of a multiple zonal jet circulation using the Oxford Jupiter GCM. Jupiter's atmosphere is simulated at 512 x 256 horizontal resolution with 33 vertical levels between 0.01 and 18 bar, putting the lowest level well below the expected water cloud base. Simulations with and without a 5.7 W/m2 interior heat source were run for 130000-150000d to allow the deep atmosphere to come into radiative-convective-dynamical equilibrium, with variants on the interior heating case including varying the initial tracer distribution, particle condensate diameter, and cloud process timescales. The cloud scheme includes simple representations of the ammonia and water cycles. Ammonia vapour changes phase to ice, and reacts with hydrogen sulphide to produce ammonium hydrosulphide. Water changes phases between vapour, liquid, and ice depending on local environmental conditions, and all condensates sediment at their respective Stokes velocities. With interior heating, clouds of ammonia ice, ammonium hydrosulphide ice, and water ice form with cloud bases around 0.4 bar, 1.5 bar, and 3 bar respectively. Without interior heating the ammonia cloud base forms in the same way, but the ammonium hydrosulphide and water clouds sediment to the bottom of the domain. The liquid water cloud is either absent or extremely sparse. Zonal structures form that correlate regions of strong latitudinal shear with regions of constant condensate concentration, implying that jets act as barriers to the mixing. Regions with locally high and low cloud concentrations also correlated with regions of upwelling and downwelling, respectively. Shortly after initialisation, the ammonia vapour distribution up to the cloud base resembles the enhanced concentration seen in Juno observations, due to strong meridional mean circulation at the equator. The resemblance decays rapidly over time, but suggests that at least some of the relevant physics is captured by the model. The comparison should improve with additional microphysics and better representation of the deep ammonia reservoir.

Investigating the semiannual oscillation on Mars using data assimilation

Icarus Elsevier 333 (2019) 404-414 )

T Ruan, N Lewis, S Lewis, L Montabone, P Read

A Martian semiannual oscillation (SAO), similar to that in the Earths tropical stratosphere, is evident in the Mars Analysis Correction Data Assimilation reanalysis dataset (MACDA) version 1.0, not only in the tropics, but also extending to higher latitudes. Unlike on Earth, the Martian SAO is found not always to reverse its zonal wind direction, but only manifests itself as a deceleration of the dominant wind at certain pressure levels and latitudes. Singular System Analysis (SSA) is further applied on the zonal-mean zonal wind in different latitude bands to reveal the characteristics of SAO phenomena at different latitudes. The second pair of principal components (PCs) is usually dominated by a SAO signal, though the SAO signal can be strong enough to manifest itself also in the first pair of PCs. An analysis of terms in the Transformed Eulerian Mean equation (TEM) is applied in the tropics to further elucidate the forcing processes driving the tendency of the zonal-mean zonal wind. The zonal-mean meridional advection is found to correlate strongly with the observed oscillations of zonal-mean zonal wind, and supplies the majority of the westward (retrograde) forcing in the SAO cycle. The forcing due to various non-zonal waves supplies forcing to the zonal-mean zonal wind that is nearly the opposite of the forcing due to meridional advection above ∼3 Pa altitude, but it also partly supports the SAO between 40 Pa and 3 Pa. Some distinctive features occurring during the period of the Mars year (MY) 25 global-scale dust storm (GDS) are also notable in our diagnostic results with substantially stronger values of eastward and westward momentum in the second half of MY 25 and stronger forcing due to vertical advection, transient waves and thermal tides

Wave number selection in the presence of noise: Experimental results

Chaos AIP Publishing 28 (2018) 053110

O Krivonosova, D Zhilenko, P Read, M Gritsevich

In this study, we consider how the wave number selection in spherical Couette flow, in the transition to azimuthal waves after the first instability, occurs in the presence of noise. The outer sphere was held stationary, while the inner sphere rotational speed was increased linearly from a subcritical flow to a supercritical one. In a supercritical flow, one of two possible flow states, each with different azimuthal wave numbers, can appear depending upon the initial and final Reynolds numbers and the acceleration value. Noise perturbations were added by introducing small disturbances into the rotational speed signal. With an increasing noise amplitude, a change in the dominant wave number from m to m ± 1 was found to occur at the same initial and final Reynolds numbers and acceleration values. The flow velocity measurements were conducted by using laser Doppler anemometry. Using these results, the role of noise as well as the behaviour of the amplitudes of the competing modes in their stages of damping and growth were determined.

An experimental investigation of blocking by partial barriers in a rotating baroclinic annulus

Geophysical and Astrophysical Fluid Dynamics (2017) 1-33

SD Marshall, PL Read

© 2017 Informa UK Limited, trading as Taylor & Francis Group We present a series of experimental investigations in which a differentially-heated annulus was used to investigate the effects of topography on rotating, stratified flows with similarities to the Earth’s atmospheric or oceanic circulation. In particular, we compare and investigate blocking effects via partial mechanical barriers to previous experiments by the authors utilising azimuthally-periodic topography. The mechanical obstacle used was an isolated ridge, forming a partial barrier, employed to study the difference between partially blocked and fully unblocked flow. The topography was found to lead to the formation of bottom-trapped waves, as well as impacting the circulation at a level much higher than the top of the ridge. This produced a unique flow structure when the drifting flow and the topography interacted in the form of an “interference” regime at low Taylor number, but forming an erratic “irregular” regime at higher Taylor number. The results also showed evidence of resonant wave-triads, similar to those noted with periodic wavenumber-3 topography by Marshall and Read (Geophys. Astrophys. Fluid Dyn., 2015, 109), though the component wavenumbers of the wave-triads and their impact on the flow were found to depend on the topography in question. With periodic topography, wave-triads were found to occur between both the baroclinic and barotropic components of the zonal wavenumber-3 mode and the wavenumber-6 baroclinic component, whereas with the partial barrier two nonlinear resonant wave-triads were noted, each sharing a common wavenumber-1 mode.

A Chorus of the WindsOn Saturn!


PL Read

Atmospheric Dynamics of Terrestrial Planets

in Handbook of Exoplanets, Springer (2018)

P Read, GK Vallis

<p style="text-align:justify;">The solar system presents us with a number of planetary bodies with shallow atmospheres that are sufficiently Earth-like in their form and structure to be termed “terrestrial.” These atmospheres have much in common, in having circulations that are driven primarily by heating from the Sun and radiative cooling to space, which vary markedly with latitude. The principal response to this forcing is typically in the form of a (roughly zonally symmetric) meridional overturning that transports heat vertically upward and in latitude. But even within the solar system, these planets exhibit many differences in the types of large-scale waves and instabilities that also contribute substantially to determining their respective climates. Here we argue that the study of simplified models (either numerical simulations or laboratory experiments) provides considerable insights into the likely roles of planetary size, rotation, thermal stratification, and other factors in determining the styles of global circulation and dominant waves and instability processes. We discuss the importance of a number of key dimensionless parameters, for example, the thermal Rossby and the Burger numbers as well as nondimensional measures of the frictional or radiative timescales, in defining the type of circulation regime to be expected in a prototypical planetary atmosphere subject to axisymmetric driving. These considerations help to place each of the solar system terrestrial planets into an appropriate dynamical context and also lay the foundations for predicting and understanding the climate and circulation regimes of (as yet undiscovered) Earth-like extrasolar planets. However, as recent discoveries of “super-Earth” planets around some nearby stars are beginning to reveal, this parameter space is likely to be incomplete, and other factors, such as the possibility of tidally locked rotation and tidal forcing, may also need to be taken into account for some classes of extrasolar planet.</p>

Descent rate models of the synchronization of the Quasi-Biennial Oscillation by the annual cycle in tropical upwelling

Journal of the Atmospheric Sciences American Meteorological Society 75 (2018) 2281–2297-

K Rajendran, I Moroz, S Osprey, PL Read

The response of the Quasi-Biennial Oscillation (QBO) to an imposed mean upwelling with a periodic modulation is studied, by modelling 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. Using a simple transformation, it is then demonstrated 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 non-autonomous ordinary differential equation captures much of the synchronization behaviour 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 analyse 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 behaviour.

Superrotation on Venus, on Titan, and elsewhere

Annual Review of Earth and Planetary Sciences Annual Reviews 46 (2018) 175-202

P Read, S Lebonnois

The superrotation of the atmospheres of Venus and Titan has puzzled dynamicists for many years and seems to put these planets in a very different dynamical regime from most other planets. In this review, we consider how to define superrotation objectively and explore the constraints that determine its occurrence. Atmospheric superrotation also occurs elsewhere in the Solar System and beyond, and we compare Venus and Titan with Earth and other planets for which wind estimates are available. The extreme superrotation on Venus and Titan poses some difficult challenges for numerical models of atmospheric circulation, much more difficult than for more rapidly rotating planets such as Earth or Mars. We consider mechanisms for generating and maintaining a superrotating state, all of which involve a global meridional overturning circulation. The role of nonaxisymmetric eddies is crucial, however, but the detailed mechanisms may differ between Venus, Titan, and other planets.

Comparative terrestrial atmospheric circulation regimes in simplified global circulation models. Part I: From cyclostrophic super‐rotation to geostrophic turbulence

Quarterly Journal of the Royal Meteorological Society Wiley 144 (2018) 2537-2557

Y Wang, P Read, F Tabataba-Vakili, RMB Young

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.

Comparative terrestrial atmospheric circulation regimes in simplified global circulation models: II. energy budgets and spectral transfers

Quarterly Journal of the Royal Meteorological Society Wiley 144 (2018) 2558-2576

F Tabataba-Vakili, Y Wang, P Augier, E Lindborg, A Valeanu, PL Read, RMB Young

The energetics of possible global atmospheric circulation patterns in an Earth-like atmosphere are explored using a simplified GCM based on the University of Hamburg’s Portable University Model for the Atmosphere (designated here as PUMA-S), forced by linear relaxation towards a prescribed temperature field and subject to Rayleigh surface drag and hyperdiffusive dissipation. Results from a series of simulations, obtained by varying planetary rotation rate Ω with an imposed equator-to-pole temperature difference, were analysed to determine the structure and magnitude of the heat transport and other contributions to the energy budget for the time-averaged, equilibrated flow. These show clear trends with rotation rate, with the most intense Lorenz energy cycle for an Earth-sized planet occurring with a rotation rate around half that of the present day Earth (i.e. Ω* = Ω/ΩE = 1/2, where ΩE is the rotation rate of the Earth). KE and APE spectra, EK(n) and EA(n) (where n is total spherical wavenumber), also show clear trends with rotation rate, with n^-3 enstrophy-dominated spectra around Ω* = 1 and steeper (~ n^-5) slopes in the zonal mean flow with little evidence for the n^-5/3 spectrum anticipated for an inverse KE cascade. Instead, both KE and APE spectra become almost flat at scales larger than the internal Rossby radius, Ld, and exhibit near-equipartition at high wavenumbers. At Ω* &lt;&lt; 1, the spectrum becomes dominated by KE with EK(n) ~ (2 - 3)EA(n) at most wavenumbers and a slope that tends towards n^-5/3 across most of the spectrum. Spectral flux calculations show that enstrophy and APE are almost always cascaded downscale, regardless of rotation rate. KE cascades are more complicated, however, with downscale transfers across almost all wavenumbers, dominated by horizontally divergent modes, for Ω* ≲ 1/4. At higher rotation rates, transfers of KE become increasingly dominated by rotational (horizontally non-divergent) components with strong upscale transfers (dominated by eddy-zonal flow interactions) for scales larger than Ld and weaker downscale transfers for scales smaller than Ld.

A hexagon in Saturn’s northern stratosphere surrounding the emerging summertime polar vortex

Nature Communications Springer Nature 9 (2018) 3564

LN Fletcher, GS Orton, JA Sinclair, S Guerlet, PL Read, A Antunano, RK Achterberg, P Irwin, FM Flasar, GL Bjoraker, BE Hesman, J Hurley, M Segura, A Mamoutkine, N Gorius, SB Calcutt

Saturn’s polar stratosphere exhibits the seasonal growth and dissipation of broad, warm vortices poleward of ~75° latitude, which are strongest in the summer and absent in winter. The longevity of the exploration of the Saturn system by Cassini allows the use of infrared spectroscopy to trace the formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced temperatures and elevated hydrocarbon abundances at millibar pressures. We constrain the timescales of stratospheric vortex formation and dissipation in both hemispheres. Although the NPSV formed during late northern spring, by the end of Cassini’s reconnaissance (shortly after northern summer solstice), it still did not display the contrasts in temperature and composition that were evident at the south pole during southern summer. The newly formed NPSV was bounded by a strengthening stratospheric thermal gradient near 78°N. The emergent boundary was hexagonal, suggesting that the Rossby wave responsible for Saturn’s long-lived polar hexagon—which was previously expected to be trapped in the troposphere—can influence the stratospheric temperatures some 300 km above Saturn’s clouds.

A rotating annulus driven by localized convective forcing: a new atmosphere-like experiment

Experiments in Fluids Springer Berlin Heidelberg 2017 (2017) 75

H Scolan, PL Read

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 subject 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 whilst also allowing stably-stratified baroclinic motion in the central zone of the annulus, as in midlatitude 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 spatiotemporal 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.

Phase synchronization of baroclinic waves in a differentially heated rotating annulus experiment subject to periodic forcing with a variable duty cycle

Chaos AIP Publishing 27 (2017) 127001

A Castrejon Pita, X Morice-Atkinson, EJ Allen, PL Read

A series of laboratory experiments in a thermally driven, rotating fluid annulus are presented that investigate the onset and characteristics of phase synchronization and frequency entrainment between the intrinsic, chaotic, oscillatory amplitude modulation of travelling baroclinic waves and a periodic modulation of the (axisymmetric) thermal boundary conditions, subject to time-dependent coupling. The time-dependence is in the form of a prescribed duty cycle in which the periodic forcing of the boundary conditions is applied for only a fraction ߜ of each oscillation. For the rest of the oscillation, the boundary conditions are held fixed. Two profiles of forcing were investigated that capture different parts of the sinusoidal variation and ߜ was varied over the range 0.1 ൑ ߜ ൑ 1. Reducing ߜ was found to act in a similar way to a reduction in a constant coupling coefficient in reducing the width of the interval in forcing frequency or period over which complete synchronization was observed (the “Arnol’d tongue”) with respect to the detuning, though for the strongest pulselike forcing profile some degree of synchronization was discernible even at ߜ ൌ 0.1. Complete phase synchronization was obtained within the Arnol’d tongue itself, though the strength of the amplitude modulation of the baroclinic wave was not significantly affected. These experiments demonstrate a possible mechanism for intraseasonal and/or interannual “teleconnections” within the climate system of the Earth and other planets that does not rely upon Rossby wave propagation across the planet along great circles.

Regimes of axisymmetric flow and scaling laws in a rotating annulus with local convective forcing

Fluids MDPI 2 (2017) 41

S Wright, S Su, R Young, H Scolan, PL Read

We present a numerical study of axisymmetric flow in a rotating annulus in which local thermal forcing, via a heated annular ring on the outside of the base and a cooled circular disk in the centre of the top surface, drives convection. This new configuration is a variant of the classical thermally-driven annulus, where uniform heating and cooling are applied through the outer and inner sidewalls respectively. The annulus provides an analogue to a planetary circulation and the new configuration, with its more relaxed vertical thermal boundary conditions, is expected to better emulate vigorous convection in the tropics and polar regions as well as baroclinic instability in the mid-latitude baroclinic zone. Using the Met Office/Oxford Rotating Annulus Laboratory (MORALS) code, we have investigated a series of equilibrated, two dimensional axisymmetric flows across a large region of parameter space. These are characterized in terms of their velocity and temperature fields. When rotation is applied several distinct flow regimes may be identified for different rotation rates and strengths of differential heating. These regimes are defined as a function of the ratio of the horizontal Ekman layer thickness to the non-rotating thermal boundary layer thickness and are found to be similar to those identified in previous annulus experiments. Convection without rotation is also considered and the scaling of the heat transport with Rayleigh number is calculated. This is then compared with existing work on the classical annulus as well as horizontal and Rayleigh-Bénard convection. As with previous studies on both rotating and non-rotating convection the system’s behaviour is found to be aspect ratio dependent. This dependence is seen in the scaling of the non-rotating Nusselt number and in transitions between regimes in the rotating case although further investigation is required to fully explain these observations.

Forward and inverse kinetic energy cascades in Jupiter’s turbulent weather layer

Nature Physics Nature Publishing Group 13 (2017) 1135–1140-

RMB Young, PL Read

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 with the first baroclinic Rossby deformation radius to the global jet scale, but also a forward cascade of kinetic energy from the deformation radius to smaller scales. The latter 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

Space Science Reviews 212 (2017) 1541-1616

A Sánchez-Lavega, S Lebonnois, T Imamura, P Read, D Luz

© 2017, Springer Science+Business Media B.V. 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 ∼100ms−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 ∼1–3ms−1 in a layer of thickness ∼10km close to the surface. Meridional motions with peak speeds of ∼15ms−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 ∼1–3ms−1 occur in the statically unstable layer between altitudes of ∼50–55km. 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 martian planetary boundary layer

in The Atmosphere and Climate of Mars, (2017) 172-202

PL Read, B Galperin, SE Larsen, SR Lewis, A Määttänen, A Petrosyan, N Rennó, H Savijärvi, T Siili, A Spiga, A Toigo, L Vázquez

The global circulation

in The Atmosphere and Climate of Mars, (2017) 229-294

JR Barnes, RM Haberle, RJ Wilson, SR Lewis, JR Murphy, PL Read