P. L. Read

Improved general circulation models of the Martian atmosphere from the surface to above 80 km

We describe a set of two “new generation” general circulation models of the Martian atmosphere derived from the models we originally developed in the early 1990s. The two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations. The vertical resolution near the surface has been refined, and the vertical domain has been extended to above 80 km. These changes are accompanied by the inclusion of state-of-the-art parameterizations to better simulate the dynamical and physical processes near the surface (boundary layer scheme, subgrid-scale topography parameterization, etc.) and at high altitude (gravity wave drag). In addition, radiative transfer calculations and the representation of polar processes have been significantly improved. We present some examples of zonal-mean fields from simulations using the model at several seasons. One relatively novel aspect, previously introduced by Wilson [1997], is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi-global, angular-momentum conserving Hadley cell which has no terrestrial equivalent. Within such a cell the Coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km. This winter polar warming had been observed but not properly modeled until recently. In fact, thermal inversions are generally predicted above one, and often both, poles around 60–70 km. However, the Mars middle atmosphere above 40 km is found to be very model-sensitive and thus difficult to simulate accurately in the absence of observations.

A climate database for Mars

A database of statistics which describe the climate and surface environment of Mars has been constructed directly on the basis of output from multiannual integrations of two general circulation models developed jointly at Laboratoire de Meteorologie Dynamique du Centre National de la Recherche Scientifique, France, and the University of Oxford, United Kingdom, with support from the European Space Agency. The models have been developed and validated to reproduce the main features of the meteorology of Mars, as observed by past spacecraft missions. As well as the more standard statistical measures for mission design studies, the Mars Climate Database includes a novel representation of large-scale variability, using empirical eigenfunctions derived from an analysis of the full simulations, and small-scale variability using parameterizations of processes such as gravity wave propagation. The database may be used as a tool for mission planning and also provides a valuable resource for scientific studies of the Martian atmosphere. The database is described and critically compared with a representative range of currently available observations.

Estimation of dynamical invariants without embedding by recurrence plots.

In this paper we show that two dynamical invariants, the second order Renyi entropy and the correlation dimension, can be estimated from recurrence plots (RPs) with arbitrary embedding dimension and delay. This fact is interesting as these quantities are even invariant if no embedding is used. This is an important advantage of RPs compared to other techniques of nonlinear data analysis. These estimates for the correlation dimension and entropy are robust and, moreover, can be obtained at a low numerical cost. We exemplify our results for the Rossler system, the funnel attractor and the Mackey-Glass system. In the last part of the paper we estimate dynamical invariants for data from some fluid dynamical experiments and confirm previous evidence for low dimensional chaos in this experimental system.

Structure and dynamics of the Martian lower and middle atmosphere as observed by the Mars Climate Sounder: Seasonal variations in zonal mean temperature, dust, and water ice aerosols

[1] The first Martian year and a half of observations by the Mars Climate Sounder aboard the Mars Reconnaissance Orbiter has revealed new details of the thermal structure and distributions of dust and water ice in the atmosphere. The Martian atmosphere is shown in the observations by the Mars Climate Sounder to vary seasonally between two modes: a symmetrical equinoctial structure with middle atmosphere polar warming and a solstitial structure with an intense middle atmosphere polar warming overlying a deep winter polar vortex. The dust distribution, in particular, is more complex than appreciated before the advent of these high (∼5 km) vertical resolution observations, which extend from near the surface to above 80 km and yield 13 dayside and 13 nightside pole-to-pole cross sections each day. Among the new features noted is a persistent maximum in dust mass mixing ratio at 15-25 km above the surface (at least on the nightside) during northern spring and summer. The water ice distribution is very sensitive to the diurnal and seasonal variation of temperature and is a good tracer of the vertically propagating tide.

An intense stratospheric jet on Jupiter.

The Earths equatorial stratosphere shows oscillations in which the east–west winds reverse direction and the temperatures change cyclically with a period of about two years. This phenomenon, called the quasi-biennial oscillation, also affects the dynamics of the mid- and high-latitude stratosphere and weather in the lower atmosphere. Ground-based observations have suggested that similar temperature oscillations (with a 4–5-yr cycle) occur on Jupiter, but these data suffer from poor vertical resolution and Jupiters stratospheric wind velocities have not yet been determined. Here we report maps of temperatures and winds with high spatial resolution, obtained from spacecraft measurements of infrared spectra of Jupiters stratosphere. We find an intense, high-altitude equatorial jet with a speed of ∼140 m s-1, whose spatial structure resembles that of a quasi-quadrennial oscillation. Wave activity in the stratosphere also appears analogous to that occurring on Earth. A strong interaction between Jupiter and its plasma environment produces hot spots in its upper atmosphere and stratosphere near its poles, and the temperature maps define the penetration of the hot spots into the stratosphere.

Temperature and Composition of Saturn's Polar Hot Spots and Hexagon

Saturns poles exhibit an unexpected symmetry in hot, cyclonic polar vortices, despite huge seasonal differences in solar flux. The cores of both vortices are depleted in phosphine gas, probably resulting from subsidence of air into the troposphere. The warm cores are present throughout the upper troposphere and stratosphere at both poles. The thermal structure associated with the marked hexagonal polar jet at 77°N has been observed for the first time. Both the warm cyclonic belt at 79°N and the cold anticyclonic zone at 75°N exhibit the hexagonal structure.

Saturn’s rotation period from its atmospheric planetary-wave configuration

The rotation period of a gas giants magnetic field (called the System III reference frame) is commonly used to infer its bulk rotation. Saturns dipole magnetic field is not tilted relative to its rotation axis (unlike Jupiter, Uranus and Neptune), so the surrogate measure of its long-wavelength (kilometric) radiation is currently used to fix the System III rotation period. The period as measured now by the Cassini spacecraft is up to ∼7 min longer than the value of 10 h 39 min 24 s measured 28 years ago by Voyager. Here we report a determination of Saturns rotation period based on an analysis of potential vorticity. The resulting reference frame (which we call System IIIw) rotates with a period of 10 h 34 min 13 ± 20 s. This shifted reference frame is consistent with a pattern of alternating jets on Saturn that is more symmetrical between eastward and westward flow. This suggests that Saturns winds are much more like those of Jupiter than hitherto believed.

Superrotation in a Venus general circulation model

A superrotating atmosphere with equatorial winds of ~ 35 ms-1 is simulated using a simplified Venus general circulation model (GCM). The equatorial superrotation in the model atmosphere is maintained by barotropic instabilities in the midlatitude jets which transport angular momentum toward the equator. The midlatitude jets are maintained by the mean meridional circulation, and the momentum transporting waves are qualitatively similar to observed midlatitude waves; an equatorial Kelvin wave is also present in the atmosphere. The GCM is forced by linearized cooling and friction parameterizations, with hyperdiffusion and a polar Fourier filter to maintain numerical stability. Atmospheric superrotation is a robust feature of the model and is spontaneously produced without specific tuning. A strong meridional circulation develops in the form of a single Hadley cell, extending from the equator to the pole in both hemispheres, and from the surface to 50 km altitude. The zonal jets produced by this circulation reach 45 ms-1 at 60 km, with peak winds of 35 ms-1 at the equator. A warm pole and cold collar are also found in the GCM, caused by adiabatic warming in the mean meridional circulation. Wave frequencies and zonal wind speeds are smaller than in observations by cloud tracking but are consistent with a Doppler shifting by wind speeds in the generating region of each wave. Magnitudes of polar temperature anomalies are smaller than the observed features, suggesting dynamical processes alone may not be sufficient to maintain the large observed temperature contrasts at the magnitudes and periods found in this GCM.

Wave interactions and the transition to chaos of baroclinic waves in a thermally driven rotating annulus

A series of laboratory experiments is presented investigating regular and chaotic baroclinic waves in a high–Prandtl number fluid contained in a rotating vessel and subjected to a horizontal temperature gradient. The study focuses on nonlinear aspects of mixed–mode states at moderate values of the forcing parameters within the regular wave regime. Frequency entrainment and phase locking of resonant triads and sidebands were found to be widespread. Cases were analysed in phase space reconstructions through a singular value decomposition of multi–variate time series. Four forms of mixed–mode states were found, each in well–defined regions of parameter space: (1) a nonlinear interference vacillation associated with strong phase locking through higher harmonics; (2) a modulated amplitude vacillation showing strong phase coherence in triads involving the long wave; (3) an intermittent bursting of secondary modes; (4) an attractor switching flow, where the dominant wave number switched at irregular intervals between two possible wave numbers. Many of the mixed–mode states are suggested to arise from homoclinic bifurcations, whereas no secondary Hopf bifurcations were found. One of the postulated homoclinic bifurcations was consistent with a bifurcation through intermittency. The bifurcation sequences, however, were strongly affected by phase locking between different wave number components and frequency locking between drift and modulation frequencies. When all free frequencies were locked, the flow reduced to a limit cycle which subsequently became unstable through an incomplete period–doubling cascade. The only observed case of torus–doubling was also associated with strong phase locking. Most of the observed regimes were consistent with low–dimensional dynamics involving a limited number of domain–filling modes, which can be represented in phase space reconstructions and characterized by invariants such as attractor dimensions and the Lyapunov exponents. Some flows associated with a weak structural vacillation, however, were not consistent with low–dimensional dynamics. It appeared rather that they were the result of spatially localized instabilities consistent with high–dimensional dynamics, which can be parametrized as stochastic dynamics.

Inertia–Gravity Waves Emitted from Balanced Flow: Observations, Properties, and Consequences

This paper describes laboratory observations of inertia–gravity waves emitted from balanced fluid flow. In a rotating two-layer annulus experiment, the wavelength of the inertia–gravity waves is very close to the deformation radius. Their amplitude varies linearly with Rossby number in the range 0.05–0.14, at constant Burger number (or rotational Froude number). This linear scaling challenges the notion, suggested by several dynamical theories, that inertia–gravity waves generated by balanced motion will be exponentially small. It is estimated that the balanced flow leaks roughly 1% of its energy each rotation period into the inertia–gravity waves at the peak of their generation. The findings of this study imply an inevitable emission of inertia–gravity waves at Rossby numbers similar to those of the large-scale atmospheric and oceanic flow. Extrapolation of the results suggests that inertia– gravity waves might make a significant contribution to the energy budgets of the atmosphere and ocean. In particular, emission of inertia–gravity waves from mesoscale eddies may be an important source of energy for deep interior mixing in the ocean.