Exploring the Venus global super-rotation using a comprehensive General Circulation Model

The atmospheric circulation in Venus is well known to exhibit strong super-rotation. However, the atmospheric mechanisms responsible for the formation of this super-rotation are still not fully understood. In this work, we developed a new Venus general circulation model to study the most likely mechanisms driving the atmosphere to the current observed circulation. Our model includes a new radiative transfer, convection and suitably adapted boundary layer schemes and a dynamical core that takes into account the dependence of the heat capacity at constant pressure with temperature. The new Venus model is able to simulate a super-rotation phenomenon in the cloud region quantitatively similar to the one observed. The mechanisms maintaining the strong winds in the cloud region were found in the model results to be a combination of zonal mean circulation, thermal tides and transient waves. In this process, the semi-diurnal tide excited in the upper clouds has a key contribution in transporting axial angular momentum mainly from the upper atmosphere towards the cloud region. The magnitude of the super-rotation in the cloud region is sensitive to various radiative parameters such as the amount of solar radiative energy absorbed by the surface, which controls the static stability near the surface. In this work, we also discuss the main difficulties in representing the flow below the cloud base in Venus atmospheric models. Our new radiative scheme is more suitable for 3D Venus climate models than those used in previous work due to its easy adaptability to different atmospheric conditions. This flexibility of the model was crucial to explore the uncertainties in the lower atmospheric conditions and may also be used in the future to explore, for example, dynamical-radiative-microphysical feedbacks.Comment: Accepted for publication in Planet. Space Sc

Direct Numerical Simulation of structural vacillation in the transition to geostrophic turbulence

The onset of small-scale fluctuations around a steady convection pattern in a rotating baroclinic annulus filled with air is investigated using Direct Numerical Simulation. In previous laboratory experiments of baroclinic waves, such fluctuations have been associated with a flow regime termed Structural Vacillation which is regarded as the first step in the transition to fully-developed geostrophic turbulence.Comment: 6 page

DNS of bifurcations in an air-filled rotating baroclinic annulus

Three-dimensional Direct Numerical Simulation (DNS) on the nonlinear dynamics and a route to chaos in a rotating fluid subjected to lateral heating is presented here and discussed in the context of laboratory experiments in the baroclinic annulus. Following two previous preliminary studies by Maubert and Randriamampianina, the fluid used is air rather than a liquid as used in all other previous work. This study investigated a bifurcation sequence from the axisymmetric flow to a number of complex flows. The transition sequence, on increase of the rotation rate, from the axisymmetric solution via a steady, fully-developed baroclinic wave to chaotic flow followed a variant of the classical quasi-periodic bifurcation route, starting with a subcritical Hopf and associated saddle-node bifurcation. This was followed by a sequence of two supercritical Hopf-type bifurcations, first to an amplitude vacillation, then to a three-frequency quasi-periodic modulated amplitude vacillation (MAV), and finally to a chaotic MAV\@. In the context of the baroclinic annulus this sequence is unusual as the vacillation is usually found on decrease of the rotation rate from the steady wave flow. Further transitions of a steady wave with a higher wave number pointed to the possibility that a barotropic instability of the side wall boundary layers and the subsequent breakdown of these barotropic vortices may play a role in the transition to structural vacillation and, ultimately, geostrophic turbulence.Comment: 31 page

A diagnosis of low-order dynamics in the atmosphere of Mars

Introduction: There is considerable evidence that shows that the Martian atmosphere behaves in a more regular fashion than its terrestrial counterpart [1, 2, 3, 4]. This evidence leads to the hypothesis of theMartian climate attractor being of a relatively low dimension, which, in turn, would imply the possibility of describing the state of the atmosphere by means of a relatively few degrees of freedom. We explore this hypothesis by assuming that the atmospheric total energy (TE), i.e. the sum of kinetic energy and total potential energy (gravitational potential energy plus internal energy), is confined in a few coherent structures which dynamically interact nonlinearly with each other

Transient teleconnection event at the onset of a planet-encircling dust storm on Mars

We use proper orthogonal decomposition (POD) to study a transient teleconnection event at the onset of the 2001 planet-encircling dust storm on Mars, in terms of empirical orthogonal functions (EOFs). There are several differences between this and previous studies of atmospheric events using EOFs. First, instead of using a single variable such as surface pressure or geopotential height on a given pressure surface, we use a dataset describing the evolution in time of global and fully three-dimensional atmospheric fields such as horizontal velocity and temperature. These fields are produced by assimilating Thermal Emission Spectrometer observations from NASA's Mars Global Surveyor spacecraft into a Mars general circulation model. We use total atmospheric energy (TE) as a physically meaningful quantity which weights the state variables. Second, instead of adopting the EOFs to define teleconnection patterns as planetary-scale correlations that explain a large portion of long time-scale variability, we use EOFs to understand transient processes due to localised heating perturbations that have implications for the atmospheric circulation over distant regions. The localised perturbation is given by anomalous heating due to the enhanced presence of dust around the northern edge of the Hellas Planitia basin on Mars. We show that the localised disturbance is seemingly restricted to a small number (a few tens) of EOFs. These can be classified as low-order, transitional, or high-order EOFs according to the TE amount they explain throughout the event. Despite the global character of the EOFs, they show the capability of accounting for the localised effects of the perturbation via the presence of specific centres of action. We finally discuss possible applications for the study of terrestrial phenomena with similar characteristics

Assimilation of TES data from the Mars Global Surveyor scientifc mapping phase

The Thermal Emission Spectrometer (TES)aboard Mars Global Surveyor has produced data which cover almost two Martian years so far (during its scientific mapping phase). Thermal profiles for the atmosphere below 40 km and total dust opacities can be retrieved from TES nadir spectra and assimilated into a Mars general circulation model (MGCM), by using the assimilation techniques described in detail by Lewis et al. (2002). This paper describes some preliminary results from assimilations of temperature data from the period Ls=141°- 270° corresponding to late northern summer until winter solstice on Mars. Work in progress is devoted to assimilate both temperature and total dust opacity data for the full period for which they are already available

Data assimilation for the Martian atmosphere using MGS Thermal Emission Spectrometer observations

From the introduction: Given the quantity of data expected from current and forthcoming spacecraft missions to Mars, it is now possible to use data assimilation as a means of atmospheric analysis for the first time for a planet other than the Earth. Several groups have described plans to develop assimilation schemes for Mars [Banfield et al., 1995; Houben, 1999; Lewis and Read, 1995; Lewis et al., 1996, 1997; Zhang et al., 2001]. Data assimilation is a technique for the analysis of atmospheric observations which combines currently valid information with prior knowledge from previous observations and dynamical and physical constraints, via the use of a numerical model. Despite the number of new missions, observations of the atmosphere of Mars in the near future are still likely to be sparse when compared to those of the Earth, perhaps comprising one orbiter and a few surface stations at best at any one time. Data assimilation is useful as a means to extract the maximum information from such observations, both by a form of interpolation in space and time using model constraints and by the combination of information from different observations, e.g. temperature profiles and surface pressure measurements which may be irregularly distributed. The procedure can produce a dynamically consistent set of meteorological fields and can be used directly to test and to refine an atmospheric model against observations

The atmospheric circulation and dust activity in different orbital epochs on Mars

A general circulation model is used to evaluate changes to the circulation and dust transport in the martian atmosphere for a range of past orbital conditions. A dust transport scheme, including parameterized dust lifting, is incorporated within the model to enable passive or radiatively active dust transport. The focus is on changes which relate to surface features, as these may potentially be verified by observations. Obliquity variations have the largest impact, as they affect the latitudinal distribution of solar heating. At low obliquities permanent CO2 ice caps form at both poles, lowering mean surface pressures. At higher obliquities, solar insolation peaks at higher summer latitudes near solstice, producing a stronger, broader meridional circulation and a larger seasonal CO2 ice cap in winter. Near-surface winds associated with the main meridional circulation intensify and extend polewards, with changes in cap edge position also affecting the flow. Hence the model predicts significant changes in surface wind directions as well as magnitudes. Dust lifting by wind stress increases with obliquity as the meridional circulation and associated near-surface winds strengthen. If active dust transport is used, then lifting rates increase further in response to the larger atmospheric dust opacities (hence circulation) produced. Dust lifting by dust devils increases more gradually with obliquity, having a weaker link to the meridional circulation. The primary effect of varying eccentricity is to change the impact of varying the areocentric longitude of perihelion, l, which determines when the solar forcing is strongest. The atmospheric circulation is stronger when l aligns with solstice rather than equinox, and there is also a bias from the martian topography, resulting in the strongest circulations when perihelion is at northern winter solstice. Net dust accumulation depends on both lifting and deposition. Dust which has been well mixed within the atmosphere is deposited preferentially over high topography. For wind stress lifting, the combination produces peak net removal within western boundary currents and southern midlatitude bands, and net accumulation concentrated in Arabia and Tharsis. In active dust transport experiments, dust is also scoured from northern midlatitudes during winter, further confining peak accumulation to equatorial regions. As obliquity increases, polar accumulation rates increase for wind stress lifting and are largest for high eccentricities when perihelion occurs during northern winter. For dust devil lifting, polar accumulation rates increase (though less rapidly) with obliquity above o=25°, but increase with decreasing obliquity below this, thus polar dust accumulation at low obliquities may be increasingly due to dust lifted by dust devils. For all cases discussed, the pole receiving most dust shifts from north to south as obliquity is increased