D. Luz, F. Hourdin, P. Rannou, and S. Lebonnois. Latitudinal transport by barotropic waves in Titan's stratosphere.. II. Results from a coupled dynamics-microphysics-photochemistry GCM. Icarus, 166:343-358, 2003. [ bib | DOI | ADS link ]
We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C 2H 2, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions.
E. Millour, G. Labrosse, and E. Tric. Sensitivity of binary liquid thermal convection to confinement. Physics of Fluids, 15:2791-2802, 2003. [ bib | DOI | ADS link ]
The stable axisymmetric convective states of a binary liquid enclosed in a vertical cylinder heated from below are exhaustively and accurately identified by pseudo-spectral numerical integration. In order to gain some insight on the influence that nearby boundaries can exert on flow dynamics, three aspect ratios (1/2, 1, and 2), as well as two types of lateral kinematic boundary conditions (either no-slip or free-slip) are investigated. The ranges over which stable quiescent, oscillatory and steady convective states extend and coexist are given. The bifurcations leading to transitions from one branch of solutions to another, as well as those that occur along the oscillatory branch, are analyzed. The most significant effect of varying boundary conditions and aspect ratio involves the route from oscillatory to steady convection. For a given configuration, that route consists of a period doubling cascade followed by chaos, or a subcritical generalized Hopf (or Neimark-Sacker) bifurcation, or a homoclinic bifurcation. The dynamics of thermal convection of enclosed binary mixtures is clearly very sensitive to both boundary conditions and aspect ratio.
G. Tobie, F. Forget, and F. Lott. Numerical simulation of the winter polar wave clouds observed by Mars Global Surveyor Mars Orbiter Laser Altimeter. Icarus, 164:33-49, 2003. [ bib | DOI | ADS link ]
In 1998, the Mars Orbiter Laser Altimeter revealed the presence of isolated or quasi-periodic thick clouds during the martian polar night. They are believed to be composed of CO 2 ice particles and to be tilted against the wind direction, a feature characteristic of vertically propagating orographic gravity waves. To support that interpretation, we present here numerical simulations with a two-dimensional anelastic model of stratified shear flow that includes simple CO 2 ice microphysics. In some of the simulations presented, the orography is an idealized trough, with dimensions characteristic of the many troughs that shape the Mars polar cap. In others, it is near the real orography. In the polar night conditions, our model shows that gravity waves over the north polar cap are strong enough to induce adiabatic cooling below the CO 2 frost point. From this cooling, airborne heterogeneous nucleation of CO 2 ice particles occurs from the ground up to the altitude of the polar thermal inversion. Although the model predicts that clouds can be present above 15 km, only low altitude clouds can backscatter the Laser beams of MOLA at a detectable level. Accordingly, the shape of the Laser echoes is related to the shape of the clouds at low level, but do not necessarily coincide with the top of the clouds. The model helps to interpret the cloud patterns observed by MOLA. Above an isolated orographic trough, an isolated extended sloping cloud tilted against the wind is obtained. The model shows that the observed quasi-periodic clouds are due to the succession of small-scale topographic features, rather than to the presence of resonant trapped lee waves. Indeed, the CO 2 condensation greatly damps the buoyancy force, essential for the maintenance of gravity waves far from their sources. Simulations with realistic topography profiles show the cloud response is sensitive to the wind direction. When the wind is directed upslope of the polar cap, on the one hand, a large scale cloud, modulated by small-scale waves, forms just above the ground. On the other hand, when the wind is directed downslope, air is globally warmed, and periodic ice clouds induced by small-scale orography form at altitudes higher than 3-5 km above the ground. In both cases, a good agreement between the simulated echoes and the observed one is obtained. According to our model, we conclude that the observed clouds are quasi-stationary clouds made of moving ice particles that successively grow and sublimate by crossing cold and warm phases of orographic gravity waves generated by the successive polar troughs. We also find that the rate of ice precipitation is relatively weak, except when there is a large scale air dynamical cooling.
S. Lebonnois, F. Hourdin, P. Rannou, D. Luz, and D. Toublanc. Impact of the seasonal variations of composition on the temperature field of Titan's stratosphere. Icarus, 163:164-174, 2003. [ bib | DOI | ADS link ]
We investigate the role of seasonal variations of Titan's stratospheric composition on the temperature. We use a general circulation model coupled with idealized chemical tracers that reproduce variations of ethane (C 2H 6), acetylene (C 2H 2), and hydrogen cyanide (HCN). Enhancement of the mole fractions of these compounds, at high latitudes in the winter hemisphere relative to their equatorial values, induces a relative decrease in temperature above approximately 0.2 mbar, with a peak amplitude around -20 K, and a relative increase in temperature below, around 1 mbar, with a peak amplitude around +7 K. These thermal effects are mainly due to the variations of the cooling to space induced by the varying distributions. The ethane, acetylene, and hydrogen cyanide variations affect the cooling rates in a similar way, with the dominant effect being due to ethane, though its latitudinal variations are small.
N. Mangold, F. Costard, and F. Forget. Debris flows over sand dunes on Mars: Evidence for liquid water. Journal of Geophysical Research (Planets), 108:5027, 2003. [ bib | DOI | ADS link ]
This study focuses on the formation and physical properties of the gullies observed over large Martian dunes, especially those of the Russell crater (54degS, 347degW). Geomorphic features like sinuosities and connections of the channels show that gullies over dunes involve flows with a significant proportion of liquid. The occurrence of levees implies that these flows are debris flows with a yield strength characteristics of Bingham plastic materials. We apply terrestrial methods to estimate viscosity and velocity of these flows from levee size and sinuosities. We obtain average velocities in the range of 1 to 7 m s-1 and apparent viscosities of 2.8 to 46,000 Pa s, with an average at 740 Pa s, compared with the 0.001 Pa s of pure water. These viscosities and velocities are in the range of terrestrial debris flows with a proportion of 10 to 40% of H2O. These properties are typical of water-holding debris flows but not of pure water surface runoff or CO2 driven flows. The debris flows over dunes are oriented on south-facing slopes like other recent gullies. Meltwater from ground ice formed during a recent period of high obliquity is the more likely explanation for the formation of such flows over dunes.
S. é. Lebonnois, E. L. O. Bakes, and C. P. McKay. Atomic and molecular hydrogen budget in Titan's atmosphere. Icarus, 161:474-485, 2003. [ bib | DOI | ADS link ]
Using a one-dimensional model, we investigate the hydrogen budget and escape to space in Titan's atmosphere. Our goal is to study in detail the distributions and fluxes of atomic and molecular hydrogen in the model, while identifying sources of qualitative and quantitative uncertainties. Our study confirms that the escape of atomic and molecular hydrogen to space is limited by the diffusion through the homopause level. The H distribution and flux inside the atmosphere are very sensitive to the eddy diffusion coefficient used above altitude 600 km. We chose a high value of this coefficient 1 × 10 8 cm 2 s -1 and a homopause level around altitude 900 km. We find that H flows down significantly from the production region above 500 km to the region [300-500] km, where it recombines into H 2. Production of both H and H 2 also occurs in the stratosphere, mostly from photodissociation of acetylene. The only available observational data to be compared are the escape rate of H deduced from Pioneer 11 and IUE observations of the H torus 1-3 × 10 9 cm -2 s -1 and the latest retrieved value of the H 2 mole fraction in the stratosphere: (1.1 0.1) × 10 -3. Our results for both of these values are at least 50-100% higher, though the uncertainties within the chemical schemes and other aspects of the model are large. The chemical conversion from H to H 2 is essentially done through catalytic cycles using acetylene and diacetylene. We have studied the role of this diacetylene cycle, for which the associated reaction rates are poorly known. We find that it mostly affects C 4 species and benzene in the lower atmosphere, rather than the H profile and the hydrogen budget. We have introduced the heterogenous recombination of hydrogen on the surface of aerosol particles in the stratosphere, and this appears to be a significant process, comparable to the chemical processes. It has a major influence on the H distribution, and consequently on several other species, especially C 3H 4, C 4H 2 and C 6H 6. Therefore, this heterogenous process should be taken into account when trying to understand the stratospheric distribution of these hydrocarbons.
E. L. O. Bakes, S. é. Lebonnois, C. W. Bauschlicher, and C. P. McKay. The role of submicrometer aerosols and macromolecules in H 2 formation in the titan haze. Icarus, 161:468-473, 2003. [ bib | DOI | ADS link ]
Previous studies of the photochemistry of small molecules in Titan's atmosphere found it difficult to have hydrogen atoms removed at a rate sufficient to explain the observed abundance of unsaturated hydrocarbons. One qualitative explanation of the discrepancy nominated catalytic aerosol surface chemistry as an efficient sink of hydrogen atoms, although no quantitative study of this mechanism was attempted. In this paper, we quantify how haze aerosols and macromolecules may efficiently catalyze the formation of hydrogen atoms into H 2. We describe the prompt reaction model for the formation of H 2 on aerosol surfaces and compare this with the catalytic formation of H 2 using negatively charged hydrogenated aromatic macromolecules. We conclude that the PRM is an efficient mechanism for the removal of hydrogen atoms from the atmosphere to form H 2 with a peak formation rate of 70 cm -3 s -1 at 420 km. We also conclude that catalytic H 2 formation via hydrogenated anionic macromolecules is viable but much less productive (a maximum of 0.1 cm -3 s -1 at 210 km) than microphysical aerosols.