Jonathan L. Mitchell

Improved Cosmological Constraints from Gravitational Lens Statistics

We combine the Cosmic Lens All-Sky Survey (CLASS) with new Sloan Digital Sky Survey (SDSS) data on the local velocity dispersion distribution function of E/S0 galaxies, � (� ), to derive lens statistics constraints on � � and � m. Previous studies of this kind relied on a combination of the E/S0 galaxy luminosity function and the FaberJackson relation to characterize the lens galaxy population. However, ignoring dispersion in the Faber-Jackson relation leads to a biased estimate of � (� ) and therefore biased and overconfident constraints on the cosmological parameters. The measured velocity dispersion function from a large sample of E/S0 galaxies provides a more reliable method for probing cosmology with strong lens statistics. Our new constraints are in good agreement with recent results from the redshift-magnitude relation of Type Ia supernovae. Adopting the traditional assumption that the E/S0 velocity function is constant in comoving units, we find a maximum likelihood estimate of � � ¼ 0:74 0:78 for a spatially flat universe (where the range reflects uncertainty in the number of E/S0 lenses in the CLASS sample) and a 95% confidence upper bound of � � < 0:86. If � (� ) instead evolves in accord with the extended PressSchechter theory, then the maximum likelihood estimate for � � becomes 0.72‐0.78, with the 95% confidence upper bound � � < 0:89. Even without assuming flatness, lensing provides independent confirmation of the evidence from Type Ia supernovae for a nonzero dark energy component in the universe. Subject headingg cosmological parameters — cosmology: observations — cosmology: theory — gravitational lensing

The dynamics behind Titan's methane clouds

We present results of an axisymmetric global circulation model of Titan with a simplified suite of atmospheric physics forced by seasonally varying insolation. The recent discovery of midlatitude tropospheric clouds on Titan has caused much excitement about the roles of surface sources of methane and the global circulation in forming clouds. Although localized surface sources, such as methane geysers or “cryovolcanoes,” have been invoked to explain these clouds, we find in this work that clouds appear in regions of convergence by the mean meridional circulation and over the poles during solstices, where the solar forcing reaches its seasonal maximum. Other regions are inhibited from forming clouds because of dynamical transports of methane and strong subsidence. We find that for a variety of moist regimes, i.e., with the effect of methane thermodynamics included, the observed cloud features can be explained by the large-scale dynamics of the atmosphere. Clouds at the solsticial pole are found to be a robust feature of Titans dynamics, whereas isolated midlatitude clouds are present exclusively in a variety of moist dynamical regimes. In all cases, even without including methane thermodynamics, our model ceases to produce polar clouds ≈4–6 terrestrial years after solstices.

The Mechanical Greenhouse: Burial of Heat by Turbulence in Hot Jupiter Atmospheres

The intense irradiation received by hot Jupiters suppresses convection in the outer layers of their atmospheres and lowers their cooling rates. “Inflated” hot Jupiters, i.e., those with anomalously large transit radii, require additional sources of heat or suppressed cooling. We consider the effect of forced turbulent mixing in the radiative layer, which could be driven by atmospheric circulation or by another mechanism. Due to stable stratification in the atmosphere, forced turbulence drives a downward flux of heat. Weak turbulent mixing slows the cooling rate by this process, as if the planet was irradiated more intensely. Stronger turbulent mixing buries heat into the convective interior, provided the turbulence extends to the radiative-convective boundary. This inflates the planet until a balance is reached between the heat buried into and radiated from the interior. We also include the direct injection of heat due to the dissipation of turbulence or other effects. Such heating is already known to slow planetary cooling. We find that dissipation also enhances heat burial from mixing by lowering the threshold for turbulent mixing to drive heat into the interior. Strong turbulent mixing of heavy molecular species such as TiO may be necessary to explain stratospheric thermal inversions. We show that the amount of mixing required to loft TiO may overinflate the planet by our mechanism. This possible refutation of the TiO hypothesis deserves further study. Our inflation mechanism requires a deep stratified layer that only exists when the absorbed stellar flux greatly exceeds the intrinsic emitted flux. Thus it would be less effective for more luminous brown dwarfs and for longer period gas giants, including Jupiter and Saturn. Subject headings: diffusion – opacity – planet-star interactions – planets and satellites: atmospheres – radiative transfer – turbulence

Elastic ice shells of synchronous moons: Implications for cracks on Europa and non-synchronous rotation of Titan

A number of synchronous moons are thought to harbor water oceans beneath their outer ice shells. A subsurface ocean frictionally decouples the shell from the interior. This has led to proposals that a weak tidal or atmospheric torque might cause the shell to rotate differentially with respect to the synchronously rotating interior. Applications along these lines have been made to Europa and Titan. However, the shell is coupled to the ocean by an elastic torque. As a result of centrifugal and tidal forces, the ocean would assume an ellipsoidal shape with its long axis aligned toward the parent planet. Any displacement of the shell away from its equilibrium position would induce strains thereby increasing its elastic energy and giving rise to an elastic restoring torque. In the investigation reported on here, the elastic torque is compared with the tidal torque acting on Europa and the atmospheric torque acting on Titan. Regarding Europa, it is shown that the tidal torque is far too weak to produce stresses that could fracture the ice shell, thus refuting an idea that has been widely advocated. Instead, it is suggested that the cracks arise from time-dependent stresses due to non-hydrostatic gravity anomalies from tidally driven, episodic convection in the satellite’s interior. Two years of Cassini RADAR observations of Titan’s surface have been interpreted as implying an angular displacement of ~0.24° relative to synchronous rotation. Compatibility of the amplitude and phase of the observed non-synchronous rotation with estimates of the atmospheric torque requires that Titan’s shell be decoupled from its interior. We find that the elastic torque balances the seasonal atmospheric torque at an angular displacement ≾0.05°, effectively coupling the shell to the interior. Moreover, if Titan’s surface were spinning faster than synchronous, the tidal torque tending to restore synchronous rotation would almost certainly be larger than the atmospheric torque. There must either be a problem with the interpretation of the radar observations, or with our basic understanding of Titan’s atmosphere and/or interior.

High regional climate sensitivity over continental China constrained by glacial-recent changes in temperature and the hydrological cycle

The East Asian monsoon is one of Earth’s most significant climatic phenomena, and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales. Quantitative constraints on the climate changes associated with these past variations are limited, yet are needed to constrain sensitivity of the region to changes in greenhouse gas levels. Here, we show central China is a region that experienced a much larger temperature change since the Last Glacial Maximum than typically simulated by climate models. We applied clumped isotope thermometry to carbonates from the central Chinese Loess Plateau to reconstruct temperature and water isotope shifts from the Last Glacial Maximum to present. We find a summertime temperature change of 6–7 °C that is reproduced by climate model simulations presented here. Proxy data reveal evidence for a shift to lighter isotopic composition of meteoric waters in glacial times, which is also captured by our model. Analysis of model outputs suggests that glacial cooling over continental China is significantly amplified by the influence of stationary waves, which, in turn, are enhanced by continental ice sheets. These results not only support high regional climate sensitivity in Central China but highlight the fundamental role of planetary-scale atmospheric dynamics in the sensitivity of regional climates to continental glaciation, changing greenhouse gas levels, and insolation.

Titan's asymmetric lake distribution mediated by methane transport due to atmospheric eddies

The hemispheric asymmetry of Titans surface methane has been proposed to be a consequence of orbital forcing affecting Titans hydrologic cycle, but the mechanism behind asymmetrical transport of moisture remains to be examined. Using general circulation model simulations of Titans atmosphere, we show that atmospheric moisture transport by three-dimensional tropospheric eddies is critical in generating Titans surface liquid asymmetry. Comparison of axisymmetric and three-dimensional simulations demonstrates that a significant asymmetry only develops in the latter case. Analysis of the components of the three-dimensional moisture transport reveals that nonaxisymmetric eddies transport methane away from the poles and into the midlatitudes, where they transfer moisture into the cross-equatorial transport by the mean meridional circulation, producing an atmospheric “bucket brigade.” Because these high-latitude, baroclinic eddies are more intense in the south than in the north, the net transport is preferentially northward, with the northern hemisphere gaining surface liquid at the expense of the southern hemisphere.

Spontaneous Superrotation and the Role of Kelvin Waves in an Idealized Dry GCM

The nondimensional parameter space of an idealized dry primitive equation model is explored to find superrotating climate states. The model has no convective parameterization and is forced using a simple thermal relaxation to a prescribed radiative equilibrium temperature. It is demonstrated that, of four nondimensional parameters that determine the model’s state, only the thermal Rossby number has a significant effect on superrotation. The mode that drives the transition to superrotation in an intermediate-thermalRossby-number atmosphere is shown to behave like a Kelvin wave in the tropics.

Planetary ageostrophic instability leads to superrotation

We demonstrate the existence of a global-scale, linear instability in the atmospheres of slowly rotating and/or small planets that spontaneously emerges and produces momentum convergence at the equator, thus supporting the development of planetary superrotation. We identify the instability as being barotropic, ageostrophic in nature, coupling an equatorial Kelvin wave with midlatitude or high-latitude Rossby waves. This coupling requires a frequency matching of the Doppler-shifted wave components and moderate spatial overlap between them, which are determined by two nondimensional parameters, the Rossby and Froude numbers. By diagnosing these parameters, we find that this instability is an essential and necessary process to obtain superrotation in dry atmospheric, general circulation models with axisymmetric forcing. The Rossby and Froude numbers for Solar System bodies are consistent with the presence or absence of superrotation, which suggests that they provide useful diagnostics for predicting the emergence of superrotation in the atmospheres of terrestrial planets.

Abrupt reorganization of North Pacific and western North American climate during the last deglaciation

Dramatic hydroclimate shifts occurred in western North America during the last deglaciation, but the timing and mechanisms driving these changes are uncertain and debated, and previous modeling has largely relied on linear interpolations between equilibrium snapshot simulations. Using a published transient climate simulation and a range of proxy records, we analyze the regions climate evolution in order to identify the mechanisms governing hydroclimate shifts. A rapid loss of ice around 14,000 years ago causes an abrupt reorganization of the circulation, which precipitates drying and moistening of southwestern and northwestern North America, respectively. The atmospheric circulation transitions between two states on a timescale of decades to centuries, during which time the westerly jet shifts north by about seven degrees. In contrast to previous studies, we find that changes in the water budget of western North America prior to this event are not attributable to variations in the position of the jet, but rather to the intensity of moisture transport into the continent.

COUPLING CONVECTIVELY DRIVEN ATMOSPHERIC CIRCULATION TO SURFACE ROTATION: EVIDENCE FOR ACTIVE METHANE WEATHER IN THE OBSERVED SPIN RATE DRIFT OF TITAN

A large drift in the rotation rate of Titan observed by Cassini provided the first evidence of a subsurface ocean isolating the massive core from the icy crust. Seasonal exchange of angular momentum between the surface and atmosphere accounts for the magnitude of the effect, but observations lag the expected signal by a few years. We argue that this time lag is due to the presence of an active methane weather cycle in the atmosphere. An analytic model of the seasonal cycle of atmospheric angular momentum is developed and compared with time-dependent simulations of Titans atmosphere with and without methane thermodynamics. The disappearance of clouds at the summer pole suggests the drift rate has already switched direction, signaling the change in season from solstice to equinox.