Linking the Climate and Thermal Phase Curve of 55 Cancri e

The thermal phase curve of 55 Cancri e is the first measurement of the temperature distribution of a tidally locked Super-Earth, but raises a number of puzzling questions about the planet's climate. The phase curve has a high amplitude and peak offset, suggesting that it has a significant eastward hot-spot shift as well as a large day-night temperature contrast. We use a general circulation model to model potential climates, and investigate the relation between bulk atmospheric composition and the magnitude of these seemingly contradictory features. We confirm theoretical models of tidally locked circulation are consistent with our numerical model of 55 Cnc e, and rule out certain atmospheric compositions based on their thermodynamic properties. Our best-fitting atmosphere has a significant hot-spot shift and day-night contrast, although these are not as large as the observed phase curve. We discuss possible physical processes which could explain the observations, and show that night-side cloud formation from species such as SiO from a day-side magma ocean could potentially increase the phase curve amplitude and explain the observations. We conclude that the observations could be explained by an optically thick atmosphere with a low mean molecular weight, a surface pressure of several bar and a strong eastward circulation, with night-side cloud formation a possible explanation for the difference between our model and the observations.Comment: Accepted for publication in Ap

Surface Quasigeostrophic Turbulence : The Study of an Active Scalar

We study the statistical and geometrical properties of the potential temperature (PT) field in the Surface Quasigeostrophic (SQG) system of equations. In addition to extracting information in a global sense via tools such as the power spectrum, the g-beta spectrum and the structure functions we explore the local nature of the PT field by means of the wavelet transform method. The primary indication is that an initially smooth PT field becomes rough (within specified scales), though in a qualitatively sparse fashion. Similarly, initially 1D iso-PT contours (i.e., PT level sets) are seen to acquire a fractal nature. Moreover, the dimensions of the iso-PT contours satisfy existing analytical bounds. The expectation that the roughness will manifest itself in the singular nature of the gradient fields is confirmed via the multifractal nature of the dissipation field. Following earlier work on the subject, the singular and oscillatory nature of the gradient field is investigated by examining the scaling of a probability measure and a sign singular measure respectively. A physically motivated derivation of the relations between the variety of scaling exponents is presented, the aim being to bring out some of the underlying assumptions which seem to have gone unnoticed in previous presentations. Apart from concentrating on specific properties of the SQG system, a broader theme of the paper is a comparison of the diagnostic inertial range properties of the SQG system with both the 2D and 3D Euler equations.Comment: 26 pages, 11 figures. To appear in Chao

The Decay of Passive Scalars Under the Action of Single Scale Smooth Velocity Fields in Bounded 2D Domains : From non self similar pdf's to self similar eigenmodes

We examine the decay of passive scalars with small, but non zero, diffusivity in bounded 2D domains. The velocity fields responsible for advection are smooth (i.e., they have bounded gradients) and of a single large scale. Moreover, the scale of the velocity field is taken to be similar to the size of the entire domain. The importance of the initial scale of variation of the scalar field with respect to that of the velocity field is strongly emphasized. If these scales are comparable and the velocity field is time periodic, we see the formation of a periodic scalar eigenmode. The eigenmode is numerically realized by means of a deterministic 2D map on a lattice. Analytical justification for the eigenmode is available from theorems in the dynamo literature. Weakening the notion of an eigenmode to mean statistical stationarity, we provide numerical evidence that the eigenmode solution also holds for aperiodic flows (represented by random maps). Turning to the evolution of an initially small scale scalar field, we demonstrate the transition from an evolving (i.e., {\it non} self similar) pdf to a stationary (self similar) pdf as the scale of variation of the scalar field progresses from being small to being comparable to that of the velocity field (and of the domain). Furthermore, the {\it non} self similar regime itself consists of two stages. Both the stages are examined and the coupling between diffusion and the distribution of the Finite Time Lyapunov Exponents is shown to be responsible for the pdf evolution.Comment: 21 pages (2 col. format), 11 figures. Accepted, to appear in PR

The runaway greenhouse on subNeptune waterworlds

The implications of the water vapor runaway greenhouse phenomenon for water-rich subNeptunes are developed. In particular, the nature of the post-runaway equilibration process for planets that have an extremely high water inventory is addressed. Crossing the threshold from sub-runaway to super-runaway conditions leads to a transition from equilibrated states with cold deep liquid oceans and deep interior ice-X phases to states with hot supercritical fluid interiors. There is a corresponding marked inflation of radius for a given mass, similar to the runaway greenhouse radius inflation effect noted earlier for terrestrial planets, but in the present case the inflation involves the entire interior of the planet. The calculation employs the AQUA equation of state database to simplify the internal structure calculation. Some speculations concerning the effect of $\mathrm{H_2}$ admixture, silicate cores and hot vs. cold start evolution trajectories are offered. Observational implications are discussed, though the search for the mass-radius signature of the phenomena considered is limited by degeneracies and by lack of data.Comment: Submitted to Astrophysical Journa

The Importance of Ice Vertical Resolution for Snowball Climate and Deglaciation

Sea ice schemes with a few vertical levels are typically used to simulate the thermodynamic evolution of sea ice in global climate models. Here it is shown that these schemes overestimate the magnitude of the diurnal surface temperature cycle by a factor of 2–3 when they are used to simulate tropical ice in a Snowball earth event. This could strongly influence our understanding of Snowball termination, which occurs in global climate models when the midday surface temperature in the tropics reaches the melting point. A hierarchy of models is used to show that accurate simulation of surface temperature variation on a given time scale requires that a sea ice model resolve the e-folding depth to which a periodic signal on that time scale penetrates. This is used to suggest modifications to the sea ice schemes used in global climate models that would allow more accurate simulation of Snowball deglaciation

CO2 Ocean Bistability on Terrestrial Exoplanets

Cycling of carbon dioxide between the atmosphere and interior of rocky planets can stabilize global climate and enable planetary surface temperatures above freezing over geologic time. However, variations in global carbon budget and unstable feedback cycles between planetary sub‐systems may destabilize the climate of rocky exoplanets toward regimes unknown in the Solar System. Here, we perform clear‐sky atmospheric radiative transfer and surface weathering simulations to probe the stability of climate equilibria for rocky, ocean‐bearing exoplanets at instellations relevant for planetary systems in the outer regions of the circumstellar habitable zone. Our simulations suggest that planets orbiting G‐ and F‐type stars (but not M‐type stars) may display bistability between an Earth‐like climate state with efficient carbon sequestration and an alternative stable climate equilibrium where CO(2) condenses at the surface and forms a blanket of either clathrate hydrate or liquid CO(2). At increasing instellation and with ineffective weathering, the latter state oscillates between cool, surface CO(2)‐condensing and hot, non‐condensing climates. CO(2) bistable climates may emerge early in planetary history and remain stable for billions of years. The carbon dioxide‐condensing climates follow an opposite trend in pCO(2) versus instellation compared to the weathering‐stabilized planet population, suggesting the possibility of observational discrimination between these distinct climate categories

The Equatorial Jet Speed on Tidally Locked Planets: I -- Terrestrial Planets

The atmospheric circulation of tidally locked planets is dominated by a superrotating eastward equatorial jet. We develop a predictive theory for the formation of this jet, proposing a mechanism in which the three-dimensional stationary waves induced by the day-night forcing gradient produce an equatorial acceleration. This is balanced in equilibrium by an interaction between the resulting jet and the vertical motion of the atmosphere. The three-dimensional structure of the zonal acceleration is vital to this mechanism. We demonstrate this mechanism in a hierarchy of models. We calculate the three-dimensional stationary waves induced by the forcing on these planets, and show the vertical structure of the zonal acceleration produced by these waves, which we use to suggest a mechanism for how the jet forms. GCM simulations are used to confirm the equilibrium state predicted by this mechanism, where the acceleration from these waves is balanced by an interaction between the zonal-mean vertical velocity and the jet. We derive a simple model of this using the "Weak Temperature Gradient" approximation, which gives an estimate of the jet speed on a terrestrial tidally locked planet. We conclude that the proposed mechanism is a good description of the formation of an equatorial jet on a terrestrial tidally locked planet, and should be useful for interpreting observations and simulations of these planets. The mechanism requires assumptions such as a large equatorial Rossby radius and weak acceleration due to transient waves, and a different mechanism may produce the equatorial jets on gaseous tidally locked planets.Comment: Accepted by Ap