163 research outputs found
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
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
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