14,832 research outputs found
Atmospheric circulation of hot Jupiters: Coupled radiative-dynamical general circulation model simulations of HD 189733b and HD 209458b
We present global, three-dimensional numerical simulations of HD 189733b and
HD 209458b that couple the atmospheric dynamics to a realistic representation
of non-gray cloud-free radiative transfer. The model, which we call the
Substellar and Planetary Atmospheric Radiation and Circulation (SPARC) model,
adopts the MITgcm for the dynamics and uses the radiative model of McKay,
Marley, Fortney, and collaborators for the radiation. Like earlier work with
simplified forcing, our simulations develop a broad eastward equatorial jet,
mean westward flow at higher latitudes, and substantial flow over the poles at
low pressure. For HD 189733b, our simulations without TiO and VO opacity can
explain the broad features of the observed 8 and 24-micron light curves,
including the modest day-night flux variation and the fact that the planet/star
flux ratio peaks before the secondary eclipse. Our simulations also provide
reasonable matches to the Spitzer secondary-eclipse depths at 4.5, 5.8, 8, 16,
and 24 microns and the groundbased upper limit at 2.2 microns. However, we
substantially underpredict the 3.6-micron secondary-eclipse depth, suggesting
that our simulations are too cold in the 0.1-1 bar region. Predicted temporal
variability in secondary-eclipse depths is ~1% at Spitzer bandpasses,
consistent with recent observational upper limits at 8 microns. We also show
that nonsynchronous rotation can significantly alter the jet structure. For HD
209458b, we include TiO and VO opacity; these simulations develop a hot (>2000
K) dayside stratosphere. Despite this stratosphere, we do not reproduce current
Spitzer photometry of this planet. Light curves in Spitzer bandpasses show
modest phase variation and satisfy the observational upper limit on day-night
phase variation at 8 microns. (abridged)Comment: 20 pages (emulate-apj format), 21 figures, final version now
published in ApJ. Includes expanded discussion of radiative-transfer methods
and two new figure
Radiation-Hydrodynamics of Hot Jupiter Atmospheres
Radiative transfer in planetary atmospheres is usually treated in the static
limit, i.e., neglecting atmospheric motions. We argue that hot Jupiter
atmospheres, with possibly fast (sonic) wind speeds, may require a more
strongly coupled treatment, formally in the regime of radiation-hydrodynamics.
To lowest order in v/c, relativistic Doppler shifts distort line profiles along
optical paths with finite wind velocity gradients. This leads to flow-dependent
deviations in the effective emission and absorption properties of the
atmospheric medium. Evaluating the overall impact of these distortions on the
radiative structure of a dynamic atmosphere is non-trivial. We present
transmissivity and systematic equivalent width excess calculations which
suggest possibly important consequences for radiation transport in hot Jupiter
atmospheres. If winds are fast and bulk Doppler shifts are indeed important for
the global radiative balance, accurate modeling and reliable data
interpretation for hot Jupiter atmospheres may prove challenging: it would
involve anisotropic and dynamic radiative transfer in a coupled
radiation-hydrodynamical flow. On the bright side, it would also imply that the
emergent properties of hot Jupiter atmospheres are more direct tracers of their
atmospheric flows than is the case for Solar System planets.
Radiation-hydrodynamics may also influence radiative transfer in other classes
of hot exoplanetary atmospheres with fast winds.Comment: 25 pages, 4 figures, accepted for publication in ApJ (minor
revisions
Assessing radiative transfer models trained by numerical weather forecasts using sun-tracking radiometric measurements for satellite link characterization up to W band
Radio communications, and in particular Earth-to-satellite
links, are worldwide used for delivering digital services.
The bandwidth demand of such services is increasing
accordingly to the advent of more advanced applications
(e.g., multimedia services, deep-space explorations, etc.)
thus pushing the scientific community toward the
investigation of channel carriers at higher frequencies.
When using carrier frequencies above X band, the main
drawback is how to tackle the impact of tropospheric
processes (i.e., rain, cloud, water vapor). This work
assesses the joint use of weather forecast models, radiative
transfer models and Sun-tracking radiometric
measurements to explore their potential benefits in
predicting path attenuation and sky noise temperature for
slant paths at frequencies between K and W band, thus
paving the way to the optimization of satellite link-budgets
A General Circulation Model for Gaseous Exoplanets with Double-Gray Radiative Transfer
We present a new version of our code for modeling the atmospheric circulation
on gaseous exoplanets, now employing a "double-gray" radiative transfer scheme,
which self-consistently solves for fluxes and heating throughout the
atmosphere, including the emerging (observable) infrared flux. We separate the
radiation into infrared and optical components, each with its own absorption
coefficient, and solve standard two-stream radiative transfer equations. We use
a constant optical absorption coefficient, while the infrared coefficient can
scale as a powerlaw with pressure. Here we describe our new code in detail and
demonstrate its utility by presenting a generic hot Jupiter model. We discuss
issues related to modeling the deepest pressures of the atmosphere and describe
our use of the diffusion approximation for radiative fluxes at high optical
depths. In addition, we present new models using a simple form for magnetic
drag on the atmosphere. We calculate emitted thermal phase curves and find that
our drag-free model has the brightest region of the atmosphere offset by ~12
degrees from the substellar point and a minimum flux that is 17% of the
maximum, while the model with the strongest magnetic drag has an offset of only
~2 degrees and a ratio of 13%. Finally, we calculate rates of numerical loss of
kinetic energy at ~15% for every model except for our strong-drag model, where
there is no measurable loss; we speculate that this is due to the much
decreased wind speeds in that model.Comment: 29 pages, 12 figures, 2 tables, submitted to Ap
Feasibility of quasi-random band model in evaluating atmospheric radiance
The use of the quasi-random band model in evaluating upwelling atmospheric radiation is investigated. The spectral transmittance and total band adsorptance are evaluated for selected molecular bands by using the line by line model, quasi-random band model, exponential sum fit method, and empirical correlations, and these are compared with the available experimental results. The atmospheric transmittance and upwelling radiance were calculated by using the line by line and quasi random band models and were compared with the results of an existing program called LOWTRAN. The results obtained by the exponential sum fit and empirical relations were not in good agreement with experimental results and their use cannot be justified for atmospheric studies. The line by line model was found to be the best model for atmospheric applications, but it is not practical because of high computational costs. The results of the quasi random band model compare well with the line by line and experimental results. The use of the quasi random band model is recommended for evaluation of the atmospheric radiation
Simulations of Contrail Optical Properties and Radiative Forcing for Various Crystal Shapes
The aim of this study is to investigate the sensitivity of radiative-forcing computations to various contrail
crystal shape models. Contrail optical properties in the shortwave and longwave ranges are derived using
a ray-tracing geometric method and the discrete dipole approximation method, respectively. Both methods
present good correspondence of the single-scattering albedo and the asymmetry parameter in a transition
range (3–8 µm). There are substantial differences in single-scattering properties among 10 crystal models
investigated here (e.g., hexagonal columns and plates with different aspect ratios, and spherical particles). The
single-scattering albedo and the asymmetry parameter both vary by up to 0.1 among various crystal shapes.
The computed single-scattering properties are incorporated in the moderate-resolution atmospheric radiance
and transmittance model(MODTRAN) radiative transfer code to simulate solar and infrared fluxes at the top
of the atmosphere. Particle shapes have a strong impact on the contrail radiative forcing in both the shortwave
and longwave ranges. The differences in the net radiative forcing among optical models reach 50% with
respect to the mean model value. The hexagonal-column and hexagonal-plate particles show the smallest net
radiative forcing, and the largest forcing is obtained for the spheres. The balance between the shortwave
forcing and longwave forcing is highly sensitive with respect to the assumed crystal shape and may even
change the sign of the net forcing. The optical depth at which the mean diurnal radiative forcing changes sign
from positive to negative varies from 4.5 to 10 for a surface albedo of 0.2 and from 2 to 6.5 for a surface albedo
of 0.05. Contrails are probably never that optically thick (except for some aged contrail cirrus), however, and
so will not have a cooling effect on climate
A non-grey analytical model for irradiated atmospheres. II: Analytical vs. numerical solutions
The recent discovery and characterization of the diversity of the atmospheres
of exoplanets and brown dwarfs calls for the development of fast and accurate
analytical models. We quantify the accuracy of the analytical solution derived
in paper I for an irradiated, non-grey atmosphere by comparing it to a
state-of-the-art radiative transfer model. Then, using a grid of numerical
models, we calibrate the different coefficients of our analytical model for
irradiated solar-composition atmospheres of giant exoplanets and brown dwarfs.
We show that the so-called Eddington approximation used to solve the angular
dependency of the radiation field leads to relative errors of up to 5% on the
temperature profile. We show that for realistic non-grey planetary atmospheres,
the presence of a convective zone that extends to optical depths smaller than
unity can lead to changes in the radiative temperature profile on the order of
20% or more. When the convective zone is located at deeper levels (such as for
strongly irradiated hot Jupiters), its effect on the radiative atmosphere is
smaller. We show that the temperature inversion induced by a strong absorber in
the optical, such as TiO or VO is mainly due to non-grey thermal effects
reducing the ability of the upper atmosphere to cool down rather than an
enhanced absorption of the stellar light as previously thought.
Finally, we provide a functional form for the coefficients of our analytical
model for solar-composition giant exoplanets and brown dwarfs. This leads to
fully analytical pressure-temperature profiles for irradiated atmospheres with
a relative accuracy better than 10% for gravities between 2.5m/s^2 and 250
m/s^2 and effective temperatures between 100 K and 3000 K. This is a great
improvement over the commonly used Eddington boundary condition.Comment: Accepted in A&A, models are available at
http://www.oca.eu/parmentier/nongrey or in CD
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