1,521 research outputs found
A non-linear optimal estimation inverse method for radio occultation measurements of temperature, humidity and surface pressure
An optimal estimation inverse method is presented which can be used to
retrieve simultaneously vertical profiles of temperature and specific humidity,
in addition to surface pressure, from satellite-to-satellite radio occultation
observations of the Earth's atmosphere. The method is a non-linear, maximum
{\it a posteriori} technique which can accommodate most aspects of the real
radio occultation problem and is found to be stable and to converge rapidly in
most cases. The optimal estimation inverse method has two distinct advantages
over the analytic inverse method in that it accounts for some of the effects of
horizontal gradients and is able to retrieve optimally temperature and humidity
simultaneously from the observations. It is also able to account for
observation noise and other sources of error. Combined, these advantages ensure
a realistic retrieval of atmospheric quantities.
A complete error analysis emerges naturally from the optimal estimation
theory, allowing a full characterisation of the solution. Using this analysis a
quality control scheme is implemented which allows anomalous retrieval
conditions to be recognised and removed, thus preventing gross retrieval
errors.
The inverse method presented in this paper has been implemented for bending
angle measurements derived from GPS/MET radio occultation observations of the
Earth. Preliminary results from simulated data suggest that these observations
have the potential to improve NWP model analyses significantly throughout their
vertical range.Comment: 18 (jgr journal) pages, 7 figure
Reflected Spectra and Albedos of Extrasolar Giant Planets I: Clear and Cloudy Atmospheres
The reflected spectra of extrasolar giant planets are primarily influenced by
Rayleigh scattering, molecular absorption, and atmospheric condensates. We
present model geometric albedo and phase integral spectra and Bond albedos for
planets and brown dwarfs with masses between 0.8 and 70 Jupiter masses.
Rayleigh scattering predominates in the blue while molecular absorption removes
most red and infrared photons. Thus cloud-free atmospheres, found on giant
planets with effective temperatures exceeding about 400 K, are quite dark in
reflected light beyond 0.6 microns. In cooler atmospheres first water clouds
and then other condensates provide a bright reflecting layer. Only planets with
cloudy atmospheres will be detectable in reflected light beyond 1 micron.
Thermal emission dominates the near-infrared for warm objects with clear
atmospheres. However the presence of other condensates, not considered here,
may brighten some planets in reflected near-infrared light and darken them in
the blue and UV. Bond albedos, the ratio of the total reflected to incident
power, are sensitive to the spectral type of the primary. Most incident photons
from early type stars will be Rayleigh scattered, while most incident photons
from late type stars will be absorbed. The Bond albedo of a given planet thus
may range from 0.4 to 0.05, depending on the primary type. Condensation of a
water cloud increases the Bond albedo of a given planet by up to a factor of
two. The spectra of cloudy planets are strongly influenced by poorly
constrained cloud microphysical properties, particularly particle size and
supersaturation. Both Bond and geometric albedos are comparatively less
sensitive to variations in planet mass and effective temperature.Comment: AASTeX; 23 pages, 2 tables, 18 figures; ApJ in press; typo fixe
Precipitating Condensation Clouds in Substellar Atmospheres
We present a method to calculate vertical profiles of particle size
distributions in condensation clouds of giant planets and brown dwarfs. The
method assumes a balance between turbulent diffusion and sedimentation in
horizontally uniform cloud decks. Calculations for the Jovian ammonia cloud are
compared with results from previous methods. An adjustable parameter describing
the efficiency of sedimentation allows the new model to span the range of
predictions made by previous models. Calculations for the Jovian ammonia cloud
are consistent with observations. Example calculations are provided for water,
silicate, and iron clouds on brown dwarfs and on a cool extrasolar giant
planet. We find that precipitating cloud decks naturally account for the
characteristic trends seen in the spectra and colors of L- and T-type ultracool
dwarfs.Comment: 33 pages including 7 figures; AASTex; Accepted for publication in
Astrophysical Journal, tentatively scheduled for v556 n1 July 20, 2001 .
Plotting error in Fig 5 corrected; slight modification to Fig 5 dicussion in
tex
Evolution of the nuclear modification factors with rapidity and centrality in d+Au collisions at $\sqrt{s_{NN}} = 200 GeV
We report on a study of the transverse momentum dependence of nuclear
modification factors for charged hadrons produced in deuteron + gold
collisions at GeV, as a function of collision centrality
and of the pseudorapidity () of the produced hadrons. We
find significant and systematic decrease of with increasing rapidity.
The midrapidity enhancement and the forward rapidity suppression are more
pronounced in central collisions relative to peripheral collisions. These
results are relevant to the study of the possible onset of gluon saturation at
RHIC energies.Comment: Four pages, four figures. Published in PRL. Figures 1 and 2 have been
updated, and several changes made to the tex
Recent Results from the BRAHMS Experiment
We present recent results obtained by the BRAHMS experiment at the
Relativistic Heavy Ion Collider (RHIC) for the systems of Au + Au and Cu + Cu
at \rootsnn{200} and at 62.4 GeV, and p + p at \rootsnn{200}. Nuclear
modification factors for Au + Au and Cu + Cu collisions are presented. Analysis
of anti-particle to particle ratios as a function of rapidity and collision
energy reveal that particle populations at the chemical freeze-out stage for
heavy-ion reactions at and above SPS energies are controlled by the baryon
chemical potential. From the particle spectra we deduce significant radial
expansion ( 0.75), as expected for systems created with a large
initial energy density. We also measure the elliptic flow parameter
versus rapidity and \ptn. We present rapidity dependent ratios within
for Au + Au and Cu + Cu at \rootsnn{200}. \Raa is found to increase
with decreasing collision energy, decreasing system size, and when going
towards more peripheral collisions. However, \Raa shows only a very weak
dependence on rapidity (for ), both for pions and protons.Comment: 16 pages and 14 figures, proceedings for plenary talk at Quark Matter
2005, Budapest, Hungar
Upper atmospheres and ionospheres of planets and satellites
The upper atmospheres of the planets and their satellites are more directly
exposed to sunlight and solar wind particles than the surface or the deeper
atmospheric layers. At the altitudes where the associated energy is deposited,
the atmospheres may become ionized and are referred to as ionospheres. The
details of the photon and particle interactions with the upper atmosphere
depend strongly on whether the object has anintrinsic magnetic field that may
channel the precipitating particles into the atmosphere or drive the
atmospheric gas out to space. Important implications of these interactions
include atmospheric loss over diverse timescales, photochemistry and the
formation of aerosols, which affect the evolution, composition and remote
sensing of the planets (satellites). The upper atmosphere connects the planet
(satellite) bulk composition to the near-planet (-satellite) environment.
Understanding the relevant physics and chemistry provides insight to the past
and future conditions of these objects, which is critical for understanding
their evolution. This chapter introduces the basic concepts of upper
atmospheres and ionospheres in our solar system, and discusses aspects of their
neutral and ion composition, wind dynamics and energy budget. This knowledge is
key to putting in context the observations of upper atmospheres and haze on
exoplanets, and to devise a theory that explains exoplanet demographics.Comment: Invited Revie
Midlatitude and high‐latitude electron density profiles in the ionosphere of Saturn obtained by Cassini radio occultation observations
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95024/1/jgra19781.pd
Centrality dependence of charged-particle pseudorapidity distributions from d+Au collisions at sqrt(s_{NN})=200 GeV
Charged-particle pseudorapidity densities are presented for the d+Au reaction
at sqrt{s_{NN}}=200 GeV with -4.2 <= eta <= 4.2$. The results, from the BRAHMS
experiment at RHIC, are shown for minimum-bias events and 0-30%, 30-60%, and
60-80% centrality classes. Models incorporating both soft physics and hard,
perturbative QCD-based scattering physics agree well with the experimental
results. The data do not support predictions based on strong-coupling,
semi-classical QCD. In the deuteron-fragmentation region the central 200 GeV
data show behavior similar to full-overlap d+Au results at sqrt{s_{NN}}=19.4
GeV.Comment: 4 pages, 3figures; expanded discussion of uncertainties; added 60-80%
centrality range; added additional discussion on centrality selection bia
Scanning the phases of QCD with BRAHMS
BRAHMS has the ability to study relativistic heavy ion collisions from the
final freeze-out of hadrons all the way back to the initial wave-function of
the gold nuclei. This is accomplished by studying hadrons with a very wide
range of momenta and angles. In doing so we can scan various phases of QCD,
from a hadron gas, to a quark gluon plasma and perhaps to a color glass
condensate.Comment: 8 pages, 6 figures, proceedings of plenary talk at Quark Matter 2004
conferenc
High Pt Hadron Spectra at High Rapidity
We report the measurement of charged hadron production at different
pseudo-rapidity values in deuteron+gold as well as proton+proton collisions at
= 200GeV at RHIC. The nuclear modification factors and
are used to investigate new behaviors in the deuteron+gold system as
function of rapidity and the centrality of the collisions respectively.Comment: Nine pages 4 figures to be published in the QM2004 Proceedings, typos
corrected and one reference adde
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