9,775 research outputs found
Sensible and latent heating of the atmosphere as inferred from DST-6 data
The average distribution of convective latent heating, boundary layer sensible heat flux, and vertical velocity are determined for the winter 1976 DST period from GLAS model diagnostics. Key features are the regions of intense latent heating over Brazil, Central Africa, and Indonesia; and the regions of strong sensible heating due to air mass modification over the North Atlantic and North Pacific Oceans
The effect of clouds on the earth's radiation balance
The effect of global cloudiness on the radiation balance at the top of the atmosphere is studied in general circulation model experiments. Wintertime simulations were conducted with clouds that had realistic optical properties, and were compared with simulations in which the clouds were transparent to either solar or thermal radiation. Clouds increase the net balance by limiting longwave loss to space, but decrease it by reflecting solar radiation. It is found that the net result of cloudiness is to maintain net radiation which is less than would be realized under clear conditions: Clouds cause the net radiation at the top of the atmosphere to increase due to longwave absorption, but to decrease even more due to cloud reflectance of solar radiation
Diluted maximum-likelihood algorithm for quantum tomography
We propose a refined iterative likelihood-maximization algorithm for
reconstructing a quantum state from a set of tomographic measurements. The
algorithm is characterized by a very high convergence rate and features a
simple adaptive procedure that ensures likelihood increase in every iteration
and convergence to the maximum-likelihood state.
We apply the algorithm to homodyne tomography of optical states and quantum
tomography of entangled spin states of trapped ions and investigate its
convergence properties.Comment: v2: Convergence proof adde
Deformation of Silica Aerogel During Fluid Adsorption
Aerogels are very compliant materials - even small stresses can lead to large
deformations. In this paper we present measurements of the linear deformation
of high porosity aerogels during adsorption of low surface tension fluids,
performed using a Linear Variable Differential Transformer (LVDT). We show that
the degree of deformation of the aerogel during capillary condensation scales
with the surface tension, and extract the bulk modulus of the gel from the
data. Furthermore we suggest limits on safe temperatures for filling and
emptying low density aerogels with helium.Comment: 8 pages, 5 figures, submitted to PR
Superiorization: An optimization heuristic for medical physics
Purpose: To describe and mathematically validate the superiorization
methodology, which is a recently-developed heuristic approach to optimization,
and to discuss its applicability to medical physics problem formulations that
specify the desired solution (of physically given or otherwise obtained
constraints) by an optimization criterion. Methods: The underlying idea is that
many iterative algorithms for finding such a solution are perturbation
resilient in the sense that, even if certain kinds of changes are made at the
end of each iterative step, the algorithm still produces a
constraints-compatible solution. This property is exploited by using permitted
changes to steer the algorithm to a solution that is not only
constraints-compatible, but is also desirable according to a specified
optimization criterion. The approach is very general, it is applicable to many
iterative procedures and optimization criteria used in medical physics.
Results: The main practical contribution is a procedure for automatically
producing from any given iterative algorithm its superiorized version, which
will supply solutions that are superior according to a given optimization
criterion. It is shown that if the original iterative algorithm satisfies
certain mathematical conditions, then the output of its superiorized version is
guaranteed to be as constraints-compatible as the output of the original
algorithm, but it is superior to the latter according to the optimization
criterion. This intuitive description is made precise in the paper and the
stated claims are rigorously proved. Superiorization is illustrated on
simulated computerized tomography data of a head cross-section and, in spite of
its generality, superiorization is shown to be competitive to an optimization
algorithm that is specifically designed to minimize total variation.Comment: Accepted for publication in: Medical Physic
Black Hole Horizons and Complementarity
We investigate the effect of gravitational back-reaction on the black hole
evaporation process. The standard derivation of Hawking radiation is
re-examined and extended by including gravitational interactions between the
infalling matter and the outgoing radiation. We find that these interactions
lead to substantial effects. In particular, as seen by an outside observer,
they lead to a fast growing uncertainty in the position of the infalling matter
as it approaches the horizon. We argue that this result supports the idea of
black hole complementarity, which states that, in the description of the black
hole system appropriate to outside observers, the region behind the horizon
does not establish itself as a classical region of space-time. We also give a
new formulation of this complementarity principle, which does not make any
specific reference to the location of the black hole horizon.Comment: Some minor modifications in text and the title chang
Phase Space Tomography of Matter-Wave Diffraction in the Talbot Regime
We report on the theoretical investigation of Wigner distribution function
(WDF) reconstruction of the motional quantum state of large molecules in de
Broglie interference. De Broglie interference of fullerenes and as the like
already proves the wavelike behaviour of these heavy particles, while we aim to
extract more quantitative information about the superposition quantum state in
motion. We simulate the reconstruction of the WDF numerically based on an
analytic probability distribution and investigate its properties by variation
of parameters, which are relevant for the experiment. Even though the WDF
described in the near-field experiment cannot be reconstructed completely, we
observe negativity even in the partially reconstructed WDF. We further consider
incoherent factors to simulate the experimental situation such as a finite
number of slits, collimation, and particle-slit van der Waals interaction. From
this we find experimental conditions to reconstruct the WDF from Talbot
interference fringes in molecule Talbot-Lau interferometry.Comment: 16 pages, 9 figures, accepted at New Journal of Physic
Thermal emission spectroscopy of the middle atmosphere
The general objective of this research is to obtain, via remote sensing, simultaneous measurements of the vertical distributions of stratospheric temperature, ozone, and trace constituents that participate in the catalytic destruction of ozone (NO(sub y): NO, NO2, NO3, HNO3, ClONO2, N2O5, HNO4; Cl(sub x): HOCl), and the source gases for the catalytic cycles (H2O, CH4, N2O, CF2Cl2, CFCl3, CCl4, CH3Cl, CHF2Cl, etc.). Data are collected during a complete diurnal cycle in order to test our present understanding of ozone chemistry and its associate catalytic cycles. The instrumentation employed is an emission-mode, balloon-borne, liquid-nitrogen-cooled Michelson interferometer-spectrometer (SIRIS), covering the mid-infrared range with a spectral resolution of 0.020 cm(exp -1). Cryogenic cooling combined with the use of extrinsic silicon photoconductor detectors allows the detection of weak emission features of stratospheric gaseous species. Vertical distributions of these species are inferred from scans of the thermal emission of the limb in a sequence of elevation angles. The fourth SIRIS balloon flight was carried out from Palestine, Texas on September 15-16, 1986 with 9 hours of nighttime data (40 km). High quality data with spectral resolution 0.022 cm(exp -1), were obtained for numerous limb sequences. Fifteen stratospheric species have been identified to date from this flight: five species from the NO(sub y) family (HNO3, NO2, NO, ClONO2, N2O5), plus CO2, O3, H2O, N2O, CH4, CCl3F, CCl2F2, CHF2Cl, CF4, and CCl4. The nighttime values of N2O5, ClONO2, and total odd nitrogen have been measured for the first time, and compared to model results. Analysis of the diurnal variation of N2O5 within the 1984 and 1986 data sets, and of the 1984 ClONO2 measurements, were presented in the literature. The demonstrated ability of SIRIS to measure all the major NO(sub y) species, and therefore to determine the partitioning of the nitrogen family over a continuous diurnal cycle, is a powerful tool in the verification and improvement of photochemical modeling
First-principles calculation of the temperature dependence of the optical response of bulk GaAs
A novel approach has been developed to calculate the temperature dependence
of the optical response of a semiconductor. The dielectric function is averaged
over several thermally perturbed configurations that are extracted from
molecular dynamic simulations. The calculated temperature dependence of the
imaginary part of the dielectric function of GaAs is presented in the range
from 0 to 700 K. This approach that explicitly takes into account lattice
vibrations describes well the observed thermally-induced energy shifts and
broadening of the dielectric function.Comment: 6 pages, 3 figure
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