The Experts below are selected from a list of 294 Experts worldwide ranked by ideXlab platform
Yoshihisa Iio - One of the best experts on this subject based on the ideXlab platform.
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Seismic Source Spectrum of microearthquakes
Bulletin of the Seismological Society of America, 1992Co-Authors: Yoshihisa IioAbstract:The seismic Source spectra of microearthquakes having seismic moments between 1014 and 1018 dyne cm were investigated by using local recordings from an excellent hard-rock site. The P -wave velocity near the site was estimated as about 6 km/sec, even immediately below the ground surface. The effect of anelastic attenuation was thought to be very small, since predominant frequencies of greater than 100 Hz were detected in seismograms recorded at focal distances greater 10 km. Many seismograms with S-P times of less than 0.6 sec were observed. The first cycle of the P -wave velocity seismogram was used in this study. The waveforms after the first cycle are likely formed near the site, since their periods are exactly the same for earthquakes that have different Source processes. In the high-frequency portion of the estimated displacement Source spectra, the slopes of the fall-off have values much greater than 2. The Source process of microearthquakes is assumed to be very slow and smooth.
Michael W. Vannier - One of the best experts on this subject based on the ideXlab platform.
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A robust method of x-ray Source Spectrum estimation from transmission measurements: Demonstrated on computer simulated, scatter-free transmission data
Journal of Applied Physics, 2005Co-Authors: Emil Y. Sidky, Xiaochuan Pan, Yu Zou, Michael W. VannierAbstract:Knowledge of the x-ray Spectrum in diagnostic imaging is important for dose calculations, correction for beam-hardening artifacts, and dual-energy computed tomography. One way to determine the x-ray Source Spectrum is to estimate it from transmission data of a known phantom. Although such an approach is experimentally simple, Spectrum estimation from transmission data is known to be an ill-conditioned problem. The contribution of this work is twofold. First, we present a systematic method of designing the transmission measurements to reduce the mathematical instability of Spectrum estimation. Second, we apply the expectation-maximization (EM) method, which is known to yield robust solutions to positive, linear integral equations such as the one that describes Spectrum estimation from transmission data. The proposed EM method is compared to other algorithms in the literature on simulated data from x-ray spectra typical to mammography and computed tomography. The EM method appears to outperform existing alg...
Elisa Manzini - One of the best experts on this subject based on the ideXlab platform.
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Intercomparison of gravity wave parameterizations: Hines Doppler-spread and Warner and McIntyre ultra-simple schemes
Journal of the Meteorological Society of Japan, 2002Co-Authors: Martin Charron, Elisa Manzini, C. D. WarnerAbstract:The ‘‘three-part ultra-simple’’ gravity wave parameterization of Warner and McIntyre is compared with the ‘‘Doppler-spread’’ parameterization of Hines. The two parameterizations are tested on a background state at rest with constant buoyancy frequency, as well as on background states defined by the CIRA86 data at 70N in July and January. To achieve as clean a comparison as possible between the two parameterizations, two approaches are taken. The first approach is to adjust the free parameters to obtain the same Source level momentum fluxes, and as similar a Source Spectrum shape, as is possible. The second approach is to adjust the Source level momentum fluxes to obtain the same momentum fluxes at mesospheric altitudes. The resulting vertical profiles of the momentum fluxes, of the wave-induced forces, and of the energy dissipation rates produced by the two parameterizations are compared. When a similar gravity wave Source Spectrum is used, specifically the Source Spectrum recommended by Hines, momentum deposition generally tends to occur lower in the atmosphere for the Warner and McIntyre parameterization than for the Hines Doppler-spread parameterization. In order to obtain similar wave-induced forces, and dissipation rates in the mesosphere from the two parameterizations, it has been found that the Warner and McIntyre parameterization requires the Source Spectrum to be scaled so that the net momentum flux in the lower stratosphere is an order of magnitude higher than the Hines Doppler-spread parameterization.
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the effect of varying the Source Spectrum of a gravity wave parameterization in a middle atmosphere general circulation model
Journal of Geophysical Research, 1998Co-Authors: Elisa Manzini, N A McfarlaneAbstract:Climate simulations of the middle atmosphere circulation with general circulation models are now starting to include parameterizations of the momentum flux deposition due to unresolved gravity wave spectra. A current uncertainty in the application of such parameterizations is the specification of the imposed gravity wave Spectrum. The aim of this work is to quantify the effect of varying within a realistic range the Source Spectrum of a gravity wave parameterization in a general circulation model. Results from two simulations with the gravity wave Spectrum launched at two different heights, the surface and the 110-hPa pressure level, respectively, are compared. Noteworthy differences found in the simulated middle atmosphere response include the following (1) The average temperature in the southern winter upper stratosphere is about 40 K warmer in the experiment with the surface as the launching height, virtually eliminating the typical cold polar bias that affects many general circulation models. (2) Stronger easterlies in the subtropical summer mesosphere, again in the experiment with the surface as the launching height. Diagnostics of the parameterized gravity waves indicate that in the experiment with the surface as the launching height, the net zonal momentum flux transported by the gravity waves is negative just above the troposphere at middle latitudes. This negative net momentum flux facilitates the deceleration of the mesospheric winter westerlies. The meridional circulation induced by such deceleration is thereafter responsible for the substantial polar winter warming. In contrast, in summer the negative net momentum flux limits the upper mesospheric deceleration of the easterlies. In the experiment with launching height at 110-hPa, the gravity wave net momentum flux is instead zero by construction at the launching height.
N A Mcfarlane - One of the best experts on this subject based on the ideXlab platform.
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the effect of varying the Source Spectrum of a gravity wave parameterization in a middle atmosphere general circulation model
Journal of Geophysical Research, 1998Co-Authors: Elisa Manzini, N A McfarlaneAbstract:Climate simulations of the middle atmosphere circulation with general circulation models are now starting to include parameterizations of the momentum flux deposition due to unresolved gravity wave spectra. A current uncertainty in the application of such parameterizations is the specification of the imposed gravity wave Spectrum. The aim of this work is to quantify the effect of varying within a realistic range the Source Spectrum of a gravity wave parameterization in a general circulation model. Results from two simulations with the gravity wave Spectrum launched at two different heights, the surface and the 110-hPa pressure level, respectively, are compared. Noteworthy differences found in the simulated middle atmosphere response include the following (1) The average temperature in the southern winter upper stratosphere is about 40 K warmer in the experiment with the surface as the launching height, virtually eliminating the typical cold polar bias that affects many general circulation models. (2) Stronger easterlies in the subtropical summer mesosphere, again in the experiment with the surface as the launching height. Diagnostics of the parameterized gravity waves indicate that in the experiment with the surface as the launching height, the net zonal momentum flux transported by the gravity waves is negative just above the troposphere at middle latitudes. This negative net momentum flux facilitates the deceleration of the mesospheric winter westerlies. The meridional circulation induced by such deceleration is thereafter responsible for the substantial polar winter warming. In contrast, in summer the negative net momentum flux limits the upper mesospheric deceleration of the easterlies. In the experiment with launching height at 110-hPa, the gravity wave net momentum flux is instead zero by construction at the launching height.
Emil Y. Sidky - One of the best experts on this subject based on the ideXlab platform.
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A robust method of x-ray Source Spectrum estimation from transmission measurements: Demonstrated on computer simulated, scatter-free transmission data
Journal of Applied Physics, 2005Co-Authors: Emil Y. Sidky, Xiaochuan Pan, Yu Zou, Michael W. VannierAbstract:Knowledge of the x-ray Spectrum in diagnostic imaging is important for dose calculations, correction for beam-hardening artifacts, and dual-energy computed tomography. One way to determine the x-ray Source Spectrum is to estimate it from transmission data of a known phantom. Although such an approach is experimentally simple, Spectrum estimation from transmission data is known to be an ill-conditioned problem. The contribution of this work is twofold. First, we present a systematic method of designing the transmission measurements to reduce the mathematical instability of Spectrum estimation. Second, we apply the expectation-maximization (EM) method, which is known to yield robust solutions to positive, linear integral equations such as the one that describes Spectrum estimation from transmission data. The proposed EM method is compared to other algorithms in the literature on simulated data from x-ray spectra typical to mammography and computed tomography. The EM method appears to outperform existing alg...
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A novel method for determining Source Spectrum/detector spectral response in X-ray imaging
2003 IEEE Nuclear Science Symposium. Conference Record (IEEE Cat. No.03CH37515), 1Co-Authors: Emil Y. Sidky, Xiaochuan PanAbstract:Knowledge of the X-ray Spectrum in diagnostic imaging is important for dose calculations and correction for beam-hardening artifacts. This work investigates the possibility of determining the X-ray Source Spectrum from transmission data of a known phantom. In addition to the design of the phantom, the novel aspect of this work is the application of the expectation-maximization method, widely used in tomographic image reconstruction, in solving the ill-posed problem of finding the X-ray Spectrum from transmission data.
