Simple Slab Model Research Articles

K‐Shell Photoabsorption of Oxygen Ions

Extensive calculations of the atomic data required for the spectral modelling of the K-shell photoabsorption of oxygen ions have been carried out in a multi-code approach. The present level energies and wavelengths for the highly ionized species (electron occupancies 2 4, lack of measurements, wide experimental scatter, and discrepancies among theoretical values are handicaps in reliable accuracy assessments. The radiative and Auger rates are expected to be accurate to 10% and 20%, respectively, except for transitions involving strongly mixed levels. Radiative and Auger dampings have been taken into account in the calculation of photoabsorption cross sections in the K-threshold region, leading to overlapping lorentzian shaped resonances of constant widths that cause edge smearing. The behavior of the improved opacities in this region has been studied with the XSTAR modelling code using simple constant density slab models, and is displayed for a range of ionization parameters.

The effects of internal refractive index variation in near-infrared optical tomography: a finite element modelling approach

Near-infrared (NIR) tomography is a technique used to measure light propagation through tissue and generate images of internal optical property distributions from boundary measurements. Most popular applications have concentrated on female breast imaging, neonatal and adult head imaging, as well as muscle and small animal studies. In most instances a highly scattering medium with a homogeneous refractive index is assumed throughout the imaging domain. Using these assumptions, it is possible to simplify the model to the diffusion approximation. However, biological tissue contains regions of varying optical absorption and scatter, as well as varying refractive index. In this work, we introduce an internal boundary constraint in the finite element method approach to modelling light propagation through tissue that accounts for regions of different refractive indices. We have compared the results to data from a Monte Carlo simulation and show that for a simple two-layered slab model of varying refractive index, the phase of the measured reflectance data is significantly altered by the variation in internal refractive index, whereas the amplitude data are affected only slightly.

A Simple Model to Determine In Situ Mixing Height Growth from Surface Measurements

A generic In Situ Mixing Height Growth (IMG) model, capable of predicting the real-time growth of the mixed layer and its diurnal evolution from routinely observed simple surface meteorological is developed. The algorithm for the determination of temporally growing daytime mixing height includes both convective and mechanical turbulence contributions. It accounts for neutral as well as height varying potential temperature gradients above the mixed layer. For thermally stable and mechanically dominated unstable night time Atmospheric Boundary Layer (ABL) the module uses similarity formulae based on the wind velocity [1]), the Monin—Obukhov length [2], and the Coriolis parameter. In the convective case simple slab model is integrated, based on initial lapse rate and the surface heat flux. The lapse rate is evaluated on the basis of vertical atmospheric stability, surface type and surface temperature. This differentiates the IMG model from other existing mixing height models that need initial measured lapse rate for calculation. IMG model is site specific as it calculates the radiative incoming heat flux depending on the solar declination estimates based on-site latitude and longitude. The IMG model is applied to calculate mixing height for India by using surface data (viz. wind speed, surface temperature, surface type) from 152 surface meteorological stations. Results have been evaluated with radiosonde mixing height data procured from 18 upper air stations. Sensitivity analysis of the model with respect to various parameters is performed. The model is formulated after reviewing presently available radiosonde mixing height data in India and can satisfactorily provide an alternative means of estimating mixing height for air pollution dispersion models.

Light Scattering in Some Order-Disorder Type Ferroelectrics Due to Phase Transitions

Computer model of Light Scattering and Light Propagation from KDP type crystal's domain structure were obtained by mean field method. We discuss numerical aspects of computations specific for slab and present results of benchmark computations for a simple slab model. Then we report results of extensive computations for poly-disperse, randomly rough 2D surface with refractive index of KDP and ADP.These computations parallel to our previous study of light scattering by poly-disperse randomly oriented spheroid and our used to compare scattering properties of the two classes of simple convex particles.

Using shear wave amplitude patterns to detect metastable olivine in subducted slabs

Olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to β phase. This may produce a tapering low seismic velocity wedge within the slab in the transition zone; transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes. Detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic. Motivated by previous two‐dimensional numerical calculations for upgoing and downgoing seismic waves, a three‐dimensional Gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions. Five slab velocity structures are considered: a thermal model with horizontal phase transitions at the same depths as in ambient mantle, two models incorporating equilibrium transitions within the slab, and two models with tongues of metastable olivine. A depth‐ and temperature‐dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to β phase are included in the thermal modeling. Three‐dimensional dynamic ray tracing and Gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models. For sources near 400 km depth the presence of low‐velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models. Comparison with observed amplitudes for four events in the Kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models. This indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs.

Nearest-Grid-Point Interpolation in Gyrokinetic Particle-in-Cell Simulation

Linear interpolation or other higher order interpolation schemes are almost always used in particle-in-cell simulation because of their lower noise characteristics and accuracy relative to the nearest-grid-point method. The higher order interpolation schemes are chosen because of their optimal performance, balancing a smaller number of particles against more computer operations per particle per time step. However, this is not the case in large-scale gyrokinetic particle simulations, where sometimes nearest-grid-point interpolation is used with results virtually identical to those of linear interpolation using the same number of particles. Here, a comparison and analysis of nearest-grid-point and linear interpolation schemes is given showing why nearest-grid-point interpolation can be optimal. A simple two-dimensional slab model of the ion-temperature-gradient instability and associated generation of zonal flows is used to compare the interpolation schemes. Significant improvements in performance are possible by using nearest-grid-point interpolation because of the large number of gathers/scatters associated with gyroaveraging.

The patchy accretion disc in HT Cassiopeiae

We have reconstructed the temperatures and surface densities in the quiescent accretion disc in HT Cas by performing a Physical Parameter Eclipse Mapping analysis of archival UBVR observations. Using a simple hydrogen slab model and demanding a smooth, maximally artefact-free reconstruction, we derive a formal distance to HT Cas of 207 +- 10 pc, significantly larger than the 133 +- 14 pc we derive from a re-analysis of the data in the literature. We argue that the larger derived distance is probably incorrect but is not produced by inaccuracies in our spectral model or optimization method. The discrepancy can be resolved if the emission regions on the disc are patchy with a filling factor of about 40% of the disc's surface. This solves the problem with the high effective temperatures in the disc -- reducing them to around 6 500 K within a radius of 0.2 R_L1 -- and reduces the derived temperature of the white dwarf and/or boundary layer from 22600 to 15500 K. We discuss several possible sources of the chromospheric emission and its patchiness, including irradiation of the disc, thermal instabilities, spiral-wave-like global structures, and magnetically active regions associated with dynamo action and/or Balbus-Hawley instabilities.

Coupling an Advanced Land Surface–Hydrology Model with the Penn State–NCAR MM5 Modeling System. Part II: Preliminary Model Validation

A number of short-term numerical experiments conducted by the Penn State‐NCAR fifth-generation Mesoscale Model (MM5) coupled with an advanced land surface model, alongside the simulations coupled with a simple slab model, are verified with observations. For clear sky day cases, the MM5 model gives reasonable estimates of radiation forcing at the surface with solar radiation being slightly overestimated probably due to the lack of aerosol treatment in the current MM5 radiation scheme. The improvements in the calculation of surface latent and sensible heat fluxes with the new land surface model (LSM) are very apparent, and more importantly, the new LSM captures the correct Bowen ratio. Evaporation obtained from the simple slab model is significantly lower than observations. Having time-varying soil moisture is important for capturing even short-term evolution of evaporation. Due to the more reasonable diurnal cycle of surface heat fluxes in the MM5‐LSM, its nearsurface temperature and humidity are closer to the FIFE observations. In particular, the MM5‐slab model has a systematic warm bias in 2-m temperature. Both the slab model and the new LSM were coupled with the nonlocal Medium-Range Forecast model PBL parameterization scheme and they reproduced the depth of the morning surface inversion in the stable boundary layer well. The observed mixed layer in the late morning deepens faster than both models, despite the fact that both models have high bias in surface sensible heat fluxes. Presumably, such a rapid development of convective mixed layer is due to some effects induced by small-scale heterogeneity or large-scale advection that the MM5 failed to capture. Both surface models reasonably reproduce the daytime convective PBL growth and, in general, the temperature difference between the two models and observations is less than 28. The simulations of two rainfall events are not conclusive. Both models produce a good forecast of rainfall for 24 June 1997 and have similar problems for the event of 4 July 1997, although the simulations with the new LSM have slightly improved results in some 3-h rainfall accumulations. It seems that the new LSM does not have unexpected influences in situations for which the land surface processes are secondary, but that it may have subtle, though complex, effects on the model behavior because of heterogeneity introduced by soil moisture, vegetation effects, and soil type, which are all lacking in the slab model.

Modelling Regional Scale Surface Energy Exchanges And Cbl Growth In A Heterogeneous, Urban-Rural Landscape

Over the last decade, simple models of theconvective boundary layer (CBL) have beensuggested as an approach to inferring regionallyaveraged land-air exchanges of heat, water and tracegases, because the properties of the CBL respond toan average of the underlying small-scaleheterogeneity. This paper explores the use of anintegral CBL method to infer regionally averagedfluxes in a landscape that has at least three majorsources of heterogeneity – irrigated andnon-irrigated rural land use and a large urban area(Sacramento region, California). The first part of the paper assesses the validity ofthe simple slab model of the CBL – this isintegrated forwards in time using local-scalemeasured heat and water vapour fluxes, to predictmixed-layer depth, temperature and humidity. Of thefour different CBL growth schemes used, the Tennekesand Driedonks model is found to give the bestperformance. Evaluation of the model performancewith different weightings of heat and water vapourfluxes based on the land use characteristics in theregion suggest that the source area for theboundary-layer sonde measurements is larger thanphysically-based estimates would suggest. Finally, measured time series of potentialtemperature are used to infer regionally averagedsensible heat fluxes using an integral CBL (ICBL)method. These ICBL fluxes are compared with thosemeasured at the local scale over the three land usetypes that comprise the region of interest. They arefound to be closest to the heat fluxes calculated byappropriately weighting the measured heat fluxes inthe source area calculated for the ICBL. We concludethat the integral CBL budget method providesadequate estimates of regionally-averaged surfaceheat fluxes in a landscape that is characterised bysurface types with distinctly different surfaceenergy budgets.

Accretion‐induced Lithium Line Enhancements in Classical T Tauri Stars: RW Aurigae

It is widely accepted that much of the stochastic variability of T Tauri stars is due to accretion by a circumstellar disk. The emission-line spectrum as well as the excess continuum emission are common probes of this process. In this communication, we present additional probes of the circumstellar environment in the form of resonance lines of low ionization potential elements. Using a set of 14 high-resolution echelle observations of the classical T Tauri star (CTTS) RW Aur, taken between 1986 and 1996, we carefully measure the continuum veiling at each epoch by comparing more than 500 absorption lines with those of an appropriate template. This allows us to accurately subtract out the continuum emission and to recover the underlying photospheric spectrum. In doing so, we find that selected photospheric lines are enhanced by the accretion process, namely, the resonance lines of Li I and K I. A resonance line of Ti I and a low excitation potential line of Ca I also show weak enhancements. Simple slab models and computed line bisectors lead us to propose that these line enhancements are markers of cool gas at the beginning of the accretion flow which provides an additional source of line opacity. These results suggest that published values of surface lithium abundances of classical T Tauri stars are likely to be overestimated. This would account for the various reports of surface lithium abundances in excess of meteoritic values among the extreme CTTSs. Computing LTE lithium abundances of RW Aur in a low and then high accretion state yields abundances which vary by 1 order of magnitude. The low accretion state lithium abundance is consistent with theoretical predictions for a star of this age and mass, while the high accretion state spectrum yields a supermeteoritic lithium abundance.

Response of the deep ocean internal wave field to traveling midlatitude storms as observed in long‐term current measurements

It has been demonstrated in a recent numerical experiment that double‐inertial frequency internal waves may play a crucial role in diapycnal mixing processes in the deep ocean, with the energy effectively transferred across the internal wave spectrum down to small dissipation scales by nonlinear wave‐wave interactions [Hibiya et al., 1998]. To examine whether or not such double‐inertial frequency waves are actually generated in the real deep ocean, current meter data from long‐term moorings in the northwest Pacific basin are analyzed together with global sea surface wind data. By incorporating the wind data into a simple damped slab model, predominant inertial currents are shown to be excited in the mixed layer in the northwest Pacific basin by traveling midlatitude storms during fall and winter. The multiple filter analysis demonstrates that double‐inertial frequency waves as well as near‐inertial frequency waves are significantly amplified in the deep ocean internal wave field during the periods strong inertial currents are excited in the mixed layer. This suggests that in addition to near‐inertial frequency waves, double‐inertial frequency waves are actually excited by strong atmospheric disturbances through nonlinear effects as demonstrated in the numerical experiment by Niwa and Hibiya [1997]. Double‐inertial frequency waves thus excited seem to propagate over horizontal distances of the order of 1000 km from their source region while feeding their energy to the local internal wave field, consistent with the theoretical prediction based on the magnitudes of group velocity and nonlinear interaction time [Olbers, 1983].

A model of the circulation on the outer Scotian Shelf with open boundary conditions inferred by data assimilation

The circulation on Western Bank is described using data collected in spring 1991 and 1992 as part of an interdisciplinary study of the early life history of Atlantic cod (Gadus morhua). The mean circulation over the crest of the bank is weak, of the order of a few centimeters per second, and is flanked to the south and west by an anticyclonic flow with a speed of about 10 cm s−1. A thermal wind calculation shows the mean circulation is due primarily to horizontal variations in the density field. The M2 tidal constituent is dominant and reaches speeds of about 20 cm s−1. The tidal residuals have a standard deviation of about 10 cm s−1 and characteristic time and length scales of several days and tens of kilometers, respectively. In order to help collect and interpret biological data from Western Bank, we developed a hydrodynamic model of the near‐surface flow that could be used operationally. The model is based on a decomposition of the flow into components driven by (1) local wind stress, (2) horizontal density gradients, and (3) flows through the open boundaries of the model. The wind‐driven component is calculated using a simple slab model driven by the observed wind. The other two components are estimated through the assimilation of observed bottom pressures, dynamic heights, and currents into the hydrodynamic model. The hindcast skill of the model is quantified by cross validation and shown to be higher than that of four simple, statistically based schemes.

The wind-forced response on a buoyant coastal current: Observations of the western Gulf of Maine plume

The freshwater plume in the western Gulf of Maine is being studied as part of an interdisciplinary investigation of the physical transport of a toxic alga. A field program was conducted in the springs of 1993 and 1994 to map the spatial and temporal patterns of salinity, currents and algal toxicity. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman drift dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to as little as 10 km. Using a simple slab model, we find qualitative agreement between the observed variations of plume width and those predicted by Ekman theory for short time scales of integration. Near surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate a dominant Ekman balance within the plume. A significant correlation between alongshore currents and winds suggests that interfacial drag may be important, although inclusion of a Raleigh drag term does not significantly improve the alongshore momentum balance.

Observations of strong inertial oscillations after the passage of Tropical Cyclone Ofa

One week after Tropical Cyclone Ofa passed near Samoa, near‐inertial oscillations of almost 1 m s−1 in amplitude were observed with a shipboard acoustic Doppler current profiler (ADCP). The velocity was nearly uniform with depth in the upper 80 m. The strongest currents in this layer were found about 300 km from Ofa's track, farther than expected from previous observations and models. Below 80 m was a layer 50–100 m thick in which the shear exceeded 0.01 s−1. Strong near‐inertial oscillations, with currents over 0.6 m s−1 from 130 to 200 m, extended through the shear layer to at least 300 m, the limit of the ADCP measurements. The phase of the oscillations propagated upward, consistent with downward propagation of energy. A simple slab model driven by an idealized wind field approximately matches the observed amplitude and phase of the oscillations in the surface layer within 150 km of Ofa's track, but not the stronger current 300 km from the track.

Effects of Interference and Oxidation on the UV/Visible Rejection Properties of Filters for Soft X-ray Detectors

We report on UV/Visible transmission measurements of aluminum coated Lexan filters designed as UV blocking filters for soft x-ray detectors. Transmission of the filters in the 2300-8000 A wavelength range is significantly higher than expected. It cannot be accounted for applying a simple slab model of the transmission and adopting material properties reported in the literature. We show that this is due to interference effects which are strongly dependent on the filter geometry, and to oxidation of exposed aluminum surfaces and/or chemical interaction with the plastic support. The results of this work have led to the redesign of the Advanced X-ray Astrophysics Facility High Resolution Camera UV blocking filters.

Shielding of resonant magnetic perturbations by rotation

Magnetic perturbations that drive islands are shielded from a rotating plasma by either a torque limit or an equilibrium limit. The physics of the shielding is given in a simple slab model along with the mechanism by which the shielding currents are maintained. Shielding currents in a channel of width Δ are damped by a resistance η/Δ with η the resistivity. If an island is shielded in a nearly ideal plasma, η/Δ=(μ0/π)VpAM2p with VpA the poloidal Alfvén speed, Mp≡v⊥/VpA, and v⊥ the speed of the cross-field flow at the resonance surface.

Low-Frequency Variability of Surface Air Temperature in a 1000-Year Integration of a Coupled Atmosphere-Ocean-Land Surface Model

Abstract The study analyzes the variability of surface air temperature (SAT) and sea surface temperature (SST) obtained from a 1000-yr integration of a coupled atmosphere-ocean-land surface model, which consists of general circulation models of the atmosphere and oceans and a heat and water budget model of land surface. It also explores the role of oceans in maintaining the variability of SAT by comparing the long-term integration of the coupled model with those of two simpler models. They are 1 ) a “mixed layer model,” that is, the general circulation model of the atmosphere combined with a simple slab model of the mixed layer ocean, and 2) a “fixed SST model,” that is, the same atmosphere model overlying seasonally varying, prescribed SST. With the exception of the tropical Pacific, both the coupled and mixed layer models are capable of approximately simulating the standard deviations of observed annual and 5-yr-mean anomalies of local SAT. The standard deviation tends to be larger over continents than ...

Young oxygen-rich supernova remnants. 1: Observations of three southern remnants

view Abstract Citations (30) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Young Oxygen-rich Supernova Remnants. I. Observations of Three Southern Remnants Sutherland, Ralph S. ; Dopita, Michael A. Abstract Narrowband imaging and two-dimensional spectrophotometery are combined in a series of observations of N132D, a young, oxygen-rich supernova remnant in the Large Magellanic Cloud. Comparison spectra of two galactic remnants, Pup A and SNR G292.0+ 1.8 are also examined. The observations permit the determination of some important physical parameters of the N132D remnant in particular. An expansion age of 2350 +/- 520 yr is obtained, significantly greater than the ~1300 yr obtained by Lasker (1980). Estimates of the densities and temperatures in the observed oxygen-rich material are made on the basis of simple homogeneous slab models. The models indicate that such simple models are unlikely to be appropriate; nevertheless they do highlight the difficulties of interpretation when the diagnostic plot trends lie in similar directions to that expected from reddening effects. Line fluxes are given in raw form, and estimates of the reddening that may be applicable are given. This forms the first paper in a short series, in the second paper an excitation mechanism for the oxygen-rich material, based on an R-type ionization front model, is presented. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175180 Bibcode: 1995ApJ...439..365S Keywords: Chronology; Density Distribution; Magellanic Clouds; Oxygen; Supernova Remnants; Temperature Profiles; Flux Density; Image Analysis; Spectrum Analysis; Stellar Spectrophotometry; Velocity Distribution; Astronomy; GALAXIES: MAGELLANIC CLOUDS; ISM: SUPERNOVA REMNANTS full text sources ADS | data products SIMBAD (3) NED (1) Related Materials (1) Part 2: 1995ApJ...439..381S

A similarity solution to the kinematic alpha-squared-omega dynamo equation with time-dependent coefficients

The indication of an association between the galactic magnetic field strength and the current star formation rate in some nearby galaxies, and the possible time variation of star formation activities in a galaxy, stimulates studies on a dynamo with time-dependent parameters. A simplified kinematic α 2 w dynamo equation is set up, and a similarity solution to this equation is presented. The temporal variation of the field lies solely in the temporal variation of a length scale of the dynamo. When the solution is applied to a simple slab model, it is found that only even modes exist. An example shows that the time dependence of the field can be very different from a constant parameter kinematic dynamo

Microwave vegetation indexes for detecting biomass and water conditions of agricultural crops

Experimental and theoretical investigations have shown that microwave emission from canopies depends on crop type and physical conditions. However, since the sensitivity of the brightness temperature to vegetation parameters is rather low, accurate observations and suitable models are necessary to extract geophysical parameters from radiometer data. The most significant results, obtained over the past years on the basis of measurements carried out with passive microwave sensors at 10 GHz and 36 GHz, are summarized in this paper. The relations between the microwave vegetation indexes and plant biophysical parameters are discussed, using a simple slab model that considers vegetation as a uniform absorbing and scattering medium upon the soil surface. It is shown that simultaneous observations at 10 GHz and 36 GHz, in V and H polarizations, especially if combined with thermal infrared measurements, are useful in separating different crops and in identifying moisture conditions of the surfaces. The difference ( δT N ), between the normalized temperatures at 36 GHz and 10 GHz, and the polarization index (PI) at 10 GHz are functions of biomass and can be related to the leaf area index of some crops.