Graduation date: 1994

One method of estimating the longwave radiative heating of the atmosphere is to combine satellite observations of emitted radiances with those computed from synoptic temperature and humidity profiles. Modeled and observed radiances are brought into agreement by altering cloud properties or even by adjusting the temperature and water vapor profiles. Here this strategy is examined in an exploratory study using global meteorological data sets and a radiative transfer model typical of those found in general circulation models. Calculated radiances are compared to those observed by the Earth Radiation Budget Satellite (ERBS). Input for the model is obtained from the National Meteorological Center (NMC) in the form of vertical profiles of temperature and relative humidity. The comparisons are limited to clear sky as deduced by ERBE algorithms, and additional filtering which requires homogeneous surface type for a 3 x 3 array of ERBS scanner fields of view. Observations are obtained from 60° N to 60° S that lie within 30 minutes of the NMC analysis time. Following the work of Ramanathan and Downey (1986), comparisons are separated into climatologically distinct groups as well as by satellite viewing angle. This separation is an attempt to distinguish between biases in the radiation model and those in the NMC data set. Results are presented for the months of July 1985, and January 1986. A comparison of the present radiation model's output with that obtained from a Geophysical Fluid Dynamics Laboratory (GFDL) model shows a bias of nearly 3% in the present model for a standard mid-latitude summer profile. Global results show a negative bias in the modeled values for nearly all scenes, except for nighttime desert. The nighttime desert bias may be a result of a skin-air temperature difference not resolved by the NMC analyses. The overall negative bias may be a result of an overestimation of water vapor for regions with low relative humidity. / Graduation date: 1991

The atmospheric radiative transfer model MOCARAT was developed and is presented in this thesis. MOCARAT employs a Monte Carlo Technique for the accurate modeling of band radiances and reflectances in an atmospheric system with a ruffled ocean surface as a lower boundary. The atmospheric radiative transfer is modeled with consideration of molecular Rayleigh scattering, Mie Scattering and absorption on particulate matter, as well as band absorption by molecules in the wavelength channels of interest. The bidirectional reflection of downwelling light at the ocean surface is computed using the empirical relationship between surface wind field and the slope distribution of wave facets derived by Cox and Munk (1954a). A method is proposed to use the oceanic sun glint for remote sensing applications. The sensitivity of channel correlations to aerosol burden and type as well as other atmospheric and observational parameters is assessed. Comparisons of observed correlations with model results are used to check the consistency of the calibration of the airborne Multichannel Cloud Radiometer (MCR) that was employed during the Indian Ocean Experiment (INDOEX). The MCR calibration exhibited large variability from flight to flight. The method was applied to MODIS observations. Unlike the MCR, MODIS was stable where expected, although numerical values for some of the wavelengths appear to depart from theory. / Graduation date: 2004

For over 30 years, the Implicit Monte Carlo (IMC) method has been used to solve challenging problems in thermal radiative transfer. These problems are typically optically thick and di ffusive, as a consequence of the high degree of "pseudo-scattering" introduced to model the absorption and reemission of photons from a tightly-coupled, radiating material. IMC has several well-known features which could be improved: a) it can be prohibitively computationally expensive, b) it introduces statistical noise into the material and radiation temperatures, which may be problematic in multiphysics simulations, and c) under certain conditions, solutions can be unphysical and numerically unstable, in that they violate a maximum principle - IMC calculated temperatures can be greater than the maximum temperature used to drive the problem. We have developed a variant of IMC called "iterative thermal emission" IMC, which is designed to be more stable than IMC and have a reduced parameter space in which the maximum principle is violated. ITE IMC is a more implicit method version of the IMC in that it uses the information obtained from a series of IMC photon histories to improve the estimate for the end of time-step material temperature during a time step. A better estimate of the end of time-step material temperature allows for a more implicit estimate of other temperature dependent quantities: opacity, heat capacity, Fleck Factor (probability that a photon absorbed during a time step is not reemitted) and the Planckian emission source. The ITE IMC method is developed by using Taylor series expansions in material temperature in a similar manner as the IMC method. It can be implemented in a Monte Carlo computer code by running photon histories for several sub-steps in a given time-step and combining the resulting data in a thoughtful way. The ITE IMC method is then validated against 0-D and 1-D analytic solutions and compared with traditional IMC. We perform an in finite medium stability analysis of ITE IMC and show that it is slightly more numerically stable than traditional IMC. We find that significantly larger time-steps can be used with ITE IMC without violating the maximum principle, especially in problems with non-linear material properties. We also compare ITE IMC to IMC on a two-dimensional, orthogonal mesh, x-y geometry problem called the "crooked pipe" and show that our new method reproduces the IMC solution. The ITE IMC method yields results with larger variances; however, the accuracy of the solution is improved in comparison with IMC, for a given choice of spatial and temporal grid. / Graduation date: 2013

A two-dimensional model for predicting the rate of radiation heat transfer for the interior of an industrial furnace is described. The model is two-dimensional due to the assumptions of the heat source as an infinite radiating plane and the heat sink as rows of parallel tubes that are both infinite in length and in number. A refractory back wall, located behind the tube rows, is also included in some of the model configurations. The optical properties for the heat source, heat sink, and refractory back wall are simplified by assuming the "black-body" case: all are treated as perfect absorbers and emitters of radiation. This assumption allows three different solution techniques-a graphical, crossed-string, and numerical method-to be used in solving for the radiant transfer rate. The numerical method, an innovative Monte Carlo technique, is the one employed in this study. Hottel used a graphical technique to solve the furnace model for a two row configuration in which the tubes are arranged on equilateral triangular centers. His results, along with those produced by the crossed-string method, are used in this work to validate the numerical technique. Having been validated, the numerical method was then employed to extend Hottel's work by adding more tube rows to the original equilateral triangular configuration and by generalizing the results to isosceles arrangements. Findings of this investigation are summarized in a table that lists the direct view factors for a ten tube row configuration arranged in an equilateral triangular array. Values from this table can be used to solve the transfer rate problem for twenty different cases by assuming a nonconducting refractory back wall. Results for twelve cases are represented graphically in this document The results are used to demonstrate the importance of a refractory back wall on overall radiation absorption. Examinations of the two row and five row cases for an isosceles triangular array indicate that the tabular values can be applied to any isosceles arrangement if the ratio of row separation distance to tube center-to-center distance is 0.7 or greater. / Graduation date: 1995

In this thesis, the parallel performance of the finite element-spherical harmonics (FE-PN) method implemented in the general-purpose radiation transport code EVENT is studied both analytically and empirically. EVENT solves the coupled set of space-angle discretized FE-PN equations using a parallel block-Jacobi domain decomposition method. As part of the analytical study, the thesis presents complexity results for EVENT when solving for a 3D criticality benchmark radiation transport problem in parallel. The empirical analysis is concerned with the impact of the main algorithmic factors affecting performance. Firstly, EVENT supports two solution strategies, namely MOD (Moments Over Domains) and DOM (Domains Over Moments), to solve the transport equation in parallel. The two strategies differ in the way they solve the multi-level space-angle coupled systems of equations. The thesis presents empirical evidence of which of the two solution strategies is more efficient. Secondly, different preconditioners are used in the Preconditioned Conjugate Gradient (PCG) inside EVENT. Performance of EVENT is compared when using three preconditioners, namely diagonal, SSOR(Symmetric Successive Over-Relaxation) and ILU. The other two factors, angular and spatial resolutions of the problem affect both the performance and precision of EVENT. The thesis presents comparative results on EVENTs performance as these two resolutions are increased. From the empirical performance study of EVENT, a bottleneck is identified that limits the improvement in performance as number of processors used by EVENT is increased. In some experiments, it is observed that uneven assignment of computational load among processors causes a significant portion of the total time being spent in synchronization among processors. The thesis presents two indicators that identify when such inefficiency occur; and in such a case, a load rebalancing strategy is applied that computes a new partition of the problem so that each partition corresponds to equal amount of computational load.

A general finite difference discretization of the time-dependent radiation transport equation is developed around the framework of an existing steady-state three dimensional radiation transport solver based on the slice-balance approach. Three related algorithms are outlined within the general finite difference scheme: an explicit, an implicit, and a semi-implicit approach. The three algorithms are analyzed with respect to the discretizations of each element of the phase space in the transport solver. The explicit method, despite its small computational cost per time step, is found to be unsuitable for many purposes due to its inability to accurately handle rapidly varying solutions. The semi-implicit method is shown to produce results nearly as reliable as the fully implicit solver, while requiring significantly less computational effort.

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Monte Carlo radiation transfer provides an efficient modelling tool for probing the dusty local environment of young stars. Within this thesis, such theoretical models are used to study the disk structure of objects across the mass spectrum - young low mass Brown Dwarfs, solar mass T-Tauri stars, intermediate mass Herbig Ae stars, and candidate B-stars with massive disks. A Monte Carlo radiation transfer code is used to model images and photometric data in the UV - mm wavelength range. These models demonstrate how modelling techniques have been updated in an attempt to reduce the number of unknown parameters and extend the diversity of objects that can be studied.

Hawking radiation, despite its presence in theoretical physics for over thirty years, remains elusive and undetected. It also suffers, in its original context of gravitational black holes, from conceptual difficulties. Of particular note is the trans-Planckian problem, which is concerned with the apparent origin of the radiation in absurdly high frequencies. In order to gain better theoretical understanding and, it is hoped, experimental verification of Hawking radiation, much study is being devoted to systems which model the spacetime geometry of black holes, and which, by analogy, are also thought to emit Hawking radiation. These analogue systems typically exhibit dispersion, which regularizes the wave behaviour at the horizon but does not lend itself well to analytic treatment, thus rendering Hawking’s prediction less secure. A general analytic method for dealing with Hawking radiation in dispersive systems has proved difficult to find. This thesis presents new numerical and analytic results for Hawking emission spectra in dispersive systems. It examines two black-hole analogue systems: it begins by introducing the well-known acoustic model, presenting some original results in that context; then, through analogy with the acoustic model, goes on to develop the lesser-known fibre-optical model. The following original results are presented in the context of both of these models: • an analytic expression for the low-frequency temperature is found for a hyperbolic tangent background profile, valid in the entire parameter space; it is well-known that the spectrum is approximately thermal at low frequencies, but a universally valid expression for the corresponding temperature is an original development; • an analytic expression for the spectrum, valid over almost the entire frequency range, when the velocity profile parameters lie in the regime where the low-frequency temperature is given by the Hawking prediction; previous work has focused on the low-frequency thermal spectrum and the characterization of the deviations from thermality, rather than a single analytic expression; and • a new unexplored regime where no group-velocity horizon exists is examined; the Hawking spectra are found to be non-zero here, but also highly non-thermal, and are found, in the limit of small deviations, to vary with the square of the maximum deviation; the analytic expression for the case with a horizon is found to carry over to this new regime, with appropriate modifications. Furthermore, the thesis examines the results of a classical frequency-shifting experiment in the context of fibre-optical horizons. The theory of this process is presented for both a constant-velocity and a constantly-decelerating pulse, the latter case taking account of the Raman effect. The resulting spectra are at least qualititively explained, but there is a discrepancy between theory and experiment that has not yet been accounted for.