Thesis (M.Eng.)--Memorial University of Newfoundland, 2004. / Bibliography: leaves 88-92.

published_or_final_version / abstract / Mechanical Engineering / Master / Master of Philosophy

Models of tephra fallout are used to assess volcanic hazards in advance of eruptions and in near-real-time. Current models often approximate the wind field using simplistic assumptions of the atmosphere that cannot account for typical variations in wind velocity that occur in time and three-dimensional space. Here, a widely used mesoscale atmospheric model is used to improve forecasts of the location of the major axis of dispersion for erupting plumes. The Pennsylvania State University-National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) specializes in atmospheric prediction for regions on the order of ten to hundreds of kilometers on a side. MM5 generates realistic wind fields based on the laws of conservation of mass, energy, and momentum, along with land surface data and atmospheric forecasts and observations.

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

No abstract available. / Thesis (Ph.D.)-University of Natal, Durban, 1985.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1960. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Bibliography: leaf 71.

Thesis (M. Phil.)--University of Hong Kong, 1991.

In the study of the dispersion of contaminants in the atmosphere by computational simulations, the Eulerian approach, based in the diffusionadvection equation, is largely employed. In these simulations, concentration values are calculated at each point of a fixed grid in space and time and for this to be done it is necessary to determine an appropriated eddy diffusivity to the planetary boundary layer under consideration. The aim of this work is to describe the decay of turbulence in the residual layer and to derive the corresponding eddy diffusivity for the case of a homogeneous (in the horizontal plane) and isotropic turbulent flow neglecting mechanical and thermal production effects. The decay of turbulence is described as a process in which eddies of a certain size loss their energy by the effect of an eddy viscosity associated with the smallest eddies through a cascade of interactions of local character in the sense of Heisenberg s model. The eddy diffusivities are calculated by considering the dispersive effects of the energy-containing eddies on the basis of Taylor s statistical diffusion model. Considering the hypothesis that the law of decay of the energy spectral density function associated with the ith axis of the reference system is similar to the law of decay of the tridimensional energy spectral density function, the corresponding variance of the turbulent part of the velocity and the corresponding eddy diffusivity are calculated. These quantities, in particular for the vertical axis, are compared with results of large eddy simulations (LES) showing very good agreement. Finally, incorporating the calculated eddy diffusivity in a Eulerian model, the effect of turbulence in the vertical dispersion of an passive scalar contaminant in the residual layer is described for the special case of a instantaneous bidimensional emission source (area source) at a certain height over land and zero mean wind velocity. / No estudo da difusão de um contaminante na atmosfera por meio de simulações computacionais é bastante utilizada a perspectiva euleriana, baseada na equação da difusão-advecção. Nessas simulações, os valores de concentração são calculados em cada ponto de um retículo fixo no espaço e no tempo e para que isso possa ser feito, é necessário determinar a difusividade turbulenta (coeficiente de difusão turbulento) para o tipo de camada limite planetária em questão. O objetivo desse trabalho é descrever o decaimento da turbulência na camada residual e derivar a correspondente difusividade turbulenta no caso de um escoamento turbulento homogêneo (no plano horizontal) e isotrópico, desprezando os efeitos mecânicos de cisalhamento e térmicos das correntes de convecção. O decaimento da turbulência é descrito como um processo em que os turbilhões de certo tamanho perdem energia por efeito de uma viscosidade cinemática turbulenta associada aos turbilhões menores através de interações em cascata de caráter local nos moldes do modelo de Heisenberg. A difusividade turbulenta é calculada pelos efeitos dispersivos dos turbilhões energéticos com base no modelo estatístico de Taylor. Com a hipótese de que o decaimento da função densidade espectral de energia unidimensional associada ao i-ésimo eixo do sistema de referência é similar à lei de decaimento da função densidade espectral tridimensional, calcula-se a correspondente variância da componente turbulenta da velocidade e a correspondente difusividade turbulenta e essas grandezas associadas ao eixo vertical, em particular, são comparadas com resultados obtidos por simulação numérica de grandes turbilhões (LES) mostrando uma concordância muito boa. Finalmente, incorporando a difusividade turbulenta calculada num modelo euleriano, descreve-se o efeito da turbulência na dispersão vertical de um contaminante escalar passivo na camada residual no caso especial de uma fonte bidimensional (fonte área) com emissão instantânea situada a uma certa altura acima do solo e na ausência de vento.

No abstract available. / Thesis (Ph.D.)-University of Natal, Durban, 1986.

This thesis aims to improve the understanding of transport and critical layer mixing in the troposphere and stratosphere. A dynamical approach is taken based on potential vorticity which has long been recognised as the essential field inducing the flow and thermodynamic structure of the atmosphere. Within the dynamical framework of critical layer mixing of potential vorticity, three main topics are addressed. First, an idealised model of critical layer mixing in the stratospheric surf zone is examined. The effect of the shear across the critical layer on the critical layer evolution itself is investigated. In particular it is found that at small shear barotropic instability occurs and the mixing efficiency of the critical layer increases due to the instability. The effect of finite deformation length is also considered which extends previous work. Secondly, the dynamical coupling between the stratosphere and troposphere is examined by considering the effect of direct perturbations to stratospheric potential vorticity on the evolution of midlatitude baroclinic instability. Both zonally symmetric and asymmetric perturbations to the stratospheric potential vorticity are considered, the former representative of a strong polar vortex, the latter representative of the stratospheric state following a major sudden warming. A comparison of these perturbations gives some insight into the possible influence of pre or post-sudden warming conditions on the tropospheric evolution. Finally, the influence of the stratospheric potential vorticity distribution on lateral mixing and transport into and out of the tropical pipe, the low latitude ascending branch of the Brewer-Dobson circulation, is investigated. The stratospheric potential vorticity distribution in the tropical stratosphere is found to have a clear pattern according to the phase of the quasi-biennial oscillation (QBO). The extent of the QBO influence is quantified, by analysing trajectories of Lagrangian particles using an online trajectory code recently implemented in the Met Office's Unified Model.