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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Numerical modelling of the Angola Low and the Botswana High during a neutral and two El Ni˜no summers

Morake, Dedricks Monyai 06 February 2019 (has links)
The Angola Low and Botswana High pressure systems are thought to play a crucial role in the variability of summer rainfall over southern Africa. However, very little is known about their variability during the summer half of the year and how their influence on rainfall patterns during ENSO and non-ENSO summers may vary. In simple terms, a weaker Angola Low is expected to lead to decreased rainfall as is a stronger Botswana High. This study looks at the monthly evolution of the Angola Low and the Botswana High during the neutral summer of 2012/13 and the two strong El Ni˜no summers of 1997/98 and 2015/16 using the WRF model. CFSR and CFSv2 reanalyses, satellite derived winds, GPCC rainfall and TRMM satellite-derived rainfall estimates are used to validate the model. The model was integrated from September through to April for each run with observed sea surface temperature and reanalyses as boundary conditions. During the neutral summer of 2012/13, the Angola Low became clearly evident in the model during the pentad of 6-10 October whereas during the 1997/98 and 2015/16 El Ni˜no summers, it became evident during the pentads of 6-10 November and 16-20 November respectively. In addition to these differences in onset, there were also differences in the date after which the Low was no longer present in the model fields. These dates were 26-28 February 2013 and 26-31 March 1998 while in the 2015/16 case, the Low remained present throughout the whole of March. In each year, the Botswana High was present throughout the entire summer half of the year. The WRF simulation indicated that during the strong El Ni˜no event of 1997/98, the Angola Low did not weaken whereas the Botswana High was weaker than normal. However, during the strong El Ni˜no event of 2015/16, the Angola Low was weaker and the Botswana High was relatively strong. The strengthening of the Angola Low and the weakening of the Botswana High during the strong 1997/98 El Ni˜no led to substantial rainfall over southern Africa. The near to above average rainfall over subtropical southern Africa during 1997/98 was unexpected given the strength of the El Ni˜no and the SST anomalies in the Indian and Pacific Oceans. The weaker Angola Low and stronger Botswana High during the strong 2015/16 El Ni˜no led to severe drought over the region. The study highlights the importance of modulations in the Angola Low and the Botswana High for rainfall anomalies during ENSO and non-ENSO summers as very different rainfall patterns may occur over southern Africa during similar strength ENSO events. The significance of these regional circulation systems is reinforced by the fact that during the 2012/13 neutral summer, the Angola Low was stronger than average and the Botswana High was relatively weak leading to good rainfall. The relationship between the Angola Low, the Botswana High and southern Africa rainfall is found to be relatively strong through the 1979-2017 period. Thus, monitoring and better understanding these regional circulation systems is important and complements ongoing efforts to monitor and predict ENSO.
2

Comparative Numerical Modelling of Tsunami Propagation

Geraghty, Beth Freya January 2006 (has links)
This thesis uses numerical simulations to assess the most suitable model type for simulating dispersive and non dispersive tsunami wave propagation over a range of bathymetries. These simulations are presented in two parts. The first part highlights differences between results as predicted by a fully nonlinear Boussinesq model (with its ability to predict dispersion) and a non dispersive, linear or weakly nonlinear model, for simulations of a dispersive wave incident at various idealized bathymetric features. The second part determines the efficacy in a real world application of the Boussinesq model as opposed to a nonlinear shallow water model. In addition, a discussion on the geophysical parameters which influence the choice of numerical model for simulating tsunami propagation in a particular bathymetric region is provided.
3

A coupled physical-biological model for the Clyde Sea

Lee, Jae-Young January 2002 (has links)
No description available.
4

Mathematical modelling and analysis of some problems in parasitology and ecology

Wilmott, S. January 1987 (has links)
No description available.
5

Local Source Tsunami Inundation Modelling for Poverty Bay, Gisborne

Henderson, David Ronald January 2008 (has links)
After the Boxing Day 2004 Sumatran Tsunami, a review of tsunami hazard and risk for New Zealand identified Gisborne as the urban area with the greatest risk. Gisborne could experience gt;500 fatalities and extensive damage to infrastructure during a severe tsunami. The severity of a tsunami is likely to be low for distance sources given the effectiveness of the Pacific Tsunami Warning System. However, there is a substantial risk from local sources, as no local warning system of any kind exists. Prompt evacuation is probably the most cost-effective tsunami mitigation strategy available for New Zealand coastal locations, including Gisborne. This requires both knowledge of the extent of tsunami inundation, and sufficient warning of the tsunami arrival. Hence, there are two main objectives for this investigation: 1. Determine the likely extent of tsunami inundation for Gisborne City and surrounding populated coastal locations in Poverty Bay, using a combination of hydrodynamic tsunami modelling and GIS. The modelling will simulate historical events, particularly the largest historical tsunami, the May 1947 local tsunami. Modelling will consider potential events based on the Maximum Credible Earthquake for local sources associated with the Hikurangi Deformation Front. 2. Create inundation maps of Poverty Bay that can be used for future town planning and emergency plans.
6

Modelling the impact of surface melt on the hydrology and dynamics of the Greenland Ice Sheet

Koziol, Conrad Pawel January 2018 (has links)
Increasing surface runoff from the Greenland Ice Sheet due to a warming climate not only accelerates ice mass loss by altering surface mass balance, but may also lead to increased dynamic losses. This is because surface melt draining to the bed can reduce ice-bed coupling, leading to faster ice flow. Understanding the impact of surface melt on ice dynamics is important for constraining the contribution of the Greenland Ice Sheet to sea level rise. The aim of this thesis is to numerically model the influence of surface runoff on ice velocities. Three new models are presented: an updated supraglacial hydrology model incorporating moulin and crevasse drainage, along with lake drainage over the ice surface via channel incision; an ice sheet model implementing a numerically efficient formulation of ice flow; an adjoint code of the ice flow model based on automatic differentiation. Together with a subglacial hydrology model, these represent the key components of the ice sheet system. The supraglacial hydrology model is calibrated in the Paakitsoq region. Model output shows the partitioning of melt between different drainage pathways and the spatial distribution of surface drainage. Melt season intensity is found to be a relevant factor for both. A key challenge for simulations applying a coupled ice-flow/hydrology model is state and parameter initialization. This challenge is addressed by developing a new workflow for incorporating modelled subglacial water pressures into inversions of basal drag. A current subglacial hydrology model is run for a winter season, and the output is incorporated into the workflow to invert for basal drag at the start of summer in the Russell Glacier area. Comparison of the modelled subglacial system to observations suggests that model output is more in line with summer conditions than winter conditions. A multicomponent model integrating the main components of the ice sheet system is developed and applied to the Russell Glacier area. A coupled ice-flow/hydrology model is initialized using the proposed workflow, and driven using output from the supraglacial hydrology model. Three recent melt seasons are modelled. To a first order, predicted ice velocities match measured velocities at multiple GPS sites. This affirms the conceptual model that summer velocity patterns are driven by transitions between distributed and channelized subglacial hydrological systems.
7

Pattern formation through self-organisation in diffusion-driven mechanisms

Jenkins, Michael John January 1990 (has links)
No description available.
8

Hybrid-Kinetic Modelling of Space Plasma with Application to Mercury

Paral, Jan Unknown Date
No description available.
9

The investigation of transmission-line matrix and finite-difference time-domain methods for the forward problem of ground probing radar

Giannopoulos, Antonios January 1998 (has links)
No description available.
10

Effect of mesoscale variability of water masses on acoustic wave propagation in a shallow sea

Chen, Feng January 2015 (has links)
Anthropogenic noise in the sea is now classed as a pollutant alongside chemical pollution and marine litter in accordance with the Marine Strategy Framework Directive. Noise from shipping is a major contributor to the ambient noise levels in the ocean, particularly at low (<300Hz) frequencies. The properties of sound propagation in shallow waters are highly influenced by the marine physical environment. Ocean modelling plays an important role in underwater noise studies since it can provide high resolution water column parameters over large geographic areas. This study investigates the noise patterns and their temporal variations in the Celtic Sea by using a coupled ocean model (POLCOMS) and an acoustic model (HARCAM). A method to predict noise exposure experienced by marine animals is then developed, following an application for diving seals. The ocean model is applied in the Celtic Sea to provide high-resolution 3D hourly temperature and salinity fields for the acoustic model. The model is validated against in-situ and satellite observations, giving high skills to simulate the water column structures. Sensitivity studies of modelled results to different atmospheric forcing are carried out in order to improve the accuracy of the model. The results show that the modelled sea surface temperature, stratification and water column structures are highly sensitive to the choice of surface forcing, especially in the summer time. The increase in resolution of surface forcing does not necessarily lead to more accurate results. The tidally frontal position is, however, insensitive to the forcing. The variability of noise propagation is studied using the coupled model, demonstrating high dependence on oceanographic conditions, geographic location of sound source and its depth. In summer, when the source of sound is on the inshore side of the bottom front, the sound energy is mostly concentrated in the near-bottom layer. In winter, the sound from the same source is distributed more evenly in the vertical. When the source is on the seaward side of the front, the sound level from a shallow source is nearly uniform in the vertical and the transmission loss is significantly greater (~16dB at 40km distance) in summer than in winter. In contrast, sound energy from a deep source is trapped in the bottom cold water, leading to a much lower transmission loss (~20dB) in summer than in winter. Note that ~10dB fluctuation of sound energy is found during the deterioration of the thermocline in late autumn. Shallow sources (e.g. ships) are sensitive to the surface heat flux as it changes significantly the vertical temperature gradient, while tides play an important role in determining the TL variability of deeper sources (e.g. pile driving) since they cause adjustments of positions of subsurface fronts. The seasonal noise patterns radiated by a large cargo ship are modelled by relating the AIS ship track data and the coupled model, showing a clear influence of the seasonal thermocline and associated bottom fronts on shipping noise distribution. The noise propagates much further (tens of kilometres) in winter than in summer. The predicted shipping noise exposure perceived by grey seals shows strong step changes in the sound level during their descent/ascent through the water column. Since grey seals tend to be benthic foragers, a hypothesis that the step change in sound exposure may have negative impacts on their foraging behaviour is proposed for biological specialists.

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