UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.

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Author: search.filters.author.Zhou, LeiEnd date: 2009

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    MADDEN-JULIAN OSCILLATION AND SEA SURFACE TEMPERATURE INTERACTIONS IN A MULTI-SCALE FRAMEWORK
    (2009) Zhou, Lei; Murtugudde, Raghu; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The ocean-atmosphere coupling can play a role in initiating and sustaining the Madden-Julian Oscillations (MJOs), which are the major intraseasonal oscillations in the atmosphere. In this thesis, the oceanic influence on MJOs is studied with reanalysis products, numerical models, and idealized theoretical models. The energy sources for MJOs are calculated with NCEP reanalysis. The perturbed potential energy is found to be the most important energy source for most MJO events. In some MJO events, the sea surface is warmed due to the reduced latent heat flux during the suppressed phase of MJOs. As a result, warm sea surface temperature anomalies (SSTAs) occur, which appear to prolong the life time of these MJO events. In a minority of the MJO events, warm SSTAs can drive the atmosphere actively and trigger MJO events. In these events, the warm SSTAs are attributable to the internal oceanic processes influenced by the warm Indonesian Throughflow (ITF), which spreads from the southeastern Indian Ocean to the western Indian Ocean and modifies the subtle balance between stratification and mixing in the western Indian Ocean. In addition, during the transit period between monsoon seasons, a few MJO events are sustained by the energy obtained from the mean kinetic energy. Since the MJO events have different energy sources, their mechanisms should be considered in the context of these energy sources. While the spatial scale of the SSTAs in the Indian Ocean is only of order 100 km, the scale of MJOs is of order 1000 km, raising the potential for interactions between the oceanic and the atmospheric oscillations with different scales and this is demonstrated to be possible with analytical solutions to idealized linear governing equations. With a reasonable choice of parameters, the meso-scale oceanic and the large-scale atmospheric oscillations can interact with each other and lead to unstable waves in the intraseasonal band in this linear coupled model. The coupling and frequency shifts between oscillations with different scales and the atmospheric/oceanic responses to small variations in the external forcing are also tested with numerical models. Incorporating the oceanic influence on MJOs and the multi-scale interaction appropriately in a numerical model is supposed to help improve the simulation and forecast of MJOs. The hypothesis of multi-scale interaction is also expected to have wide applications in other studies, in addition to the MJO-SST interaction. The theoretical and numerical approach adopted here should also serve as a prototype for enhancing the process understanding of intraseasonal variability and lead to improved predictive understanding.
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    DEVELOPMENT OF ION-MOBILITY AND MASS SPECTROMETRY FOR PROBING THE REACTIVITY OF NANOPARTICLES AND NANOCOMPOSITES
    (2009) Zhou, Lei; Zachariah, Michael; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Aerosols of diameter smaller than 100 nm, usually are referred as nanoparticles or ultrafines, have received considerable interests lately as a source of building blocks to novel materials. However, our capabilities for charactering these materials are greatly limited by lack of appropriate diagnostic tools. The objective of this work is to develop new aerosol-based techniques for the characterization of nanoparticles and nanocomposites. The scope of this dissertation can be categorized in two ways. First, to provide knowledge of just how reactive a material is, we develop particle ion-mobility spectrometry and Single Particle Mass Spectrometry methods to probe the intrinsic size-dependent reactivity of individual metal particles. And second, the development of a new Time-of-Flight mass spectrometer (TOFMS) combined with a temperature jump (T-Jump) technique to study particle-particle reaction, and probe the reactivity of nanocomposite materials under combustion-like condition.
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    The Value of Security Audits, Asymmetric Information and Market Impact of Security Breaches
    (2004-08-10) Zhou, Lei; Gordon, Lawrence A.; Loeb, Martin P.; Accounting and Information Assurance; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation includes two essays on the economic aspects of information security. The first essay presents a principal-agent model for assessing the value of information security audits. The issue of information security investments is confounded by control problems arising from asymmetric information and conflicting managerial interests within the firm. By analyzing the impacts of asymmetric information and security audits, this study extends the literature in three ways. First, the degree of information asymmetry is formally measured, which allows one to study how different levels of information asymmetry affect information security investment decisions. Second, the intensity of an information security audit is explicitly modeled, and the interactions between information asymmetry and security audits are examined. This analysis provides conditions under which the benefit from security audits increases with the degree of information asymmetry. Third, the current research provides an analytic model that helps to explain existing empirical findings (e.g., Gordon and Smith, 1992) concerning the relation between information asymmetry and the value of audits. The second essay examines the economic costs of publicly announced information security breaches. Similar to Campbell et al. (2003), the current study applies the event study approach, but uses a larger sample and a more sophisticated market model (Fama and French, 1993). The results confirm those of Campbell et al. (2003) that security breaches involving confidential information cause significant market reactions and security breaches not involving confidential information only cause insignificant market reactions. Further investigations also suggest that the insignificance of market reactions to non-confidential events does not seem to vary with the nature of those events.