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.
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Mixed Organic Surfactant Effects on Cloud Condensation Nuclei(2021) Mitchell, Ian Wallace; Asa-Awuku, Akua; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atmospheric aerosols affect Earth’s radiative budget through direct and indirect effects. The direct effects are well understood but the indirect effects have large uncertainty associated with them. Uncertainty is so great that even the sign of the radiative forcing associated with indirect effects is questioned. This work examines aerosol indirect behavior by assessing surfactant effects on the activation of aerosol particles into cloud droplets. Szyszkowski-Langmuir surface tension models are applied to Köhler theory to capture surfactant effects on aerosol activation behavior. Surfactant aerosols tested are succinic acid and sodium dodecyl sulfate (SDS). Results suggest that a small addition of surface active material (like SDS) to organic carboxylic acids (like succinc acid) can significantly change droplet activation behavior.Item THE PYROCONVECTIVE PATHWAY FOR STRATOSPHERIC WATER VAPOR AND AEROSOL(2019) Kablick III, George; Li, Zhanqing; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A detailed analysis of pyrocumulonimbus (pyroCb) cases is presented that explores their convective dynamics, stratospheric plume characteristics and down-stream radiative effects. Satellite observations in conjunction with ground station data and radiative transfer models are used to quantify the impact that pyroCbs have on localized stratospheric aerosol and water vapor. The initial meteorological and fire conditions are explored using a cloud- and aerosol-resolving model to determine the dominant mechanics driving the convection and their effects on microphysics. Results show that intense sensible heat fluxes are the dominant convection trigger over a wildfire in an unstable atmosphere. Direct observations by cloud profiling radar of the active convective stage of a pyroCb are analyzed for the first time, and comparisons with non-pyro meteorological deep convection in the same vicinity and season show that the pyroCb has an extreme delay in the growth of precipitation-sized cloud droplets to altitudes above the homogeneous freezing level.Stratospheric aerosol plume morphology is analyzed for several cases, and an empirical heat accumulation efficiency model is developed to describe observed radiatively-induced self-lofting in the stratosphere. The model results suggest pyroCb aerosol plumes are ∼ 30% efficient at converting shortwave radiative heating into sensible heating, thereby driving buoyant uplift once injected into the stratosphere. PyroCbs directly inject H2O vapor into the stratosphere, which is shown to be significantly large for two separate cases. The cloud-resolving model confirms a previous hypothesis that uniquely small ice particle microphysics can enhance stratospheric H2O in detrained convective anvils. Satellite retrieval evidence suggests plume water vapor anomalies are a result of inefficient removal of small ice particles within the detrained pyroCb anvil. Model-injected total water—represented as the sum of all ice and absolute humidity—shows at least 30% of H2O survives the convective detrainment stage, and diminishes within the evolving plume over the observation period when using satellite observations of H2O as a benchmark. In the plumes presented herein, pyroCb H2O anomalies are as large as 4±3 ppmv above the background in the lower stratosphere. Detailed line-by-line radiative transfer simulations suggest that these anomalies produce an instantaneous longwave radiative forcing up to +1.0 W m −2 at the tropopause.Item DECOUPLING CONSISTENCY DETERMINATION AND TRUST FROM THE UNDERLYING DISTRIBUTED DATA STORES(2018) Lekakis, Vasileios; Keleher, Peter J; Computer Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Building applications on cloud services is cost-effective and allows for rapid development and release cycles. However, relying on cloud services can severely limit applications’ ability to control their own consistency policies, and their ability to control data visibility during replication. To understand the tension between strong consistency and security guarantees on one hand and high availability, flexible replication, and performance on the other, it helps to consider two questions. First, is it possible for an application to achieve stricter consistency guarantees than what the cloud provider offers? If we solely rely on the provider service interface, the answer is no. However, if we allow the applications to determine the implementation and the execution of the consistency protocols, then we can achieve much more. The second question is, can an application relay updates over untrusted replicas without revealing sensitive information while maintaining the desired consistency guarantees? Simply encrypting the data is not enough. Encryption does not eliminate information leakage that comes from the meta-data needed for the execution of any consistency protocol. The alternative to encryption—allowing the flow of updates only through trusted replicas— leads to predefined communication patterns. This approach is prone to failures that can cause partitioning in the system. One way to answer “yes” to this question is to allow trust relationships, defined at the application level, to guide the synchronization protocol. My goal in this thesis is to build systems that take advantage of the performance, scalability, and availability of the cloud storage services while, at the same time, bypassing the limitations imposed by cloud service providers’ design choices. The key to achieving this is pushing application-specific decisions where they belong: the application. I defend the following thesis statement: By decoupling consistency determination and trust from the underlying distributed data store, it is possible to (1) support application-specific consistency guarantees; (2) allow for topology independent replication protocols that do not compromise application privacy. First I design and implement Shell, a system architecture for supporting strict consistency guarantees over eventually consistent data stores. Shell is a software layer designed to isolate consistency implementations and cloud-provider APIs from the application code. Shell consists of four internal modules and an application store, which together abstract consistency-related operations and encapsulate communication with the underlying storage layers. Apart from consistency protocols tailored to application needs, Shell provides application-aware conflict resolution without relying on generic heuristics such as the “last write wins.” Shell does not require the application to maintain dependency-tracking in- formation for the execution of the consistency protocols as other existing approaches do. I experimentally evaluate Shell over two different data-stores using real-application traces. I found that using Shell can reduce the inconsistent updates by 10%. I also measure and show the overheads that come from introducing the Shell layer. Second, I design and implement T.Rex, a system for supporting topology-independent replication without the assumption of trust between all the participating replicas. T.Rex uses role-based access control to enable flexible and secure sharing among users with widely varying collaboration types: both users and data items are assigned roles, and a user can access data only if it shares at least one role. Building on top of this abstraction, T.Rex includes several novel mechanisms: I introduce role proofs to prove role membership to others in the role without leaking information to those not in the role. Additionally, I introduce role coherence to prevent updates from leaking across roles. Finally, I use Bloom filters as opaque digests to enable querying of remote cache state without being able to enumerate it. I combine these mechanisms to develop a novel, cryptographically secure, and efficient anti-entropy protocol, T.Rex-Sync. I evaluate T.Rex on a local test-bed, and I show that it achieves security with modest computational and storage overheads.