Browsing by Subject "Stratification"
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Item DEVELOPMENT OF A COASTAL MARGIN OBSERVATION AND ASSESSMENT SYSTEM (CMOAS) TO CAPTURE THE EPISODIC EVENTS IN A SHALLOW BAY(2010-07-14) Islam, Mohammad S.Corpus Christi Bay (TX, USA) is a shallow wind-driven bay which is designated as a National Estuary due to its impact on the economy. But this bay experiences periodic hypoxia (dissolved oxygen <2 mg/l) which threatens aerobic aquatic organisms. Development of the Coastal Margin Observation and Assessment System (CMOAS) through integration of real-time observations with numerical modeling helps to understand the processes causing hypoxia in this energetic bay. CMOAS also serves as a template for the implementation of observational systems in other dynamic ecosystems for characterizing and predicting other episodic events such as harmful algal blooms, accidental oil spills, sediment resuspension events, etc. State-of-the-art sensor technologies are involved in real-time monitoring of hydrodynamic, meteorological and water quality parameters in the bay. Three different platform types used for the installation of sensor systems are: 1) Fixed Robotic, 2) Mobile, and 3) Remote. An automated profiler system, installed on the fixed robotic platform, vertically moves a suite of in-situ sensors within the water column for continuous measurements. An Integrated Data Acquisition, Communication and Control system has been configured on our mobile platform (research vessel) for the synchronized measurements and real-time visualization of hydrodynamic and water quality parameters at greater spatial resolution. In addition, a high frequency (HF) radar system has been installed on remote platforms to generate surface current maps for Corpus Christi (CC) Bay and its offshore area. This data is made available to stakeholders in real-time through the development of cyberinfrastructure which includes establishment of communication network, software development, web services, database development, etc. Real-time availability of measured datasets assists in implementing an integrated sampling scheme for our monitoring systems installed at different platforms. With our integrated system, we were able to capture evidence of an hypoxic event in Summer 2007. Data collected from our monitoring systems are used to drive and validate numerical models developed in this study. The analysis of observational datasets and developed 2-D hydrodynamic model output suggests that a depth-integrated model is not able to capture the water current structure of CC Bay. Also, the development of a threedimensional mechanistic dissolved oxygen model and a particle aggregation transport model (PAT) helps to clarify the critical processes causing hypoxia in the bay. The various numerical models and monitoring systems developed in this study can serve as valuable tools for the understanding and prediction of various episodic events dominant in other dynamic ecosystems.Item Effect of instabilities in the buoyancy-driven flow on the bottom oxygen: Applications to the Louisiana Shelf(2009-05-15) Kiselkova, ValeriyaA combination of in situ sampling and numerical modeling was used to investigate the effects of mesoscale (<50 km) circulation patterns and stratification on the evolution of hypoxia on the Louisiana Shelf. Temperature, salinity, and dissolved oxygen concentrations records reveal the presence of an alongshelf meander, which is manifested vertically and horizontally as a wave-like distribution of the properties in the water column. The observations suggest the meander is a ubiquitous characteristic of the shelf with alongshore spatial scale approximately 50 km and less, which is consistent with the locations of sandy shoals along the coast and the local deformation radius. Twelve numerical experiments using an idealized three-dimensional shelf circulation model were performed to evaluate the relative importance of the variable bottom topography and freshwater forcing on the development, evolution, and scales of the dynamic instabilities. The inclusion of the shoals into the bottom topography showed the development of the dynamic instabilities as the flow passed over the shoals and downstream. Introduction of fresh water onto the shelf resulted in greater salinity differences, and, as a consequence in the formation of the dynamically unstable salinity fronts along the plume edge. The combination of the freshwater forcing and shoaling topography produced competing and complex interactions. Six numerical experiments were analyzed in order to investigate the effect of dynamic instabilities on spatial and temporal patterns of dissolved oxygen concentrations along the shelf. Although a linear relationship between Brunt-V?is?l? frequency and dissolved oxygen deficit was expected, a nonlinear loop-like relationship was discovered that reflects the response of biochemical properties to the alongshelf variability of the density field. Comparison of the numerical modeling runs to observations of density and dissolved oxygen concentrations on the Louisiana Shelf reinforces the importance of physical processes such as topographic steering and/or freshwater forcing on the alongshore distribution of physical and biochemical properties. It suggests that the time scales of respiration (~3 days) and buoyancy transfer processes (~5-7 days), associated with the physical processes that are responsible for water column stability and ventilation, are similar to the time scales associated with the benthic respiration rates.Item The efficiency of turbulent mixing in stratified fluids(2010-08) Ebert, Guenther Wolfgang; Swinney, H. L., 1939-; Fink, ManfredMixing is a common feature of stratified fluids. In stratified fluids the density varies with the height. This is true for the most fluids in geophysical environments, like lakes, the atmosphere or the ocean. Turbulent mixing plays a crucial role for the overall energy budget of the earth and has therefore an huge impact on the global climate. By introducing the mixing efficiency, it is possible to quantify mixing. It is defined as the ratio of gain of potential energy to the injection of mechanical energy. In the ocean energy provided by tidal forces leads to turbulence and thus highly dense water is lifted up from the deep sea to the surface. For this process, a mixing efficiency of 0.2 is estimated. Until now it is not completely understood how this high value can be achieved. Thus we measured the mixing efficiency by using a Couette-Taylor system, which can produce steady-state homogeneous turbulence. This is similar to what we find in the ocean. The Couette-Taylor system consists of two concentric cylinders that can be rotated independently. In between a stratified fluid is filled using salt as a stratifying agent. In the laboratory experiment, we obtained mixing efficiencies in the order of 0.001 as a result. Moreover we found that the mixing efficiency decreases with decreasing stratification like previous laboratory experiments have shown. As this value is two orders of magnitude smaller than what we find in the ocean, further studies will be necessary.Item Examination of the relationship of river water to occurrences of bottom water with reduced oxygen concentrations in the northern Gulf of Mexico(Texas A&M University, 2007-04-25) Belabbassi, LeilaSix years of comprehensive data sets collected over the northern continental shelf and upper slope of the Gulf of Mexico during the LATEX-A and NEGOM-COH programs showed that low-oxygen waters (<2.4 mL????L-1) are found only in spring and summer and only in water depths between 10 and 60 m. Four regions in the northern Gulf show considerable differences in the occurrence of low-oxygen waters. Lowoxygen waters are observed almost exclusively in regions subject to large riverine influences: the Louisiana and Mississippi-Alabama shelves. Hypoxic waters (oxygen concentrations <1.4 mL????L-1) are found only over the Louisiana shelf. No low-oxygen water is found over the Florida shelf which has minimum riverine influence. Lowoxygen water is found at only one station on the Texas shelf; this is during spring when the volume of low-salinity water is at maximum. The distributions of low-salinity water influenced the different distributions of low-oxygen and hypoxic waters in the four regions. Low-oxygen occurrences are clearly related to vertical stratification. Lowoxygen occurred only in stable water columns with maximum Brunt-V????is????l???? frequency (Nmax) greater than 40 cycles????h-1. When Nmax exceeded 100 cycles????h-1 in summer over the Louisiana shelf, oxygen concentrations dropped below 1.4 mL????L-1, and the bottom waters became hypoxic. Salinity is more important than temperature in controlling vertical stratification. Locations where temperature influence was larger were found in summer in water depth greater than 20 m over the Louisiana shelf, along the near shore areas of the Mississippi-Alabama shelf west of 87????W, and in the inner shelf waters of the Texas shelf. The extent of oxygen removal at the bottom of these stable water columns is reflected in the amount of remineralized silicate. Silicate concentrations are highest closest to the Mississippi River Delta and decrease east and west of the Delta. EOF analyses show that more than 65% of the oxygen variance is explained by the first mode. The amplitude functions of the first EOF modes of bottom oxygen, water column Brunt- V????is????l???? maxima, and bottom silicate are well correlated, indicating that much of the variance in bottom oxygen is explained by water column stratification and bottom remineralization.Item Exit over voice in Dominican ethnoracial politics(2015-12) Contreras, Danilo Antonio; Madrid, Raúl L.,; Philpot, Tasha; Dietz, Henry; Brinks, Daniel; Mahon, JimWhat explains why ethnoracial identity is of low salience in elections in Latin America, particularly in Afro-Latin America? Marginalized individuals in ethnoracially diverse societies, especially stratified ones, would seem most likely to mobilize politically along ethnoracial lines. I argue that, under certain conditions, individuals will deal with ethnoracial discrimination and stratification through exit rather than voice. That is, they will reclassify their way out of marginalized ethnosomatic categories instead of voting for candidates and parties that share their ethnoracial identities. This tends to be the case where ethnoracial group identity is inchoate and group boundaries are permeable. High levels of stratification combined with low degrees of ethnoracial group consolidation will typically prevent the activation of ethnoracial identity in elections. Whereas ethnoracial stratification provides the incentive structure for individuals to switch ethnoracial categories, inchoate ethnoracial group identity and permeable ethnoracial boundaries lower the transaction costs to doing so. I also argue that individuals may emphasize national origin over race or ethnicity where ethnoracial group loyalties are weak and immigration is widespread. I test my argument against competing approaches using quantitative, qualitative, and experimental evidence from the Dominican Republic. The evidence suggests that the confluence of stratification and inchoate ethnoracial group identity indeed has prevented the activation of ethnoracial cleavages in elections in the DR. This same combination, however, has not impeded the activation of national origin in elections. Rather than strengthening the salience of ethnoracial cleavages in elections, nationalism has helped to redirect those cleavages.Item Hypoxia modeling in Corpus Christi Bay using a hydrologic information system(2009-05) To, Sin Chit; Maidment, David R.Hypoxia is frequently detected during summer in Corpus Christi Bay, Texas, and causes significant harm to benthic organism population and diversity. Hypoxia is associated with the density stratification in the Bay but the cause of stratification is uncertain. To support the study of hypoxia and stratification, a cyberinfrastructure based on the CUAHSI (Consortium of Universities for the Advancement of Hydrologic Science, Inc) Hydrologic Information System (HIS) is implemented. HIS unites the sensor networks in the Bay by providing a standard data language and protocol for transferring data. Thus hypoxia-related data from multiple sources can be compiled into a structured database. In Corpus Christi Bay, salinity data collected from many locations and times are synthesized into a three-dimensional space-time continuum using geostatistical methods. The three dimensions are the depth, the distance along a transect line, and time. The kriged salinity concentration in space and time illuminates the pattern of movement of a saline gravity current along the bottom of the Bay. The travel time of a gravity current in the Bay is estimated to be on the order of one week and the speed is on the order of 1 km per day. Statistical study of high-resolution wind data shows that the stratification pattern in the Bay is related to the occurrence of strong, southeasterly winds in the 5 days prior to the observation. This relationship supports the hypothesis that stratification is caused by the wind initiating hypersaline gravity currents which flow from Laguna Madre into Corpus Christi Bay. An empirical physical hypoxia model is created that tracks the fate and transport of the gravity currents. The model uses wind and water quality data from real-time sensors published by HIS to predict the extent and duration of hypoxic regions in the Bay. Comparison of model results with historical data from 2005 to 2008 shows that wind-driven gravity currents can explain the spatially heterogeneous patterns of hypoxic zones in Corpus Christi Bay.Item Laboratory and numerical studies of internal wave generation and propagation in the ocean(2010-08) King, Benjamin Thomas; Swinney, H. L., 1939-Internal waves are generated in the ocean by oscillating tidal flow over bottom topography such as ridges, seamounts, and continental slopes. They are similar to the more familiar surface waves, but not being constrained to move on the surface, propagate throughout the bulk of the world oceans. Internal waves transmit energy over thousands of kilometers, ultimately breaking and releasing their energy into turbulence and mixing. Where these internal waves are generated, as well as where and how they break and cause mixing, has important effects on the general circulation of the ocean, which is in turn a major component in earth's climate. As a first step in a more thorough understanding of the evolution of internal waves in the ocean, it is important to characterize their generation. The two-dimensional generation problem has been studied for four decades, with ample experimental, numerical, and theoretical results. Most of this past work has also been done using linear, inviscid approximations. However, wave generation in the ocean is three-dimensional (3D), and in many locations, nonlinear and viscous effects can be significant. Recent advances in experimental and numerical techniques are only now making the fully nonlinear, 3D generation process accessible. We utilize these new techniques to perform both laboratory experiments and numerical simulations on internal wave generation in 3D. We find that a significant component of the internal wave field generated by tidal flow over 3D topography is radiated in the direction perpendicular to the tidal forcing direction. This could lead to substantial improvements of global internal wave generation models. In addition, we have developed a new method for statistical analysis of ocean data sets, and have found large regions in the deep ocean where internal waves may not propagate. This will also have important effects on the way researchers study the propagation of internal waves, which, when propagating downward, were previously thought to always reflect from the sea floor.Item Modeling a gravity current in a shallow fluid system(2011-12) Kulis, Paula Sharon; Hodges, Ben R.; Maidment, David R.; Katz, Lynn E.; Reible, Danny D.; Raman, VenkatramanCorpus Christi Bay in Texas is a wind driven system, and under most conditions winds over the bay mix the water column vertically. However, seasonal, episodic, bottom-water hypoxia has been observed in the bay in conjunction with vertical salinity stratification. This stratification may be caused by dense gravity currents entering the bay. Understanding and modeling the mechanisms that result in stratification in Corpus Christi Bay may help predict hypoxia, and for this reason that is the focus of this dissertation. An evaluation of existing gravity current modeling techniques shows that most currently available models are designed to capture either phenomena local to a gravity current, such as gravity current entrainment and spreading, or larger scale phenomena such as wind mixing and large-scale circulation, but not both. Because gravity current mixing in Corpus Christi Bay is enhanced by wind-induced turbulence, both local gravity current physics and wind mixing effects are critical elements governing gravity current propagation in Corpus Christi Bay. As existing models do not represent gravity current entrainment and wind mixing together, this dissertation develops a coupled model system that accounts explicitly for turbulent wind mixing of a bottom-boundary layer, in addition to representing other local features of dense gravity current propagation such as entrainment and spreading. The coupled model system consists of a 2D depth-averaged hydrodynamic model that calculates gravity current mixing and spreading, coupled with a 3D hydrodynamic model whose domain includes a lighter ambient fluid surrounding the gravity current. The coupled models have flexible boundary conditions that allow fluid exchange to represent mixing from both gravity current entrainment and wind mixing. The coupled model system’s development, verification and application in Corpus Christi Bay advances understanding of gravity current mechanisms, and contributes to our scientific understanding of hypoxia in Corpus Christi Bay. This modeling technique has the flexibility to be applied to other density-stratified systems that are shallow and potentially wind-driven, such as shallow desalination brine disposal sites.Item The postsecondary resource trinity model : exploring the interaction between socioeconomic, academic, and institutional resources(2014-12) Giani, Matthew Shankar; Reyes, Pedro, 1954-It is frequently stated that higher education is our nation’s greatest vehicle for social mobility. But despite the dramatic expansion of the postsecondary system since the 1940s, social mobility rates have remained stagnant and significant socioeconomic disparities in college attainment have persisted. Three explanations of this phenomenon appear most prominently in the literature. The first is that low-SES students are significantly less likely to be academically prepared for college. The second is that, even when college-ready, low-SES students are more likely to attend less selective or lower quality postsecondary institutions, decreasing their odds of attainment. The third is that socioeconomic background may exert an independent effect on the likelihood of postsecondary success, independent of background ability and institutional quality. Although each explanation is supported empirically, thus far limited attempts have been made to determine how these three factors interact across educational transitions. The purpose of this study is to estimate the relative impact of SES across transitions in students’ college-to-career pathways and explore how socioeconomic disparities in rates of student outcomes vary by student ability, institutional selectivity, and the combination of the two. Sequential logit modeling, a methodological staple in research on the effects of socioeconomic background on educational progression, is used to estimate the relative effect of SES on seven postsecondary transitions, from college application through graduate school attainment. The labor market outcomes of college graduates are also analyzed to determine whether family upbringing continues to influence students even after completing college. For both the postsecondary and labor market analyses, separate models are fit for different ability groups and institutional selectivity levels to investigate how these factors impact the magnitude of socioeconomic disparities in transitions. The results of this study suggest the need for a new conceptualization of this phenomenon, which is termed the Postsecondary Resource Trinity model. This model highlights the complex interaction between socioeconomic, academic, and institutional resources and suggests the need for a re-examination of the traditional perspective that the impact of SES declines steadily for all students as they progress through postsecondary. Implications of the model for policy, practice, and future research are discussed.Item Socioeconomic stratification in the STEM pathway from college to the labor market(2014-05) Moore, Chelsea Dyann; Riegle-Crumb, CatherineFor decades, research has explored how family background shapes access to and success in postsecondary education. However, much less is known about the effect of family background on one’s educational and occupation success within specific fields. Given rapid advances in science and technology and a changing global economy, understanding these processes within science, technology, engineering, and mathematics (STEM) are particularly important to the broader understanding of stratification. Many recent studies in the U.S. stress the importance of increasing our STEM labor force to remain competitive in the global market, and demand for highly skilled workers is at an all-time high and increasing. While the demand for these jobs is high, many researchers argue that the supply of highly skilled workers is lagging behind. In order to meet these demands, many of these researchers point to increasing the talent pool by drawing from underrepresented groups. This study looks at how family socioeconomic background affects entry into STEM majors, persistence in STEM major, and early labor market outcomes among college graduates from STEM fields, and compares these patterns and processes to those in non-STEM fields. Results from this study show stronger SES differences in STEM fields than non-STEM fields at each point from college major choice to the labor market. Together, these results suggest that less socioeconomically advantaged students may be at a particular disadvantage in STEM fields.Item Universities, status groups, and hierarchies of worth among college students in Mexico(2010-05) Rojas Ruiz, Francisco Javier; Roberts, Bryan R., 1939-; Adut, Ari; Camp, Roderic A.; Green, Penny A.; Sjoberg, Gideon A.This research investigated the basis upon which undergraduates construct notions of social honor and the role higher educational institutions play in the consolidation of status group cultures in Mexico. The topics I analyzed included the criteria college students use to evaluate the worthiness of their peers and friends and the meaning they attach to studying at certain higher educational institutions. This study drew primarily on 65 in-depth interviews and four focus group sessions with 15 students enrolled at socioeconomically stratified private and public higher educational institutions in a large city. I also relied on an institutional analysis of the higher educational institutions where I did my research to complement the analysis. The findings of this research show that there are institutional similarities in relation to the moral criteria undergraduates use to evaluate the worthiness of their friends and peers. However, there are important institutional differences showing that Mexico’s system of higher education attracts and trains at least four status groups. The status groups cultures associated with educational credentials show that there are significant cultural and socioeconomic distinctions within the high cost private sector. In particular, there is a clash between an old pedigree status group for which social connections are of outmost importance and a new emerging upper-middle class that competes through the rigorous academic training its undergraduates receive. The other two status groups are composed of lay and large public universities that attract middle-class students and demand-absorption institutions that train students who did not gain admission to public universities or who want to avoid the negative stereotypes associated with public universities. Undergraduates from these two last groups occupy the bottom of the occupational and prestige hierarchy. This research also shows that most of the internal hierarchies undergraduates use to rank their peers do not transcend the walls of a specific college. However, the testimonies of high class students revealed that members from this social stratum determine the worthiness of others based on residential location. This dissertation also shows that socioeconomic and cultural boundaries provide some of the most important sources of symbolic divisions among college students in Mexico.