Browsing by Subject "Variability"
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Item A coupled model study of the remote influence of enso on tropical Atlantic sst variability(Texas A&M University, 2006-08-16) Fang, YueTo investigate the tropical Atlantic response to the remote El Nino-Southern Oscillation (ENSO) forcing, a Reduced Physics ?? Coupled Global Circulation Model (RP-CGCM) is developed, and four experiments are carried out. The results show that the RP-CGCM is capable of capturing the major features of Tropical Atlantic Variability (TAV) and its response to ENSO forcing. The SST response to the remote influence of ENSO may be divided into two stages. In stage one, the ENSO influences the tropical Atlantic SST primarily through the Troposphere Temperature (TT) mechanism, which predicts a uniform warming in the tropical Atlantic following the mature phase of El Nino. In the north tropical Atlantic (NTA), the Walker mechanism and the Pacific-North-American (PNA) mechanism work in concert with the TT-induced warming, giving rise to a robust SST response during the boreal spring in this region. In the south tropical Atlantic (STA), the southeasterly wind anomaly and increased stratus clouds work against the TT-induced warming, resulting in a much weaker SST response in this region. At this stage, the response can be largely explained by the ocean mixed layer response to changes in surface heat fluxes induced by ENSO. In stage two, ocean dynamics play a more active role in determining the evolution of SST. The cross-equatorial wind anomaly in the western to central equatorial Atlantic can change the SST in the eastern equatorial Atlantic through Bjerknes feedback and the SST in the central equatorial Atlantic through Ekman feedback. These feedback result in a cooling of SST in the equatorial south Atlantic (ESA) region which is so overwhelming that it cancels the warming effect induced by the TT mechanism and reverses the sign of the warm SST anomaly that is formed during stage one in this region. In general, the horizontal advection of heat plays a secondary role in the SST response to the remote influence of ENSO, except in the regions where the North Equatorial Countercurrent (NECC) dominates and the SST variability is strong. Entrainment is particularly important in maintaining the correct SST structure during boreal summer.Item Crop Production Variability and U.S. Ethanol Mandates(2014-07-08) Jones, Jason P.U.S. agricultural commodity prices have been volatile in recent years, attributed to many factors, including renewable fuel standard mandates (RFS). While the RFS is legislatively able to be altered, the mandate largely required the same volume of corn for ethanol in the 2012 drought year as it would have if 2012 were a normal production year. This caused corn prices to surge, bestowing significant economic ramifications throughout the agricultural industry. An important question arose from these events, was this avoidable with a RFS relaxation policy? In this work, the economic effects of such a policy that relaxes the conventional ethanol mandates in cases of major corn production shortfalls are investigated to determine the market relationships between RFS policy and commodity markets. This is done in a three step process. First the historical incidence of shortfalls is addressed by developing a stationary probability distribution of total and regional production using econometric procedures. Second, the short-run economic impact of RFS relaxation alternatives is investigated using an optimization modeling framework where crop mix and livestock breeding herds are held fixed. Third, the long-run implications of RFS relaxation are investigated by coupling the previous model with a stochastic optimization framework of ag-producer decisions with recourse. When a shortfall driven relaxation policy is in place, crop mix/livestock breeding decisions are able to adjust. The results show RFS relaxation has a significant impact on reducing price spikes and livestock production decreases due to reduced feeding costs when shortfalls occur. Although an ethanol waiver benefits consumers through decreased commodity prices, the reduction in producer welfare was found to be greater, resulting in an overall negative welfare impact when only considering agricultural impacts. In the longer-run analysis, the RFS relaxation again mitigates price spikes during production shortfall years but also stimulates a producer response of decreasing corn acreage. This caused corn prices in non-shortfall years to increase, resulting in a negligible impact on average long-run corn prices, while reducing commodity price variability. The model findings demonstrated that positive risk reduction implications could exist from a production-dependent conventional ethanol waiver, with limited long-run changes to future expected prices.Item Development of reliable pavement models(2011-08) Aguiar Moya, José Pablo, 1981-; Prozzi, Jorge Alberto; Manuel, Lance; Walton, Michael; Machemehl, Randy B.; Yilmaz, HilalAs the cost of designing and building new highway pavements increases and the number of new construction and major rehabilitation projects decreases, the importance of ensuring that a given pavement design performs as expected in the field becomes vital. To address this issue in other fields of civil engineering, reliability analysis has been used extensively. However, in the case of pavement structural design, the reliability component is usually neglected or overly simplified. To address this need, the current dissertation proposes a framework for estimating the reliability of a given pavement structure regardless of the pavement design or analysis procedure that is being used. As part of the dissertation, the framework is applied with the Mechanistic-Empirical Pavement Design Guide (MEPDG) and failure is considered as a function of rutting of the hot-mix asphalt (HMA) layer. The proposed methodology consists of fitting a response surface, in place of the time-demanding implicit limit state functions used within the MEPDG, in combination with an analytical approach to estimating reliability using second moment techniques: First-Order and Second-Order Reliability Methods (FORM and SORM) and simulation techniques: Monte Carlo and Latin Hypercube Simulation. In order to demonstrate the methodology, a three-layered pavement structure is selected consisting of a hot-mix asphalt (HMA) surface, a base layer, and subgrade. Several pavement design variables are treated as random; these include HMA and base layer thicknesses, base and subgrade modulus, and HMA layer binder and air void content. Information on the variability and correlation between these variables are obtained from the Long-Term Pavement Performance (LTPP) program, and likely distributions, coefficients of variation, and correlation between the variables are estimated. Additionally, several scenarios are defined to account for climatic differences (cool, warm, and hot climatic regions), truck traffic distributions (mostly consisting of single unit trucks versus mostly consisting of single trailer trucks), and the thickness of the HMA layer (thick versus thin). First and second order polynomial HMA rutting failure response surfaces with interaction terms are fit by running the MEPDG under a full factorial experimental design consisting of 3 levels of the aforementioned design variables. These response surfaces are then used to analyze the reliability of the given pavement structures under the different scenarios. Additionally, in order to check for the accuracy of the proposed framework, direct simulation using the MEPDG was performed for the different scenarios. Very small differences were found between the estimates based on response surfaces and direct simulation using the MEPDG, confirming the accurateness of the proposed procedure. Finally, sensitivity analysis on the number of MEPDG runs required to fit the response surfaces was performed and it was identified that reducing the experimental design by one level still results in response surfaces that properly fit the MEPDG, ensuring the applicability of the method for practical applications.Item Effects of varying the force levels and direction of force change on accuracy and force variability in a cyclic isometric pinch force tracking task(2012-08) Park, Sangsoo; Abraham, Lawrence D.; Spirduso, WaneenThis study investigated how varying the required force level and the direction of force change produced by the thumb and index finger affect the accuracy and variability of a cyclic isometric pinch force-tracking task. Accuracy was examined by both absolute error and relative error for the minimum and maximum force levels and by root mean square error (RMSE) and normalized root mean square error (normalized RMSE) for the force direction reversals. Variability was represented by coefficient of variation of error (CVE). In this study, ‘maximum force’ was defined as the highest force level of a given target force range, and ‘minimum force’ was defined as the minimum force level of the target range. In addition, ‘force increasing to decreasing’ indicated that the track ball motion changed from increasing to decreasing, requiring the performer to exert increasing force up to the maximum force level and then decreasing force to follow the track ball moving toward the minimum force level. The phrase ‘force decreasing to increasing’ indicated the opposite force direction reversal. Eighteen healthy right handed adult volunteers (nine men and nine women; mean age ± SD, 28.3±1.22 and 26.4±1.74) participated in this study. The participants performed a cyclic isometric pinch force tracking task over three different force ranges. Force range 1 was from a minimum force of 3% of maximal voluntary contraction force (MVC) to a maximum force of 6% MVC. In force range 2, the range was from 6% to 12% MVC, and force range 3 was from 12% to 24% MVC. For each force range, five practice trials and ten actual test trials were performed. Rest periods of twenty seconds between trials and one minute between sets of trials (including between practice and actual test trials) were provided to minimize fatigue effects. Absolute error uniformly increased as a function of increasing force. However, the 3% target force level showed larger relative error compared to the 12% target force level (p < 0.05). Another finding of this study was that producing forces positioned at the minimum target level in a range yielded higher absolute error and relative error compared to the same forces when placed at the maximum target level of a different force range. In terms of the reversals, RMSE values were higher at the change from force deceasing to increasing than the opposite, as well as at higher force levels, while normalized RMSE values were greater at lower force levels. CVE was not significantly different between the two reversals in this study. This might indicate that poorer performance during the change from force decreasing to increasing could originate from the effort to maintain consistent performance and additional effort was not beneficial to increase accuracy for the change from force decreasing to increasing.Item Evaluation of cowpea (Vigna unguiculata L. Walp) genotypes for adaptation to low soil-phosphorus conditions and to rock phosphate application(2009-05-15) Mahamane, SabiouCowpea (Vigna ungiculata L. Walp) is a major food and fodder legume in poor countries, particularly Sub-Saharan Africa countries. It is generally produced in sandy, acid soils, deficient in phosphorus (P) which severely limits its production. Because processed phosphate fertilizers are expensive and poorly available to farmers, rock phosphate is viewed as a cheap alternative phosphate source. The present study evaluated 696 U.S Core Collection and IITA cowpea accessions for adaptation to low soil P environments and for response to rock phosphate application. Subsequently, organic acid exudation by selected cowpea genotypes as a mechanism for P acquisition from Fe-oxide and Ca bound P was investigated. A low P soil from Nacogdoches pine forest was used to grow plants. There were two P treatments: 0 and 300 mg P/kg of soil as Tahoua (Niger) rock phosphate. At harvest, plant height, shoot and root dry weights were determined and total biomass and shoot-to-root ratios were computed. Shoot P contents of 100 selected accessions were measured. Sixteen accessions reflecting the wide array of responses observed were selected for the organic acid study. Plants were grown in a growth chamber hydroponically with no P and +P nutrient solutions for 3 weeks. Organic acids were collected in a CaCl2-KCl solution. The nature and quantity of the collected organic acids was determined. Cowpea accessions were significantly different in their ability to adapt to Pdeficiency stress and to acquire P from rock phosphate. The parameters most effective in separating the accessions were shoot mass and total biomass. This data will be potentially useful in the selection of cowpea germplasm for (1) adaptation to West African soils of low P fertility, and (2) ability to utilize P from poorly soluble rock phosphate. The predominant organic acid exuded by cowpea roots was a tricarboxylic acid not yet identified. There was surprisingly more exudation of this acid under +P than under ?P conditions. Exudation was more highly correlated to roots than to shoots.Item Lithography variability driven cell characterization and layout optimization for manufacturability(2011-05) Ban, Yong Chan; Pan, David Z.; Abraham, Jacob; Touba, Nur; Lucas, Kevin; Orshansky, MichaelStandard cells are fundamental circuit building blocks designed at very early design stages. Nanometer standard cells are prone to lithography proximity and process variations. How to design robust cells under variations plays a crucial role in the overall circuit performance and yield. This dissertation studies five related research topics in design and manufacturing co-optimization in nanometer standard cells. First, a comprehensive sensitivity metric, which seamlessly incorporates effects from device criticality, lithographic proximity, and process variations, is proposed. The dissertation develops first-order models to compute these sensitivities, and perform robust poly and active layout optimization by minimizing the total delay sensitivity to reduce the delay under the nominal process condition and by minimizing the performance gap between the fastest and the slowest delay corners. Second, a new equivalent source/drain (S/D) contact resistance model, which accurately calculates contact resistances from contact area, contact position, and contact shape, is proposed. Based on the impact of contact resistance on the saturation current, robust S/D contact layout optimization by minimizing the lithography variation as well as by maximizing the saturation current without any leakage penalty is performed. Third, this dissertation describes the first layout decomposition methods of spacer-type self-aligned double pattering (SADP) lithography for complex 2D layouts. The favored type of SADP for complex logic interconnects is a two-mask approach using a core mask and a trim mask. This dissertation describes methods for automatically choosing and optimizing the manufacturability of base core mask patterns, generating assist core patterns, and optimizing trim mask patterns to accomplish high quality layout decomposition in SADP process. Fourth, a new cell characterization methodology, which considers a random (line-edge roughness) LER variation to estimate the device performance of a sub-45nm design, is presented. The thesis systematically analyzes the random LER by taking the impact on circuit performance due to LER variation into consideration and suggests the maximum tolerance of LER to minimize the performance degradation. Finally, this dissertation proposes a design aware LER model which claims that LER is highly related to the lithographic aerial image fidelity and the neighboring geometric proximity. With a new LER model, robust LER aware poly layout optimization to minimize the leakage power is performed.Item Measuring the Effect of Uncertainty in Unit Cost and Pre-Treatment Condition on Pavement Maintenance and Rehabilitation Decisions(2014-05-23) Li, JingA pavement maintenance and rehabilitation (M&R) project normally extends over 2-10 mile long roadway segment. At the M&R planning stage, these projects are called pavement management sections, which are often comprised of multiple data collection sections. The fact that a management section is comprised of multiple data collection sections introduces variability into the condition of the pavement within each M&R project. Also, variability is often found in the cost of M&R projects of the same M&R type. These variability are poorly understood and qualified in the pavement management literature. Accounting for these uncertainties in pre-treatment pavement condition and in the M&R treatment cost is essential for obtaining realistic estimate of needed funding. This research addresses this knowledge gap by a) developing probability density functions for pavement pre-treatment condition indicators and M&R unit cost, and b) developing a novel probabilistic methodology for estimating M&R funding needs for pavement networks that accounts for these uncertainties. Data was obtained from the Bryan district pavement management plan for 2012 and from the Texas Department of Transportation (TxDOT) Pavement Management Information System (PMIS). Probability distribution functions were fitted for distress score, ride score, and unit cost using the @Risk software. Also, a simplified decision tree was developed to help simulate the maintenance and rehabilitation treatment selection process. This decision tree considers ride score, distress score, and traffic volume. After fitting the probability distributions of pavement condition indicators and unit cost, the impact of uncertainty in them on funding needs estimate was investigated using Monte Carlo simulation, The analysis shows that the needs estimate produced by TxDOT for the studied projects falls within the 90 percent confidence interval of the simulated need estimate.Item Precision pinch isometric force, force variability, accuracy, and task time among the fourth through eighth decades of life(2011-05) Herring-Marler, Trenah Lannette; Abraham, Lawrence D.; Spirduso, Waneen Wyrick; Eakin, Richard T; Griffin, Lisa; Hunter, DianaThis dissertation encompassed three studies involving precision pinch strength and 5% submaximal fine-motor control. One hundred participants (30-79 years old) were divided into 10-year categories, with 10 males and 10 females in each decade. A Manual Force Quantification System containing a platform and force-transducer apparatus, along with a computer and visual monitor, was used. Each subject performed four tasks -- maximal voluntary isometric contraction (MVIC), force-matching, tracing, and tracking -- by applying force on the transducers with the thumb and index finger while attempting to produce a desired force level or task displayed on the computer monitor. The first study measured MVIC, accuracy (rRMSE, Root Mean Square Relative Error), and force variability (Coefficient of Variation, CV) during a 5% MVIC force-matching task. The second study measured accuracy (rRMSE), task time, and group variability during a 5% MVIC tracing task. The third study measured accuracy and group variability during a 5% MVIC tracking task. Tracing and tracking were each divided into six Segments (S1-S6), three of which (S1-S3) required the increasing application of force from 50g up to 5% MVIC and the remaining three (S4-S6) requiring a release of force from MVIC down to 1% MVIC. The force-matching and force-tracking task times were scaled to each participant's MVIC, while the tracing task was performed at the participant's self-selected speed. The participants were encouraged to be accurate but also to trace the target line as quickly as possible. Declines in precision pinch strength and force control began to occur in the 70s for easier force-control tasks and in their 60s for more advanced force-tracking tasks. Men were stronger than women at all age levels. Participants in their 30s were the fastest; those in their 40s, 50s, and 60s slowed down to be accurate; and those in their 70s moved faster but were the least accurate. Three segmental factors affected error and time: low force level, releasing as opposed to applying force, and location along the target line with respect to reversal or ending points. Finally, variables for females were more heterogeneous at earlier decades than for men, and the older the age group was, the greater the variable heterogeneity was.Item Trial-to-trial dynamics and learning in generalized, redundant reaching tasks(2010-08) Smallwood, Rachel Fay; Dingwell, Jonathan B.; Abraham, Lawrence D.Trial-to-trial variability in human movement is often overlooked and averaged out, but useful information can be gleaned on the brain’s control of variability. A task can be defined by a function specifying a solution manifold along which all task variable combinations will lead to goal success – the Goal-Equivalent Manifold (GEM). We selected a reaching task with variables reach Distance (D) and reach Time (T). Two GEMs were selected: a constant D/T and constant D×T. Subjects had no knowledge of the goal prior to the experiments and were instructed only to minimize error. Subjects learned the generalized tasks by reducing errors and consolidated learning from one day to the next, generalized learning from the D×T to the D/T GEM, and had interference of learning from the D/T to the D×T GEM. Variability was structured along each GEM significantly more than perpendicular to it. Deviations resulting in errors were corrected significantly more quickly than any other deviation. Our results indicate that subjects can learn generalized reaching tasks, and the brain exploits redundancy in those tasks.Item Understanding Spatio-Temporal Variability and Associated Physical Controls of Near-Surface Soil Moisture in Different Hydro-Climates(2013-05-06) Joshi, ChampaNear-surface soil moisture is a key state variable of the hydrologic cycle and plays a significant role in the global water and energy balance by affecting several hydrological, ecological, meteorological, geomorphologic, and other natural processes in the land-atmosphere continuum. Presence of soil moisture in the root zone is vital for the crop and plant life cycle. Soil moisture distribution is highly non-linear across time and space. Various geophysical factors (e.g., soil properties, topography, vegetation, and weather/climate) and their interactions control the spatio-temporal evolution of soil moisture at various scales. Understanding these interactions is crucial for the characterization of soil moisture dynamics occurring in the vadose zone. This dissertation focuses on understanding the spatio-temporal variability of near-surface soil moisture and the associated physical control(s) across varying measurement support (point-scale and passive microwave airborne/satellite remote sensing footprint-scale), spatial extents (field-, watershed-, and regional-scale), and changing hydro-climates. Various analysis techniques (e.g., time stability, geostatistics, Empirical Orthogonal Function, and Singular Value Decomposition) have been employed to characterize near-surface soil moisture variability and the role of contributing physical control(s) across space and time. Findings of this study can be helpful in several hydrological research/applications, such as, validation/calibration and downscaling of remote sensing data products, planning and designing effective soil moisture monitoring networks and field campaigns, improving performance of soil moisture retrieval algorithm, flood/drought prediction, climate forecast modeling, and agricultural management practices.Item The ups and downs of variability : are fluctuating relationship appraisals always detrimental for long-term relationship outcomes?(2013-12) Morgan, Taylor Anne; Neff, Lisa A.Variability in daily relationship satisfaction has been shown to undermine future relationship well-being. The current study suggests that the relationship climate may moderate this effect. Namely, and in light of prior work showing that ignoring relationship issues can be detrimental for long-term relationship well-being, it is argued that when the relationship is characterized by more negative relationship experiences, variability in daily satisfaction may actually represent an adaptive acknowledgement of those experiences. Seventy-eight newly-married couples completed a 10-day daily diary task which assessed the variability of daily marital satisfaction, the positive and negative marital events taking place each day, and the daily coping strategies used to manage negative marital events. Spouses then reported on their global marital happiness as well as the severity of their marital problems every six months over the first two and a half years of marriage. Results revealed that when the marriage was characterized by more negative than positive marital events (i.e., a more negative marital climate), greater variability in daily satisfaction predicted initially lower levels of global marital happiness and more severe marital problems. However, greater variability in a more negative marital climate also was associated with less steep declines in global marital happiness and fewer increases in marital problems over time compared to low variability. Together, these findings suggest that variability in daily relationship satisfaction may temporarily feel unpleasant but over time may allow couples to address important relationship issues.Item Variability of relationship evaluations & physical health outcomes : testing the moderating role of implicit theories of relationships(2015-08) Morgan, Taylor Anne; Loving, Timothy J.; Neff, Lisa A.; Gleason, Marci E.; Anderson, Edward R.; Campbell, LorneVariability (i.e., greater fluctuations) of relationship evaluations over time undermines current and future relationship and individual well-being. To date, greater variability of relationship evaluations has not been linked to individual physical health, yet overall relationship quality (e.g., initial or mean levels) is a strong predictor of several markers of physical health. The aims of the current study are three-fold--first, to explore new methods of capturing intra-individual variability; second, to replicate and extend previous findings on the damaging effects of variability by examining variability's association with relationship fate and physical health outcomes; and third, to investigate the potential moderating role of individuals' general relationship beliefs on these associations. I drew from a 9-month longitudinal study of 202 individuals who were in new dating relationships at the start of study participation (M[subscript length] = 3.28 months). The study included baseline self-reports of relationship satisfaction and Implicit Theories of Relationships (ITRs) as well as bi-weekly self-reports, for a total of up to 20 assessments per individual, of relationship status (together or not) and quality (semantic differential) and physical health (shortened SF-36). A series of analyses were conducted to obtain de-trended levels of intra-individual variability and a previously understudied measure of intra-individual change in relationship evaluations over time -- temporal dependency (i.e., the extent to which one day's relationship evaluation is correlated with the previous day's evaluation). These two variables were subsequently entered into a Cox proportional hazards model to explain the effect of each on the likelihood of relationship dissolution. Both variability and temporal dependency of relationship evaluations (but not their interaction) predicted increased likelihood of experiencing relationship dissolution. Contrary to predictions, however, greater variability and temporal dependency (and their interaction) were not associated with changes in self-reported physical health over time; all three terms, however, predicted higher levels of average self-reported physical health across assessments. Finally, the hypothesized moderating effects of individuals' implicit theories of relationships were not supported. These findings emphasize the predictive power of individuals' patterns of relationship evaluations over time for relationship fate yet suggest additional research is needed to understand the association between variable relationship evaluations and health outcomes.