Browsing by Author "Venayagamoorthy, Subhas K., committee member"
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Item Open Access African easterly wave energetics on intraseasonal timescales(Colorado State University. Libraries, 2014) Alaka, Ghassan J., Jr., author; Maloney, Eric D., advisor; Schubert, Wayne H., committee member; Schumacher, Russ S., committee member; Venayagamoorthy, Subhas K., committee memberAfrican easterly waves (AEWs) are synoptic-scale eddies that dominate North African weather in boreal summer. AEWs propagate westward with a maximum amplitude near 700 hPa and a period of 2.5-6-days. AEWs and associated perturbation kinetic energy (PKE) exhibit significant intraseasonal variability in tropical North Africa during boreal summer, which directly impacts local agriculture and tropical cyclogenesis. This study performs a comprehensive analysis of the 30-90-day variability of AEWs and associated energetics using both reanalysis data and model output. Specifically, the PKE and perturbation available potential energy (PAPE) budgets are used to understand the factors that contribute to PKE maxima in West Africa and the extent to which these surges of AEW activity are modulated by the Madden-Julian oscillation (MJO). The role of the MJO in the intraseasonal variability of AEWs is assessed by comparing PKE sources as a function of an MJO index and a local 30-90-day West African PKE index. Since East Africa is an initiation zone for AEW activity and is modulated by the MJO, the relationship between this region and West Africa is a primary focus in this study. The intraseasonal variability of AEW energetics is first investigated in reanalysis products. While reanalysis data depicts a similar evolution of 30-90-day PKE anomalies in both the MJO and a local PKE index, the MJO index describes only a small (yet still significant) fraction of the local 30-90-day variance. In boreal summers with more significant MJO days, the correlation between the two indices is higher. Baroclinic energy conversions are important for the initiation of 30-90-day West African PKE events east of Lake Chad. In West Africa, both barotropic and baroclinic energy conversions maintain positive PKE anomalies before they propagate into the Atlantic. The primary role of diabatic heating is to destroy PAPE in a negative feedback to baroclinic energy conversions in West Africa. More frequent East Atlantic tropical cyclone generation is associated with positive PKE events than with negative PKE events. Easterly wave activity is then examined in a regional model. The Advanced Research Weather Research and Forecasting (WRF-ARW) simulates West African monsoon climatology more accurately than the WRF Nonhydrostatic Mesoscale Model (WRF-NMM). Although the WRF-NMM produces more realistic boreal summer rainfall than the WRF-ARW, it fails to accurately simulate the AEJ and other key West African monsoon features. Parameterizations within the WRF-ARW are scrutinized as well, with the WRF single-moment 6-class microphysics and the Noah land surface model outperforming Thompson microphysics and the RUC land surface model. Three ten-year WRF-ARW experiments are performed to investigate the role of external forcing on intraseasonal variability in West Africa. In addition to a control simulation, two sensitivity experiments remove 30-90-day variability from the boundary conditions (for all zonal wavenumbers and just for eastward zonal wavenumbers 0-10). Overall, intraseasonal variability of AEWs shows only modest differences after the removal of all 30-90-day input into the model boundary conditions. PKE and PAPE budgets reveal that simulated positive PKE events in West Africa are preceded by extensions of the AEJ into East Africa, which enhance barotropic and baroclinic energy conversions in this region. This jet extension is associated with warm lower-tropospheric temperature anomalies in the eastern Sahara. In West Africa, the amplitude of PKE and PAPE budget terms exhibit a similar evolution (even in the sensitivity experiments) as in the reanalysis products.Item Open Access Bulking coefficients of aerated flow during wave overtopping simulation on protected land-side slopes(Colorado State University. Libraries, 2016) Scholl, Bryan N., author; Thornton, Christopher I., advisor; Abt, Steven R., advisor; Hughes, Steven A., committee member; Venayagamoorthy, Subhas K., committee member; Kampf, Stephanie K., committee memberPost hurricane Katrina there has been more interest in erosion on the landward side of levees resulting from wave overtopping during storm events. The development of wave overtopping simulators has enabled more rigorous evaluation of levee armoring alternatives under controlled conditions similar to those on levees. Steady state overtopping studies have demonstrated a reduction in shear stress due to air entrainment in the flow. There has not been an evaluation of air entrainment during wave overtopping simulation. For this reason, a study was conducted to quantify flow bulking occurring during wave overtopping simulation. Testing was conducted at the Hydraulics Laboratory at Colorado State University at the Engineering Research Center using a wave overtopping simulator. The simulated levee was 6 ft wide. Levee geometry in the direction of flow was a 13.2 ft. horizontal crest, 30.5 ft levee face with 3:1 (horizontal:vertical) slope and 12.2 ft berm with 25:1 slope. Un-bulked flow thickness was measured with “surfboards” which hydroplane along the surface of flow. Bulked flow thicknesses were measured using visual observations of maximum flow thickness on eight staff gages along the wall of the simulated levee. Wave volumes ranged from 20 ft3/ft to 175 ft3/ft. Conservation of mass and testing repeatability is demonstrated. Bulking values range from zero for the smallest wave volumes to over 100% for the largest wave volumes. An empirical model is developed to estimate bulking on the 3:1 levee slope. A comparison is made to steady state flows with similar air entrainment. The effect of bulking on shear stress is a potential decrease in shear stress over 50% relative to un-bulked flow thickness. A method to incorporate wave overtopping bulking into design is proposed using a cumulative work approach.Item Open Access Cold pool train dynamics and transport(Colorado State University. Libraries, 2023) Neumaier, Christine Allison, author; van den Heever, Susan C., advisor; Grant, Leah D., advisor; Kreidenweis, Sonia M., committee member; Venayagamoorthy, Subhas K., committee memberConvectively generated cold air outflows, referred to as cold pools, can initiate new convection and loft aerosols, such as dust or pollen. In the BioAerosols and Convective Storms Phase I (BACS-I) field campaign, we observed multiple cold pools passing over the same location on the same day, without colliding, which we refer to as a "cold pool train". The goals of this study are to examine how the dynamics of cold pools in a cold pool train differ, how cold pools in a cold pool train affect the vertical distribution of aerosols, and how the results may change if the properties of the second cold pool change. We utilize idealized simulations of a cold pool train composed of two cold pools to investigate the dynamics of the cold pools in the train and how cold pool trains loft and transport aerosols. We test the sensitivity of the second cold pool's evolution and aerosol lofting to its initial temperature deficit and timing relative to the first cold pool, based on the cold pool trains observed during BACS-I. Passive tracers are initialized at different times to represent the background aerosols present before cold pools, aerosols newly emitted after the passage of the first cold pool in the train, and aerosols within and ahead of each cold pool, to distinguish between how cold pools loft their own air compared to distinct environmental air. We find that the first cold pool (CP1) in the cold pool train stably stratifies the environment ahead of the downshear side of the second cold pool (CP2) in the train. All else equal, this stabilization acts to decrease the height of CP2's head and increase its propagation speed. However, the stratification also increases the horizontal wind shear ahead of CP2 by decreasing the lower level wind speeds, which opposes the stability effects and acts to deepen the head of CP2. In the CONTROL case, where CP2 is initialized two hours after CP1 and with the same temperature deficit as CP1, we find that the wind profile plays a more dominant role for the dynamics of CP2 because overall, CP2's head is deeper and propagates slower compared to CP1. In the temperature deficit sensitivity experiments, we find that CP2's head depth and propagation speed decreases with decreasing temperature deficit. Finally, in the timing sensitivity tests of CP2, we find CP2 initiated 90 minutes after CP1 had the deepest head, while CP2 in the CONTROL (120 minutes) experiment propagated the slowest. Our analysis of the tracer lofting mechanisms in the simulations shows that the downshear leading edge of CP1 lofts the highest concentration of background aerosol, while the downshear leading edge of the CONTROL CP2 lofts less than half of the amount of background aerosol as CP1. However, the downshear leading edge of CP2 lofts more than double the concentration of newly emitted aerosol compared to the background aerosol lofted by CP1. The atmospheric stratification left behind by CP1 acts to trap the newly emitted aerosol near the surface, leading to greater concentrations lofted compared to the background aerosol which is well mixed in the boundary layer. Analysis of the tracers initialized within and ahead of the cold pools demonstrates that the lofted aerosol primarily originates from the air ahead of the cold pools, while the aerosol originating in the cold pools remains trapped within the cold pools. The CONTROL CP2 lofts the most aerosol of the temperature deficit sensitivity tests, and the CONTROL CP2, released the farthest apart temporally from CP1, lofts the most aerosol out of the timing sensitivity tests. Therefore, while the wind profile change ahead of CP2 plays a dominant role in its dynamics, atmospheric static stabilization plays a dominant role for the aerosol concentration lofted by CP2.Item Open Access Distributed runoff simulation of extreme monsoon rainstorms in Malaysia using TREX(Colorado State University. Libraries, 2013) Abdullah, Jazuri, author; Julien, Pierre Y., advisor; Bledsoe, Brian P., committee member; Venayagamoorthy, Subhas K., committee member; Wohl, Ellen E., committee memberMalaysia has a monsoon climate and most areas receive more than 2,500 mm of rainfall every year. For the past five years, the frequency and magnitude of floods in Malaysia have been relatively high. Floods have become the most significant type of natural disaster for Malaysia in terms of the population affected, financial losses and adverse socio-economic impact. This study uses the distributed two-dimensional TREX model to simulate infiltration, overland runoff and channel flow during extreme rainfall events. The main objective is to calibrate the distributed hydrological model to simulate monsoon floods. The second objective is to determine the affected flooding area under different rainfall events (i.e., large and extreme rainfall events). Large rainfall events cover return periods ranging from two to one hundred years. Extreme rainfall events include both the PMP and the world's largest rainfall events. The third objective is to examine the effect of rainfall duration on the magnitude of peak flood discharge as a function of watershed size. Finally, determine and produce graphs for the relationships between peak specific-discharge and watershed sizes. Three different sizes of watersheds are considered: Lui (small - 68 km2), Semenyih (medium - 236 km2) and Kota Tinggi (large - 1,635 km2). Generally, the topography of these watersheds is steep, except for the large watershed. The TREX model calibration and validation have been done using field measurements during several storm events. The performance of the model to find peak discharge, time to peak, and volume has been tested using three metrics: Relative Percentage Difference (RPD), Percentage Bias (PBIAS) and Nash-Sutcliffe Efficiency Coefficient (NSEC)) comparison. On average, the model performance was good for small (RPD - 7%, PBIAS - 14% and NSEC - 0.4) and medium watersheds (RPD - 14%, PBIAS - 28% and NSEC - 0.7). The RPD (4%), PBIAS (2%) and NSEC (0.8) for the large watershed shows that the model performance was very good. The spatial and temporal runoff distribution for overland and channel flows were successfully visualized in 3D. Both small and medium watersheds were not flooded by large events, except in the main channel. The flow depth reached 1.72 m in the valley of the small watershed only during extreme events. It was estimated that about 24% (±10%) and 83% (±5%) of the valley area exceed a flow depth of 1.72 m during PMP and world's largest events, respectively. For the medium watershed, the valley area was covered with water in excess of 4.49 m under the world's largest events. The visualization tool shows that the valley areas are prone to severe flooding (in excess of 4.49 m of flow depth) under this event (±5%). For the large watershed, the low land areas (i.e., along the tributaries and channels) are more likely to be flooded during large and extreme events. The water depths covered more than 2.8 m in these areas. The maximum estimated discharges (MED) for large rainfall events were highest for rainfall durations of 3 to 5 hours on small watersheds. However, the MED values for medium watersheds were obtained for rainfall durations between 5 and 12 hours. The MED values for extreme rainfall events were highest for rainfall durations between 10 and 13 hours on both watersheds. For the large watershed, the MED values of large and extreme events were obtained for a rainfall duration of 168 hour. The main conclusions of this study are: (1) rainfall intensity (i.e., hourly data) is one of the main factors that contribute to the magnitude of flooding on small and medium watersheds (watershed size less than 1,000 km2). The flooding events on large watersheds (watershed size more than 1,000 km2) result from longer rainfall durations (i.e., multi-day rainstorms), (2) for all size watersheds, the average magnitude of peak discharge for the PMP and the world's largest events are approximately 5 and 12 times larger than a 100-year rainfall event, (3) the peak specific-discharge (cms/km2) decreased as the watershed size (km2) increased, and (4) the runoff coefficient C increased significantly (i.e., a factor of three) from the 100-year rainfall event to the PMP and the world's largest events for all watersheds (CPMP,CWGR > 0.7).Item Open Access Interflow dynamics and three-dimensional modeling of turbid density currents in Imha Reservoir, South Korea(Colorado State University. Libraries, 2011) An, Sang Do, author; Julien, Pierre Y., advisor; Thornton, Christopher I., committee member; Venayagamoorthy, Subhas K., committee member; Wohl, Ellen E., committee memberThis study reports a detailed research identifying the turbid density flow regimes and propagation dynamics of density currents in Imha Reservoir in South Korea during Typhoon Ewiniar. We employ a high resolution 3-D numerical model (FLOW-3D), based on nonhydrostatic Navier-Stokes equations, to investigate the propagation of density flows resulting from the complicated reservoir morphometry and various mixing processes. The 3-D numerical model was modified to simulate particle-driven density currents. The particle dynamics algorithm builds upon the original FLOW-3D code in two ways: (1) improve the original buoyant flow model to compute the changes in density via particle deposition; and (2) include multiple sediment sizes in mixtures as a function of particle size. The influences of inflow characteristics and seasonal changes of thermal structure of the reservoir on the turbid density currents intruding into Imha Reservoir are studied. A series of numerical simulations of lock-exchange are validated with laboratory experiments on: (1) gravity currents propagating into a two-layered fluid; (2) gravity currents propagating into a stratified fluid; and (3) particle-driven gravity currents. The model predictions of propagation speed compared very well with laboratory experiments and analytical solutions. Two numerical approaches (Reynolds Averaged Navier-Stokes model and large-eddy simulation) are equally effective and robust in predicting propagation speed and interfacial instability compared to the laboratory experiments. The simulation of gravity currents intruding into a stratified fluid matched the theoretical solution derived from an energy model. The modified FLOW-3D model successfully captured the decreasing propagation speed due to the different deposition rates of different particle sizes, compared to experimental measurements. We extended our simulations to include the effects of particle sizes on the propagation dynamics of gravity currents. The type of gravity currents depends on particle sizes and can be subdivided into three zones: (1) When ds, is less than about 10 μm, the particle-driven gravity currents behave like IGC (Intrusive Gravity Currents) and all sediments can remain in suspension. Thus the suspended sediments can increase the density of the currents enough to travel a longer distance; (2) When ds > 40 μm, particles will rapidly settle, resulting in a decrease in excess density of the gravity currents. So, such density currents lose their momentum quickly and rapidly vanish; and (3) When 10 μm ds 40 μm, some particles will settle quickly, but others remain suspended for a long time, affecting the propagation dynamics of the currents. Modeling gravity currents in this regime particle sizes must account for particle dynamics and settling. We applied the FLOW-3D coupled with the particle dynamics algorithm to Imha Reservoir in South Korea. The model application was validated against field measurements during Typhoon Ewiniar in 2006. In the field validation, absolute mean error (AME) and root mean squared error (RMSE) for the prediction in water temperature profiles were calculated to be 1.0 oC and 1.3 oC, respectively. For turbidity predictions, AME and RMSE were 37 and 47 NTU (nephelometric turbidity units) between the simulated and the measured turbidity at stations G3, G4, and G5. We showed the influence of inflow characteristics (discharge, temperature, sediment concentration, and particle size distribution) on the fate of density currents in Imha Reservoir. Two threshold values in particle size (10 μmand 40 μm ) were identified, consistent with previous findings from the simulations of Gladstone's experiments. The simulations indicate that when the particle sizes ds are less than 10 μm, most of the sediment inflows at the inlet point (G2) will be transported to Imha Dam (G4) in suspension by interflows. When the particle sizes ds are greater than 40 μm, they will rapidly settle before reaching the dam. Therefore, highly concentrated turbid interflows could only occur when ds is less than the threshold value of 10 μm. The numerical results also present three flow regimes determining the intrusion types of density currents: (1) river inflows will form interflows when the sediment concentration Ci is less than 2000 mg/l; (2) when Ci is between 2000 mg/l and 3000 mg/l, they will form multiple intrusions (i.e., interflows and underflows); and (3) when Ci is greater than 3000 mg/l, they will plunge and propagate as underflows. These threshold values (2000 mg/l and 3000mg/l) can be used to practically predict the formation of turbid density currents, flow type, and intrusion level in Imha Reservoir.Item Open Access Investigating best management practices to reduce selenium and nitrate contamination in a regional scale irrigated agricultural groundwater system: Lower Arkansas River Valley, southeastern Colorado(Colorado State University. Libraries, 2015) Tummalapenta, Ravi Kumar, author; Bailey, Ryan T., advisor; Gates, Timothy K., committee member; Venayagamoorthy, Subhas K., committee member; Bauder, Troy A., committee memberThe Lower Arkansas River Valley (LARV) is well known for its rich agricultural production, with 109,000 ha of irrigated area. Due to agricultural production extending for more than 100 years, the LARV now faces challenges of soil salinity, water logging from shallow groundwater tables, and a high concentration of selenium (Se, both within the alluvial aquifer system and within the Arkansas River and its tributaries). Se originates primarily from bedrock and outcropped marine shale, released due to chemical oxidation in the presence of dissolved oxygen and nitrate. Se is a dynamic element that is biologically essential for plants, animals and humans. However, it is known that Se can be harmful at elevated concentrations. Therefore, elevated concentration levels in the surface water and groundwater in the LARV are considered problematic, and methods must be found to decrease groundwater concentrations and Se loadings from the aquifer to the Arkansas River. This thesis assesses plausible methods that will decrease Selenium (Se) contamination in groundwater and surface water in the LARV. Best management practices (BMPs) to reduce selenium and nitrate mass loadings to the River Arkansas in a 55,200 ha area downstream of John Martin reservoir in the LARV were explored and analyzed using 18 scenarios. The UZF-MODFLOW and UZF-RT3D numerical models, calibrated against extensive sets of field data in the region, were used to simulate groundwater flow and the physical and chemical processes governing the fate and transport of Se and N species. Specific BMPs include reduction in the seasonal application of N fertilizer; decrease in concentration of selenate (CSeO4) and nitrate (CNO3) in canal water, representing treatment of water before application as irrigation water; reduction in irrigation application volumes; and combinations of these practices, along with fallowing of irrigated land. These practices are applied for a long term period (40 years) to observe the effects of each BMP on groundwater CSeO4 and CNO3 and on mass loadings from the aquifer to the Arkansas River. The BMPs are applied at varying levels: less aggressive (20%) to very aggressive (40%) of each practice. Results indicate that the highest aggressive combined scenario 40% reduction in N fertilizer reduction, 40% reduction in canal concentration, and 35% reduction in irrigation volume, with 25% irrigated land fallowing result in the highest decrease of mass loadings of SeO4 into the Arkansas River with 22.7%, followed by the less aggressive and highest aggressive combined scenarios of N fertilizer reduction, canal concentration reduction, and irrigation volume reduction with land fallowing showing decrease of mass loadings from 15% to 21%. For individual scenarios: the irrigation volume reduction scenario (13.1% to 13.4%) is followed by the canal concentration reduction scenario (3% to 6%); whereas the N fertilizer reduction scenario shows a minimum percent reduction (1.5% to 2.7%) as compared to the Baseline (“do-nothing” scenario). Similarly for NO3, results show that the highest aggressive combined scenario 40% reduction in N fertilizer reduction, 40% reduction in canal concentration, and 35% reduction in irrigation volume, with 25% irrigated land fallowing result in the highest decrease of mass loadings of NO3 to the Arkansas River with 34.7% followed by the less aggressive and very aggressive combined scenarios of N fertilizer reduction, canal concentration reduction, and irrigation volume reduction with land fallowing showing reduction of mass loadings from 15.5% to 30%. The results of individual BMPs is as follows: 35% irrigation volume reduction scenario (14.9%) is followed by 40% N fertilizer reduction scenario (14.5%); 20% irrigation volume reduction scenario (12%); 20% N fertilizer reduction scenario (8.3%); whereas 20% and 40% canal concentration reduction scenarios show minimum percent reduction (0.6% to 1.1%). The results are compared with results from a similar study recently performed in the Upstream Study Region of the LARV to observe the differences in BMP practices and their reduction of Se contamination in the study areas.Item Open Access Mechanics of sediment plug formation in the Middle Rio Grande, NM(Colorado State University. Libraries, 2013) Park, Kiyoung, author; Julien, Pierre Y., advisor; Thornton, Christopher I., committee member; Venayagamoorthy, Subhas K., committee member; Wohl, Ellen E., committee memberThe Rio Grande is a dynamic river system which has experienced significant hydraulic and geomorphic changes through recorded history from the early 1900's to the present. These changes stem, for the most part, from natural and human interventions to the river system, which experienced channel bed elevation changes, lateral migration, straightening, channel realignment, etc. Sediment plugs have formed in the Tiffany area near San Marcial in 1991, 1995, and 2005, and in the Bosque Reach 14 miles upstream from the Tiffany plug location in 2008. Many authors have investigated the cause of sediment plugs in the Middle Rio Grande but the previous studies do not provide a complete criteria for sediment plug formation. Better understanding of the complex mechanics of plug formation on the Middle Rio Grande is therefore pursed. Based on the historic flow and geometric characteristics of plug areas, seven parameters were identified as major causing factors of sediment plug formation in the Middle Rio Grande: (1) two geometric factors: variability of channel widths and roughness; (2) two water and sediment loss factors: perching/overbanking and sediment concentration distribution profiles; and (3) three backwater effect factors: backwater effects from a reservoir, a bridge, and sharp bends. The purpose of this research is to analyze possible sediment plug parameters and to assess the primary causing factors. The specific objectives are to: (1) investigate the mechanics of sedimentation effect due to each factor; (2) simulate the historic sediment plugs using a numerical aggradation/degradation program; and (3) determine which factors contribute the most to the formation of sediment plugs. Geometric factors show that the channel has narrowed 40% between 1962 and 2002 and channel capacity has 77% decreased over time. The representative composite roughness increased 50 % between 1992 and 2002. Accordingly sediment transport capacity has decreased 45%. The narrowing (40%) with increase in roughness (50%) causes considerable loss of sediment transport capacity (45%). Therefore geometric factors induce more overbank flows and channel bed aggradation. Sedimentation factors show that the perching ratio increased from 13% to 87% between 1992 and 2002. Bank depth has decreased 51% between 1992 and 2002. The perching and lower bank depth facilitated more overbank flows and 13 ~ 20% loss of water. As particle sizes have coarsened (0.2mm in 1992 → 0.25mm in 2002) and width/depth ratios have increased (129 in 1992 → 229 in 2002), leading to higher rouse numbers and more near-bed concentration profiles. High Rouse number (Ro >1.2) and near-bed sediment concentration profile speed up the aggradation rates (4 ~ 7 times faster) than for a uniform-concentration profile. The high near-bed concentrations shorten the plug formation time from 90 to 20 days. Since snowmelt floods exceed bankfull discharges less than 2 months, the acceleration factors are essential for sediment plugs to form. Backwater effects from the Elephant Butte Reservoir influenced the upstream channel bed elevation over time. At an average flow discharge (1,550cfs), the aggradation (up to 7ft) time to fill the 25.5 mile long channel is roughly 10 years. The historic Tiffany plug area has been influenced by the reservoir levels, but with a lag time of several years. Around the San Marcial Railroad Bridge, channel bed elevation has aggraded consistently (12ft increased between 1979 and 1987). The pier contraction and congested abutments generate about a 1ft high backwater propagating to the Tiffany plug area. Sharp bends caused a 1.6ft high backwater which propagates roughly 1 mile upstream. As the beginning point of the Bosque plug is located 0.6 mile upstream of the sharp bends, backwater does influence the channel aggradation of the Bosque plug. The time to fill the main channel up to the bank crest was estimated as approximately 17 days. In terms of significance, perching/overbank flow and sediment concentration profiles can be evaluated as the primary causing factors of sediment plugs, followed by the backwater effects from bridge and sharp bends. Backwater effect from the reservoir has influenced the upstream channel elevation on a long-term basis (7 ft / 10 years). Channel narrowing and higher roughness promote overbank flows and decrease of sediment transport capacity. Owing to the increase of overbank flows, sediment concentration profiles speed up the rate of channel aggradation, causing a sediment plug within a matter of weeks, thus these two factors are the most significant factors (1.2 ft / 20 days). Two other factors, the backwater effect from the railroad bridge and sharp bends, explain why the historic sediment plugs formed at particular areas, therefore these two parameters can be classified as local triggering factors (1~1.6 ft / 20 days). On the other hand, causal factors can be divided into two groups depending on the plug location. The Tiffany plugs have been more affected by the backwater effect from the reservoir and railroad bridge. On the other hand, the Bosque plug was more influenced by the decrease of channel width/channel capacity, roughness, and sharp bends.Item Open Access Origins and impacts of tropopause layer cooling in tropical cyclones(Colorado State University. Libraries, 2020) Rivoire, Louis, author; Birner, Thomas, advisor; Knaff, John A., advisor; Bell, Michael M., committee member; Davis, Christopher A., committee member; Kummerow, Christian D., committee member; Venayagamoorthy, Subhas K., committee memberRemote sensing data from GPS radio occultation reveal temperatures lower than climatological average over a layer several kilometers deep near the tropopause above tropical cyclones (TCs). This signal, here referred to as tropopause layer cooling (TLC), occurs primarily during TC intensification and on spatial scales of the order of 1000 km. TLC has been hypothesized to be the result of: 1) Adiabatic expansion in cloud tops that overshoot the local level of neutral buoyancy. 2) Long wave radiative effects near the cloud top. 3) Adiabatic expansion in the TC secondary circulation. The relative role of these mechanism has not been quantified yet, perhaps pertaining to the large uncertainties and relative lack of vertical resolution of observational data sets and numerical modeling studies near the tropopause. Given the complex relationships between the thermal structure of the upper troposphere and the TC secondary circulation, determining which mechanisms are at play is paramount. TLC is also expected to destabilize the upper troposphere to convection and allow clouds to reach higher altitudes, likely leading to subtle but consequential changes in the secondary circulation and associated latent heating vertical distribution. Low temperatures near the tropopause can lead to in situ formation of cirrus clouds, which impact the radiative budget in the tropical tropopause layer. Lastly, low temperatures above convective systems have been linked to dehydration of the stratosphere, prompting the question of the role of TCs on the climate. Mechanism 1 is discussed in light of existing literature and suggested to be of marginal importance. Mechanisms 2 and 3 are examined using a combination of observational and theoretical analysis, and numerical modeling. Radiative heating rates calculated using cloud properties retrieved by the A-train suggest that mechanism 2 may explain up to half of TLC in the inner core, but only marginal amounts of TLC at larger radii. While reanalysis data sets suggest that mechanism 3 may explain TLC, numerical simulations of TCs with higher resolution suggest that mechanism 3 does not act in a way consistent with the secondary circulation as is typically pictured, and may need to be revisited. Other mechanisms involving processes which violate gradient wind balance near the tropopause need to be formulated. Finally, feedbacks between TLC, cloud structure, and TC dynamics are examined using parcel theory and idealized simulations. Parcel theory predicts that the TC thermal structure exerts a positive feedback on cloud top height during intensification, especially when convective entrainment is taken into account. While idealized simulations capture this general behavior, they exhibit other complex, transient behaviors which indicate breaking points in the interaction between clouds and their thermal environment.Item Open Access Quantification of hydraulic effects from transverse instream structures in channel bends(Colorado State University. Libraries, 2014) Scurlock, Stephen Michael, author; Thornton, Christopher I., advisor; Abt, Steven R., committee member; Venayagamoorthy, Subhas K., committee member; Wohl, Ellen E., committee memberMeandering river channels possess hydraulic and geomorphic characteristics that occasionally place anthropogenic interests at risk. Loss of valuable land holdings and infrastructure due to outer-bank channel encroachment from erosion processes and complications for channel-bend navigation have prompted development of techniques for reconfiguration of instream hydraulics. Transverse instream structures are one type of technique and have been implemented in channel bends to reduce outer-bank erosivity and improve navigability. Instream structures use less material and have ecological and habitat benefits over traditional revetment type bank protection. Structures are typically constructed in series, extend from the outer-bank into the channel center, and are designed with various crest heights and slopes. Current design recommendations for the structures in natural channels provide generalized ranges of geometric parameters only; no specific information pertaining to hydraulic reconfiguration is provided. Understanding specific hydraulic response to alteration of geometric structure parameters is requisite for educated structure design. Focusing on two types of transverse instream structures, the spur-dike and vane, a mathematical design tool was developed for the quantification and prediction of induced hydraulic response. A series of dimensionless groupings were formulated using parameters obtainable from field data of natural channels and grouped using dimensional analysis. Each dimensionless grouping had an identifiable hydraulic influence on induced hydraulics. A conglomerate mathematical expression was established as the framework for induced instream structure quantification. The mathematical model was tailored to produce twenty-four hydraulic relationships through regression analysis utilizing a robust physical model dataset collected within rigid-bed, trapezoidal channel bends. Average and maximum velocity and boundary shear-stress data were segmented into outer-bank, centerline, and inner-bank regions and then normalized by bend-averaged baseline conditions. Velocity equations were developed for an all-structure dataset, a spur-dike dataset, and a vane dataset. Boundary shear-stress equations were developed for spur-dike structures only. Regression equations quantified laboratory hydraulics to a high level of accuracy. Equation response to independent parameter alteration coincided with continuity principles and physical hydraulic expectations. Methods performed well in application to extraneous natural channel data from the literature. Developed methodologies from this research presented a fundamental addition to the current design procedures for the installation of structures in migrating channel bends. Quantification of the reduction of outer-bank erosive potential and increase at the shifted conveyance zone within natural channels was made possible using readily measured field data and the proposed methodology. Equations allow for previously unattainable investigation of configuration geometry combinations to meet installation objectives using simple mathematical formulas. Configuration geometry optimization to meet hydraulic design criteria using the proposed methods may hold substantial economic benefit over traditional design protocols.Item Open Access Strong and weak cold pool collisions(Colorado State University. Libraries, 2022) Falk, Nicholas Michael, author; van den Heever, Susan C., advisor; Schumacher, Russ S., committee member; Venayagamoorthy, Subhas K., committee memberCollisions between convective cold pools commonly initiate new convective storms. This occurs through enhancements to the vertical velocity through mechanical forcing, and increased water vapor content via thermodynamic forcing. The goal of this study is to investigate the impact of the following four parameters on the mechanical and thermodynamic forcing associated with cold pool collisions: (1) the initial temperature perturbation of cold pools, (2) the initial distance between cold pools, (3) the environment in which cold pools exist, and (4) the strength of atmospheric diffusion. To achieve this goal, the dynamical and thermodynamical processes of colliding pairs of cold pools is investigated using a two-dimensional, high- resolution non-hydrostatic anelastic model. The four parameters of interest were varied across a wide range of values in a model suite comprised of 11,200 large eddy simulations in total. To facilitate our analysis, a classification of cold pool collisions into categories of "mechanically strong" and "mechanically weak" is proposed. "Mechanically strong" cold pool collisions occur when the updraft velocities resulting from the collisions are greater than those produced by the flow of air forced up the leading edges of individual cold pools. In "mechanically weak" collisions, the updraft velocities produced by individual cold pools are greater than those from cold pool collisions. An analogous classification of "thermodynamically strong/weak" collisions is also proposed. The results of this analysis show that the initial temperature perturbation of the cold pools has the largest impact on mechanical and thermodynamic forcing from cold pool collisions. Colder cold pools have greater horizontal wind velocities at their heads, leading to greater near- surface horizontal convergence when they collide. This in turn leads to greater updraft velocities which are also more effective at advecting water vapor upwards. The second largest impact on mechanical and thermodynamic cold pool forcing is from the environment in which the cold pools exist. Due to a decreased vertical gradient of potential temperature, weaker low-level static stability increases mechanical forcing as the air lofted by the collisions is decelerated less by negative buoyancy. Environments with larger low-level vertical moisture gradients are associated with increased thermodynamic forcing through enhanced vertical moisture advection. The initial edge-to-edge distance between the cold pools has the third largest impact on the proxies for convective initiation. Mechanical forcing is found to peak at an optimal initial distance between cold pools of ~2.5 km due to a balance between the creation and dissipation of kinetic energy. Thermodynamic forcing, on the other hand, peaks for much greater initial cold pool distances than those associated with the mechanical forcing. This is likely a result of the faster updraft winds generated during collisions for closely spaced initial cold pools also being more effective at advecting moisture away during the collision, thereby decreasing the thermodynamic forcing. The smallest impact on the proxies for convective initiation comes from the atmospheric diffusion rate which impacts cold pool strength through mixing. Thus, this work finds that convective initiation becomes increasingly likely from a cold pool collision when the cold pools are colder, the environment is less stable and has a greater vertical water vapor gradient, the cold pools start close to some optimal separation distance, and the atmospheric diffusion rate is low.Item Open Access The dynamics of Hadley circulation variability and change(Colorado State University. Libraries, 2017) Davis, Nicholas Alexander, author; Birner, Thomas, advisor; Randall, David A., committee member; Barnes, Elizabeth A., committee member; Venayagamoorthy, Subhas K., committee member; Randel, William J., committee memberThe Hadley circulation exerts a dominant control on the surface climate of earth's tropical belt. Its converging surface winds fuel the tropical rains, while subsidence in the subtropics dries and stabilizes the atmosphere, creating deserts on land and stratocumulus decks over the oceans. Because of the strong meridional gradients in temperature and precipitation in the subtropics, any shift in the Hadley circulation edge could project as major changes in surface climate. While climate model simulations predict an expansion of the Hadley cells in response to greenhouse gas forcings, the mechanisms remain elusive. An analysis of the climatology, variability, and response of the Hadley circulation to radiative forcings in climate models and reanalyses illuminates the broader landscape in which Hadley cell expansion is realized. The expansion is a fundamental response of the atmosphere to increasing greenhouse gas concentrations as it scales with other key climate system changes, including polar amplification, increasing static stability, stratospheric cooling, and increasing global-mean surface temperatures. Multiple measures of the Hadley circulation edge latitudes co-vary with the latitudes of the eddy-driven jets on all timescales, and both exhibit a robust poleward shift in response to forcings. Further, across models there is a robust coupling between the eddy-driving on the Hadley cells and their width. On the other hand, the subtropical jet and tropopause break latitudes, two common observational proxies for the tropical belt edges, lack a strong statistical relationship with the Hadley cell edges and have no coherent response to forcings. This undermines theories for the Hadley cell width predicated on angular momentum conservation and calls for a new framework for understanding Hadley cell expansion. A numerical framework is developed within an idealized general circulation model to isolate the mean flow and eddy responses of the global atmosphere to radiative forcings. It is found that it is primarily the eddy response to greenhouse-gas-like forcings that causes Hadley cell expansion. However, the mean flow changes in the Hadley circulation itself crucially mediate this eddy response such that the full response comes about due to eddy-mean flow interactions. A theoretical scaling for the Hadley cell width based on moist static energy is developed to provide an improved framework to understand climate change responses of the general circulation. The scaling predicts that expansion is driven by increases in the surface latent heat flux and the width of the rising branch of the circulation and opposed by increases in tropospheric radiative cooling. A reduction in subtropical moist static energy flux divergence by the eddies is key, as it tilts the energetic balance in favor of expansion.Item Open Access Wind tunnel investigation of wind load on a ground mounted photovoltaic tracker(Colorado State University. Libraries, 2011) Mohapatra, Swagat, author; Bienkiewicz, Bogusz J., advisor; Venayagamoorthy, Subhas K., committee member; Sakurai, Hiroshi, committee memberWind loading is an important environmental factor to be considered in design of components and support structures of ground mounted photovoltaic tracker systems (PVT). Current understanding of action of wind on such systems is incomplete. Over the past decade, a number of investigations devoted to this topic have been carried out. However, majority of these efforts have been of proprietary nature. As a result, limited amount of data on wind loading on PV systems can be found in open literature. This study describes a wind tunnel study of wind effects on a generic ground mounted photovoltaic tracker system. The study was carried out at the Wind Engineering and Fluids Laboratory, Colorado State University. During the wind tunnel testing, the dynamic wind loading exerted on an isolated PVT system was measured and the effects of various parameters of the system on the wind loading were investigated. The investigated parameters included: the system porosity, inclination angle, wind direction and arrangement of the PV panels. A scaled model of the system was mounted on a High Frequency Force Balance (HFFB) and wind induced forces and moments were measured in a simulated atmospheric boundary layer flow. The work described herein presents an overview of the study and discusses the obtained main findings. It is concluded that certain combination of the system parameters led to a significant reduction in the exerted wind loads. A comparison of wind tunnel based design wind loads with those obtained from American Society of Civil Engineers Standard (ASCE 7-05) demonstrated reasonable agreement between the measured peak wind loads and design loads recommended by the standard.