Browsing by Author "Niemann, Jeffrey D., committee member"
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Item Open Access An investigation of an east Pacific easterly wave genesis pathway and the impact of the Papagayo and Tehuantepec wind jets on the east Pacific mean state and easterly waves(Colorado State University. Libraries, 2021) Whitaker, Justin W., author; Maloney, Eric D., advisor; Bell, Michael M., committee member; Rasmussen, Kristen L., committee member; Niemann, Jeffrey D., committee memberPart one of this dissertation investigates the transition of a Panama Bight mesoscale convective system (MCS) into the easterly wave (EW) that became Hurricane Carlotta (2012). Reanalysis, observations, and a convective-permitting Weather Research and Forecasting (WRF) model simulation are used to analyze the processes contributing to EW genesis. A vorticity budget analysis shows that convective coupling and vortex stretching are very important to the transition in this case, while horizontal advection is mostly responsible for the propagation of the system. In the model, the disturbance is dominated by stratiform vertical motion profiles and a mid-level vortex, while the system is less top-heavy and is characterized by more prominent low-level vorticity later in the transition in reanalysis. The developing disturbance starts its evolution as a mesoscale convective system in the Bight of Panama. Leading up to MCS formation the Chocó jet intensifies, and during the MCS to EW transition the Papagayo jet strengthens. Differences in the vertical structure of the system between reanalysis and the model suggest that the relatively more bottom-heavy disturbance in reanalysis may have stronger interactions with the Papagayo jet. Field observations like those collected during the Organization of Tropical East Pacific Convection (OTREC) campaign are needed to further our understanding of this east Pacific EW genesis pathway and the factors that influence it, including the important role for the vertical structure of the developing disturbances in the context of the vorticity budget. In parts two and three of this dissertation, the Weather Research and Forecasting (WRF) model is used to quantify the impact that the Papagayo and Tehuantepec wind jets have on the east Pacific mean state and east Pacific easterly waves. Specifically, a control run simulation is compared with a gaps filled simulation, where mountain gaps in the Central American mountains are "filled in" to block the Papagayo and Tehuantepec wind jets. In the absence of these wind jets, the northern half of the east Pacific mean state becomes drier, supporting a reduction in convective activity and precipitation there. Further, a 700 hPa positive vorticity feature that is linked to the Papagayo jet is reduced. An easterly wave tracking algorithm is developed and shows that easterly wave track density and genesis density are generally reduced in the eastern half of the basin for the gaps filled run. An eddy kinetic energy (EKE) budget is also calculated and highlights that EKE, barotropic conversion, and eddy available potential energy (EAPE) to EKE conversion all decrease for easterly waves when the wind jets are blocked. A composite analysis reveals that there are slight horizontal structural changes between waves in the simulations, while the waves have surprisingly similar strengths. Overall, the Papagayo and Tehuantepec wind jets are shown to be supportive influences on east Pacific easterly waves.Item Open Access Cold pool processes in different environments(Colorado State University. Libraries, 2018) Grant, Leah Danielle, author; van den Heever, Susan C., advisor; Randall, David A., committee member; Rutledge, Steven A., committee member; Niemann, Jeffrey D., committee memberCold pools are localized regions of dense air near Earth's surface. They form in association with precipitating clouds in many environments ranging from moist tropical to semi-arid continental conditions, and they play important roles in weather in climate. The overarching goal of this dissertation research is to improve our process-level understanding of cold pool interactions with different components of the Earth system, focusing on two key knowledge gaps: (1) interactions with Earth's surface in continental environments; and (2) interactions with organized convective systems in tropical oceanic environments. The primary goal of the first study conducted in this dissertation is to evaluate how surface sensible heat fluxes impact cold pool dissipation in dry continental environments via two pathways: (a) by directly heating the cold pool, and (b) by changing mixing rates between cold pool air and environmental air through altering turbulence intensity. Idealized 2D simulations of isolated cold pools are conducted with varying sensible heat flux formulations to determine the relative importance of these two mechanisms. The results demonstrate that the impact of sensible heat fluxes on mixing, i.e. mechanism (b), contributes most significantly to cold pool dissipation. Cold pool – land surface interactions in semi-arid continental conditions are investigated in the second study. Two questions are addressed: (1) how does the land surface respond to the cold pool; and (2) to what extent do land surface feedbacks modulate the cold pool evolution? Idealized 3D simulations of a cold pool evolving in a turbulent boundary layer are conducted to answer these questions. The land surface cools in response to the cold pool, resulting in suppressed sensible heat fluxes in the center of the cold pool. However, sensible heat fluxes are enhanced near the edge of the cold pool in association with higher wind speeds, leading to cold pool dissipation from the edge inwards. The land surface interactions are shown to strongly affect the cold pool, reducing its lifetime, size, and intensity by up to 50%. Preliminary analysis of a cold pool that was observed in northeastern Colorado on 17 May 2017 ("The Bees Day") during the C3LOUD-Ex field campaign is presented in the third study. The observed case exhibits similar environmental and cold pool characteristics to the first two numerical studies, thereby providing observational context for their hypotheses and conclusions. The objective of the fourth study presented in this dissertation is to determine the role of cold pools in organized tropical oceanic convective systems. To address this goal, two convective systems embedded in a weakly sheared cloud population approaching radiative-convective equilibrium are simulated at high resolution. The cold pools are weakened in the sensitivity tests by suppressing evaporation rates below cloud base. Both of the convective systems respond in a consistent manner as follows: (a) when cold pools are weakened, the convective intensity increases; and (b) the mesoscale structure, propagation speeds, and system lifetimes are insensitive to the changes in the cold pools, in contrast to the prevailing (RKW) theory that cold pools are critical to the mesoscale organization of convective systems. In summary, these high-resolution modeling and observational studies demonstrate new insights into cold pool – surface – convection interactions. The results suggest that cold pool interactions with different components of the Earth system are not all created equally; rather, these interactions depend on the environment in which the cold pools find themselves.Item Open Access Improving hydrologic modeling of ungaged basins to support environmental flow management in a heterogeneous region(Colorado State University. Libraries, 2021) Adams, Stephen K., author; Bledsoe, Brian P., advisor; Poff, N. LeRoy, committee member; Niemann, Jeffrey D., committee member; Stein, Eric D., committee memberEnvironmental streamflow management can sustain aquatic ecosystems and the services they provide by reestablishing elements of the natural flow regime that are necessary for ecological health. One of the more difficult challenges with developing environmental flow criteria is estimating streamflow at locations without gage data; however, this challenge is not unique to environmental flows. Streamflow prediction in ungaged basins is a very common problem in hydrology and engineering with no clear solution, but it is particularly difficult to model environmental streamflow metrics across heterogeneous regions with highly diverse land uses, geologic settings, and hydroclimatological processes. In this dissertation, I create a new regionalization framework, "Streamflow Regionalization with Hydrologic Model-based Classification" (SR-HMC), for modeling challenging flow metrics in ungaged basins across a heterogeneous region. I also test the efficacy of the new framework for developing environmental streamflow criteria. In Chapter 2, I explore different approaches for classifying streams with similar flow regimes and develop a novel classification technique for prioritizing regional accuracy of hydrologic models. As the precursor to SR-HMC, this "Hydrologic Model-based Classification" (HMC) groups hydrologically similar streams by determining the degree of reciprocity of calibrated parameters between a regional catalog of rainfall-runoff models as quantified through jackknife resampling. Results show that HMC complements traditional classifications based on streamflow metrics and watershed characteristics, and offer advantages over these traditional classifications when used to regionalize ungaged basins. Next, Chapter 3 describes implementation of ensemble modeling to optimize HMC into a regionalization framework for producing time series of streamflow at ungaged sites. For gaged locations, hydrologic model parameters that cannot be calculated directly can be calibrated using observed flows; however, these same model parameters are much more uncertain and difficult to estimate at ungaged locations. SR-HMC uses geographically-weighted model output averaging with regionally-calibrated parameter sets to reduce parameter uncertainty in models of ungaged basins. This new framework is tested at five sites across a large and diverse region. Results were improved using SR-HMC over standard nearest-neighbor regionalization approaches. Finally, I turn to management applications of these novel methods in ungaged basins by analyzing the statistical relationships between streamflow alteration and ecological integrity. In Chapter 4, I compare the explanatory power of simple flow-ecology relationships produced by different methods for regionalizing ungaged basins and different metrics of flow alteration. Results highlight robust modeling practices amenable to management. Development of environmental streamflow recommendations based on prediction in ungaged basins is an ongoing challenge; however, this research demonstrates how novel approaches to classification and model extrapolation can improve streamflow estimation at ungaged locations in heterogeneous regions, and thereby bolster the scientific basis of environmental flow management.Item Open Access Improving the quality of extreme precipitation estimates using satellite passive microwave rainfall retrievals(Colorado State University. Libraries, 2017) Petković, Veljko, author; Kummerow, Christian D., advisor; Vonder Haar, Thomas H., committee member; Rutledge, Steven A., committee member; Niemann, Jeffrey D., committee memberSatellite rainfall estimates are invaluable in assessing global precipitation. As a part of the Global Precipitation Measurement (GPM) mission, a constellation of orbiting sensors, dominated by passive microwave imagers, provides a full coverage of the planet approximately every 2-3 hours. Several decades of development have resulted in passive microwave rainfall retrievals that are indispensable in addressing global precipitation climatology. However, this prominent achievement is often overshadowed by the retrieval's performance at finer spatial and temporal scales, where large variability in cloud morphology poses an obstacle for accurate rainfall measurements. This is especially true over land, where rainfall estimates are based on an observed mean relationship between high frequency (e.g., 89 GHz) brightness temperature (Tb) depression (i.e., the ice-scattering signature) and rainfall rate. In the first part of this study, an extreme precipitation event that caused historical flooding over south-east Europe is analyzed using the GPM constellation. Performance of the rainfall retrieval is evaluated against ground radar and gage reference. It is concluded that satellite observations fully address the temporal evolution of the event but greatly underestimate total rainfall accumulation (by factor of 2.5). A primary limitation of the rainfall algorithm is found to be its inability to recognize variability in precipitating system structure. This variability is closely related to the structure of the precipitation regime and the large-scale environment. To address this influence of rainfall physics on the overall retrieval bias, the second part of this study utilizes TRMM radar (PR) and radiometer (TMI) observations to first confirm that the Tb-to-rain-rate relationship is governed by the amount of ice in the atmospheric column. Then, using the Amazon and Central African regions as testbeds, it demonstrates that the amount of ice aloft is strongly linked to a precipitation regime. A correlation found between the large-scale environment and precipitation regimes is then further examined. Variables such as Convective Available Potential Energy (CAPE), Cloud Condensation Nuclei (CCN), wind shear, and vertical humidity profiles are found to be capable of predicting a precipitation regime and explaining up to 40% of climatological biases. Dry over moist air conditions are favorable for developing intense, well organized systems such as MCSs in West Africa and the Sahel. These systems are characterized by strong Tb depressions and above average amounts of ice aloft. As a consequence, microwave retrieval algorithms misinterpret these non-typical systems assigning them unrealistically high rainfall rates. The opposite is true in the Amazon region, where observed raining systems exhibit relatively little ice while producing high rainfall rates. Based on these findings, in the last part of the study, the GPM operational retrieval (GPROF) for the GMI sensor is modified to offer additional information on atmospheric conditions to its Bayesian-based algorithm. When forming an estimate, the modified algorithm is allowed to use this ancillary information to filter out a priori states that do not match the general environmental condition relevant to the observation and thus reduce the difference between the assumed and observed variability in ice-to-rain ratio. The results are compared to the ground Multi-Radar Multi-Sensor (MRMS) network over the US at various spatial and temporal scales demonstrating outstanding potentials in improving the accuracy of rainfall estimates from satellite-borne passive microwave sensors over land.Item Open Access Insights into extreme short-term precipitation associated with supercells and mesovortices(Colorado State University. Libraries, 2019) Nielsen, Erik R., author; Schumacher, Russ S., advisor; van den Heever, Susan C., committee member; Bell, Michael M., committee member; Niemann, Jeffrey D., committee memberOverall, this manuscript aims to holistically evaluate the relationship between rotation and extreme precipitation processes, since radar and rain-gauge observations in several flash flooding events have suggested that the heaviest short-term rainfall accumulations were associated with supercells or mesovortices embedded within larger convective systems. A specific subclass of these events, when tornadoes and flash floods are both concurrent and collocated (referred to here at TORFF events), present a unique set of concerns, since the recommended life-saving actions for each threat are contradictory. Given this, Chapter 2 aims to evaluate the climatological and meteorological characteristics associated with TORFF events over the United States. Two separate datasets, one based on overlapping tornado and flash flood warnings and the other based on observations, were used to arrive at estimations of the instances when a TORFF event was deemed imminent and verified to have occurred, respectively. These datasets, combined with field project data, were then used to discern the geographical and meteorological characteristics of recent TORFF events. The results show that TORFF scenarios commonly occur, are not easily distinguishable from tornadic events that fail to produce collocated flash flooding, and present difficult challenges both from the perspective of forecasting and public communication. The research in Chapter 3 strives to identify the influence that rotation has on the storm-scale processes associated with heavy precipitation. Five total idealized simulations of a TORFF event, where the magnitude of the 0-1 km shear was varied, were performed to test the sensitivity of precipitation processes to rotation. In the simulations with greater environmental low-level shear and associated rotation, more precipitation fell, both in a point maximum and area-averaged sense. Intense, rotationally induced low-level vertical accelerations associated with the dynamic nonlinear perturbation vertical pressure gradient force were found to enhance the low-to-mid level updraft strength, total vertical mass flux, and allowed access to otherwise inhibited sources of moisture and CAPE in the higher shear simulations. The dynamical accelerations, which increased with the intensity of the low-level shear, dominated over buoyant accelerations in the low levels and were responsible for inducing more intense, low-level updrafts that were sustained despite a stable boundary layer. Chapter 4 aims to explore how often extreme short-term rain rates in the United States are associated with storm-scale or mesoscale vortices, since significant low-level rotation does not always yield a tornado (i.e., not all extreme rainfall events are TORFFs). Five years of METAR observations and three years of Stage-IV analyses were obtained and filtered for hourly accumulations over 75 and 100 mm, respectively. Local dual-pol radar data was then obtained for the remaining events for the hour leading up to the METAR observation. Nearly 50% of the cases were associated with low-level rotation in high-precipitation supercells and/or mesoscale vortices embedded in more organized storm modes. These results support recent modeling results, presented in Chapter 3, suggesting that rotationally induced dynamic vertical pressure accelerations are important to the precipitation formation mechanisms that lead to extreme short-term rainfall rates. The upper Texas Coast, in and around the Houston, TX area, has experienced many intense TORFF events over the recent years. The research in Chapter 5 focuses on examining the horizontally heterogeneous environmental characteristics associated with one of those events, the Tax Day flood of 2016, which was identified as a "verified" TORFF event in Chapter 2. Radar and local mesonet rain gauge observations were used to examine the storm scale characteristics to identify the locations and structures of extreme rain rate producing cells. To supplement the observational based analysis above, a WRF-ARW simulation of the Tax Day flood in 2016, based upon a real-time forecast from the HRRR, was examined. Convective cells that produced the most intense short-term (i.e., sub-hourly to hourly) accumulations within the MCS were examined for the influence of any attendant rotation on both the dynamics and microphysics of the precipitation processes. Results show that the most intense rainfall accumulations, as in the observations analysis, are associated with rotating convective elements, and the results of this chapter confirm that the processes described in Chapter 3 apply outside of the idealized framework.Item Open Access Integration of an unmanned aircraft system and ground-based remote sensing to estimate spatially distributed crop evapotranspiration and soil water deficit throughout the vegetation soil root zone(Colorado State University. Libraries, 2016) Hathaway, Jeffrey Calvin, author; Chávez, José L., advisor; Niemann, Jeffrey D., committee member; Jayasumana, Anura P., committee member; Zhang, Huihui, committee memberIrrigation is the largest consumer of fresh water and produces over 40% of the world’s food and fiber supply. As the world’s population continues to grow rapidly, the increased demands on fresh water will force the agricultural community to improve the efficiency and productivity of irrigation systems, while reducing overall water usage. In order to address the requirements of increased efficiency and productivity in agricultural water use, the agricultural community has begun to focus on the development of precision agriculture (PA) irrigation management systems for use with irrigated agriculture. Remote sensing (RS) is at the forefront of the PA movement, allowing the estimation of spatially distributed crop water requirements on a large-scale basis. Techniques using ground, aerial and space-borne RS platforms, have been developed to estimate actual crop evapotranspiration (ETa) and soil water deficit (SWD) for use in PA irrigation management systems. The ability to monitor the ETa and SWD allows irrigators to manage their irrigation to increase efficiency and decrease overall water use while maintaining crop yields goals. Historically, remote sensing data, such as spectral reflectance and thermal infrared (TIR) imagery, were provided by ground or space-borne RS platforms, like NASA’s Landsat 8 satellites. Though these methods are effective at estimating ETa over large areas, their lack of spatial and temporal resolution limit their effectiveness for application in PA irrigation management systems. In order to address the required spatial and temporal resolutions required for PA systems, Colorado State University (CSU) developed an unmanned aircraft system (UAS) RS platform capable of collecting high spatial and temporal resolution data in the TIR, near-infrared (NIR), red and green bands of the electromagnetic spectrum. During the summer of 2015, CSU conducted four flights over corn at the Agriculture Research Development and Education Center (ARDEC), near Fort Collins, CO, with the Tempest UAS RS platform in order to collect thermal and multispectral imagery. The RS data collected over the ARDEC test location were used in three studies. The first was the comparison of the raw RS data to the ground-based RS data collected during the RS overpasses. The second study used the Tempest RS data to estimate the ETa using four methods: two methods based on the surface energy balance (Two-Source Energy Balance (TSEB) and the Surface Aerodynamic Temperature (SAT)), one method based on the TIR imagery (Crop Water Stress Index (CWSI)), and one method based on the spectral reflectance imagery (reflectance-based crop coefficients (kcbrf)) and reference ET. Remote sensing derived ETa estimates were compared to ETa derived using neutron probe soil moisture sensors. The third study utilized the RS derived ETa and the Hybrid Soil Water Balance method to estimate the SWD for comparison with the neutron probe derived SWD. Results showed that the Tempest RS data was in good agreement with the ground-based data as demonstrate by the low RMSE of the raw data, ETa and SWD calculations (TIR = 5.68 oC, NIR = 5.26 % reflectance, red = 3.51 % reflectance, green = 7.31 % reflectance, TSEB ETa = 0.89 mm/d, Hybrid SWD = 16.19 mm/m). The accuracy of the results of the Tempest UAS RS platform suggests that UAS RS platforms have the potential to increase the accuracy of ETa and SWD estimation for use in the application of a PA irrigation management system.Item Open Access Intraseasonal variability in the diurnal cycle of precipitation in the Philippines(Colorado State University. Libraries, 2019) Natoli, Michael B., author; Maloney, Eric D., advisor; Bell, Michael M., committee member; Niemann, Jeffrey D., committee memberPrecipitation in the region surrounding the South China Sea (SCS) over land and coastal waters exhibits a strong diurnal cycle associated with a land-sea temperature contrast that drives a sea-breeze circulation. The boreal summer intraseasonal oscillation (BSISO) is an important modulator of the daily mean precipitation rate and the amplitude of the diurnal cycle. Using 19 years of the CMORPH precipitation product for the Philippines, it is shown that in aggregate the diurnal cycle amplitude is maximized before the arrival of the broader oceanic convective envelope associated with the BSISO. Over Luzon Island in the northern Philippines, the diurnal cycle amplitude is not in phase with daily mean precipitation, which peaks with the large-scale BSISO convection. An increase in nocturnal and morning precipitation more than compensates for the reduced precipitation rates during the afternoon peak amidst the BSISO active period. This pattern is not seen over Mindanao Island in the southern Philippines, where diurnal cycle amplitude tends to determine daily mean precipitation. A strong diurnal cycle in coastal waters west of the Philippines is evident in the transition from the inactive to active phase, due to offshore propagation of convection generated over land. This behavior is dramatically different on small spatial scales within the Philippine archipelago, depending strongly on topography. For example, the BSISO influence on the diurnal cycle on the eastern side of the high mountains of Luzon is nearly opposite to the western side. It is proposed, using wind, moisture, and radiation budget products from the ERA-Interim reanalysis, that the enhanced diurnal cycle over land and coastal waters west of the mountains during BSISO suppressed phases is a consequence of increased insolation and weaker prevailing onshore winds. Offshore propagation, and thus the diurnal cycle over the coastal waters of the SCS, is suppressed until ambient mid-level moisture increases during the transition to the active BSISO phase. In the BSISO enhanced phases, strong low level winds out of the southwest combine with increased cloudiness to suppress the sea-breeze circulation and thus the diurnal cycle of precipitation in the SCS region. Strong frictional moisture convergence leading the BSISO is not found to be concurrent with the peak in the diurnal cycle. Results are consistent when examined in other precipitation products or BSISO indices, and support conclusions derived from studies focusing on intraseasonal modulation of precipitation in other regions of the Maritime Continent, with some important local distinctions owed to geography.Item Open Access Intraseasonal variability in the tropical diurnal cycle(Colorado State University. Libraries, 2022) Natoli, Michael B., author; Maloney, Eric D, advisor; Bell, Michael M., committee member; Randall, David A., committee member; Niemann, Jeffrey D., committee memberThe relationship between large-scale intraseasonal variability in tropical convection and the local diurnal cycle on tropical islands is explored with observations and an idealized model. In part one, the impact of quasi-biweekly variability in the monsoon southwesterly winds on the precipitation diurnal cycle in the Philippines is examined using CMORPH precipitation, ERA5 reanalysis, and outgoing longwave radiation (OLR) fields. Both a case study during the 2018 Propagation of Intraseasonal Tropical Oscillations (PISTON) field campaign and a 23-year composite analysis are used to understand the effect of the QBWO on the diurnal cycle. QBWO events in the west Pacific, identified with an extended EOF index, bring increases in moisture, cloudiness, and westerly winds to the Philippines. Such events are associated with significant variability in daily mean precipitation and the diurnal cycle. It is shown that the modulation of the diurnal cycle by the QBWO is remarkably similar to that by the boreal summer intraseasonal oscillation (BSISO). The diurnal cycle reaches a maximum amplitude on the western side of the Philippines on days with average to above average moisture, sufficient insolation, and weakly offshore prevailing wind. This occurs during the transition period from suppressed to active large-scale convection for both the QBWO and BSISO. Westerly monsoon surges associated with QBWO variability generally exhibit active precipitation over the South China Sea (SCS), but a depressed diurnal cycle. These results highlight that modes of large-scale convective variability in the tropics can have a similar impact on the diurnal cycle if they influence the local scale environmental background state similarly. In part two, a specific large-scale mode is neglected, and the impact of variability in the background wind at any timescale on the local diurnal cycle is isolated. Luzon Island in the northern Philippines is used as an observational test case. Composite diurnal cycles of CMORPH precipitation are constructed based on an index derived from the first empirical orthogonal function (EOF) of ERA5 zonal wind profiles. A strong precipitation diurnal cycle and pronounced offshore propagation in the leeward direction tends to occur on days with a weak, offshore prevailing wind. Strong background winds, particularly in the onshore direction, are associated with a suppressed diurnal cycle. Idealized high resolution 2-D Cloud Model 1 (CM1) simulations test the dependence of the diurnal cycle on environmental wind speed and direction by nudging the model base-state toward to composite profiles derived from the reanalysis zonal wind index. These simulations can qualitatively replicate the observed development, strength, and offshore propagation of diurnally generated convection under varying wind regimes. Under strong background winds, the land-sea contrast is reduced, which leads to a substantial reduction in the strength of the sea-breeze circulation and precipitation diurnal cycle. Weak offshore prevailing winds favor a strong diurnal cycle and offshore leeward propagation, with the direction of propagation highly sensitive to the background wind in the lower free troposphere. Offshore propagation speed appears consistent with density current theory rather than a direct coupling to a single gravity wave mode, though several gravity wave modes apparent in the model likely contribute to a destabilization of the offshore environment. In part three, the hypotheses developed in parts one and two regarding the mechanisms regulating the diurnal cycle response are rigorously tested. A novel probabilistic framework is applied to the Luzon test case to improve the understanding of diurnal cycle variability. High amplitude diurnal cycle days tend to occur with weak to moderate offshore low-level wind and near to above average column moisture in the local environment. The transition from the BSISO suppressed phase to the active phase is most likely to produce the wind and moisture conditions supportive of a substantial diurnal cycle over western Luzon and the South China Sea (SCS). Thus, the impact of the BSISO on the local diurnal cycle can be understood in terms of the change in the probability of favorable environmental conditions. Idealized high-resolution 3-D Cloud Model 1 (CM1) simulations driven only by a base-state derived from BSISO composite profiles are able to reproduce several important features of the observed diurnal cycle variability with BSISO phase, including the strong, land-based diurnal cycle and offshore propagation in the transition phases. Background wind appears to be the primary variable controlling the diurnal cycle response, but ambient moisture distinctly reduces precipitation strength in the suppressed BSISO phase, and enhances it in the active phase. A land-breeze, lingering deep convection over land after sunset, and strong mechanical convergence appear to all be required in order to produce offshore propagation in CM1. Simulations in which the diurnal cycle of insolation is removed suggest the potential for a natural timescale for convective regeneration related to the island size.Item Open Access Salt transport in the South Platte river system: modeling, controlling factors, and management strategies(Colorado State University. Libraries, 2021) Hocking, Craig, author; Bailey, Ryan T., advisor; Ronayne, Michael J., committee member; Niemann, Jeffrey D., committee memberIncreasing salinity poses a severe threat to urban and agricultural areas. Excess salt can accumulate in soils and groundwater, thereby impacting crop growth and productivity. This thesis aims to quantify the influence of the driving forces behind salt transport in Colorado's agro-urban South Platte River network, which has an approximate drainage area of 24,300 mi2 (62,937 km2), and investigates possible mitigation strategies to reduce salinity levels in both urban and agricultural river reaches. For this study, a one-dimensional in-river salt transport model was developed for the South Platte River system utilizing StateMod (Colorado's Division of Water Resources water allocation model) to simulate streamflow. The model accounts for multiple inputs and outputs of salt within the river network, including tributaries, wastewater treatment plants, road salt, runoff return flows from irrigation, and groundwater discharge, the latter from interpolated groundwater concentration maps generated from sampling data provided by the Agricultural Water Quality database. These concentration data are combined with the StateMod-simulated streamflow to simulate salt flow through the river network. The flow and salt models were run on a monthly basis over five years between 2002 and 2006. Based on Nash-Sutcliffe Coefficient of Efficiency (NSCE) statistics for the flow and salt models, 85% of the flow model's monthly NSCE values and approximately 68% of the salt model's monthly NSCE values fell within the acceptable range of zero to one. A global sensitivity analysis was implemented to determine the controlling factors behind salt transport in the river system. Two different scenarios were run: a reach-to-reach sensitivity study where the South Platte River was divided into five different reaches, and a seasonal sensitivity study performed over the entire South Platte River for spring (March to May), summer (June to August), fall (September to November), and winter (December to February). For urban areas located in the upstream region of the basin, controlling factors include wastewater treatment plant (WWTP) effluent concentration, salt in urban return flows, the initial concentration of salinity in upstream river water, and road salt loading. For agriculture areas located in the downstream region of the basin, controlling factors include the WWTP effluent concentration, salt in urban return flows, salt in agricultural return flows, and road salt loading, indicating the influence of upstream salinity loadings on downstream river water. Based on the sensitivity studies results, an assessment of potential management practices (MPs) was carried out for both urban and agricultural reaches. A total of 256 different MP trials were run each month. The final MP results were then calculated as the averages of the individual monthly results. A point system was assigned to help rank the trials by how efficient they were at reducing salinity levels. For the urban region, the most efficient MP during the spring and summer months is to reduce WWTP effluent concentration by 35%, resulting in a salinity concentration of 340 mg/L, a decrease of 17% from the baseline value. During the fall and winter months, the most efficient MP is to reduce road salt by 35%, resulting in a salinity concentration of 730 mg/L, a decrease of 19% from the baseline value. For agricultural areas, very few MP combinations achieve an in-river salinity concentration less than 1000 mg/L, which is approximately the level in irrigation water at which crop yield decreases. The most effective MP to accomplish this consists of a 35% reduction in WWTP effluent concentration, salt in urban return flows, salt in agricultural return flows, and road salt loading. These results point to the extreme challenge of managing salinity in the South Platte River Basin and the aggressive approaches that must be implemented to sustain irrigation practices in the basin's downstream regions. In general, this thesis provides a framework for assessing salinity movement and mitigation in a large-scale urban-agricultural river basin.Item Open Access Spatial and temporal channel changes across the watershed scale following wildfire and floods(Colorado State University. Libraries, 2018) Brogan, Daniel Joseph, author; Nelson, Peter A., advisor; Kampf, Stephanie K., committee member; MacDonald, Lee H., committee member; Niemann, Jeffrey D., committee memberFires and floods are important drivers of geomorphic change. The hydrologic and sedimentologic effects of fires have been relatively well studied at the hillslope scale, but we still lack the ability to accurately quantify and predict post-fire flooding and geomorphic changes at larger scales. This lack of understanding stems primarily from two reasons. First, there is generally limited availability of repeat high-resolution topography following fires, and this limits our ability to quantify and explain changes throughout a given channel network. Second and more fundamentally, one cannot simply scale up hillslope processes to the watershed scale, or vice-versa. Since global warming is leading to more wildfires and a higher likelihood of extreme precipitation, understanding downstream flooding and sedimentation is more critical than ever for safeguarding downstream landowners, water users, and aquatic biota. This dissertation investigates these shortcomings by documenting post-fire channel changes across watershed scales and how extreme floods can alter the more typical post-fire geomorphic response. I focus on two ~15 km2 watersheds, Skin Gulch (SG) and Hill Gulch (HG), that burned in the 2012 High Park Fire, Fort Collins, Colorado, U.S.A. Over the subsequent four years I used repeat surveys of 10-11 cross sections and longitudinal profiles along the lower channel network of each watershed, and five sequential airborne laser scanning (ALS) surveys, to quantify erosion and deposition. SG was first subjected to a high-intensity convective storm just days after the fire was contained; the resulting flood caused an exceptionally large peak flow, and extensive downstream deposition of cobbles, boulders and woody debris. Fifteen months later SG and HG experienced catastrophic stripping and bed coarsening due to an unusually rare and widespread mesoscale storm, with much greater changes in SG. These events and the data used to document their effects set up the basis of three separate, yet interdependent comparisons. First, I compare and contrast the peak flows and the effects of the two distinctly different flood disturbances in SG: the short-term peak flow and substantial deposition caused by the convective flood immediately after burning; and the widespread channel and valley bottom erosion caused by the mesoscale flood. Peak flows were estimated using three independent techniques: 1) slope-area method, 2) critical flow, and 3) 2D hydrodynamic modeling. The peak flow estimates for the 2013 flood had a higher relative uncertainty and this stemmed from whether I used pre- or post-flood channel topography. The results document the extent to which a high and moderate severity wildfire can greatly increase peak flows and alter channel morphology, illustrate how indirect peak flow estimates have larger errors than is generally assumed, and indicate that the magnitude of post-fire floods and geomorphic change can be affected by the timing, magnitude, duration, and sequence of rainstorms. Second, I use the repeat surveys of the cross sections and longitudinal profiles to quantify the channel response to the 2012 wildfire, summer thunderstorms, spring snowmelt, and the mesoscale flood in both SG and HG. The varying response between the two watersheds during the mesoscale flood necessitated further investigation. Discussions with a local landowner indicated that a flood in 1976 caused tremendous channel erosion and widening in the lower portion of HG. Geomorphic changes in HG after the fire and the mesoscale flood were much smaller than in SG, and this can be attributed to: greater post-fire, pre-mesoscale flood deposition in SG; reduced sensitivity in HG as a result of the large erosional flood in 1976; and the spatial distribution of burn severity leading to a lower peak flow in HG from the mesoscale flood. These results suggest that fires can trigger significant and dynamic channel changes over sub-decadal timescales, but unusually long or intense rainstorms can cause larger and more persistent watershed-scale changes regardless of whether a catchment has recently burned. I propose a state-and-transition conceptual model to relate landscape sensitivity to geomorphic changes according to its history of fires and floods. Third, I use the repeat ALS data to quantify spatial and temporal patterns of erosion and deposition throughout the channel networks of SG and HG. These volumes of change are related to valley and basin morphology, precipitation amounts and intensities, and burn severity. The results suggest that the amount and location of stored sediment in the valleys is critical for evaluating potential locations of erosion and deposition. Morphometric characteristics, when combined with burn severity and a specified storm, can indicate the relative likelihood and locations of post-fire erosion and deposition risks. Taken together, this body of work demonstrates: 1) how the timing and sequence of different disturbances affect the relative sensitivity of watersheds to downstream channel changes; 2) that the effects of extreme floods are longer lasting and more dominant than the effects of wildfires; and 3) that the amount and location of stored sediment in the valleys is critical for predicting potential geomorphic change. This information can help resource managers assess downstream risks and prioritize areas for post-fire hillslope rehabilitation treatments.Item Open Access The impacts of mineral dust on organized mesoscale deep convection(Colorado State University. Libraries, 2012) Seigel, Robert Brian, author; van den Heever, Susan C., advisor; Kreidenweis, Sonia M., committee member; Schubert, Wayne H., committee member; Niemann, Jeffrey D., committee memberTo view the abstract, please see the full text of the document.Item Open Access The innovative application of random packing material to enhance the hydraulic disinfection efficiency of small scale water systems(Colorado State University. Libraries, 2021) Baker, Jessica L., author; Venayagamoorthy, Subhas Karan, advisor; De Long, Susan K., advisor; Niemann, Jeffrey D., committee member; Leisz, Stephen J., committee memberIn a world where the quality of our water supplies is declining and our infrastructure is deteriorating, let alone the lack of available water in arid regions, the treatment of drinking water is becoming ever more challenging – especially for small scale systems that lack technical and financial support. The innovative application of random packing material (RPM) has been proposed as a possible tool to aid small water treatment systems (SWTSs) improve their disinfection contact systems in order to meet the Safe Drinking Water Act (SDWA) standards and provide the communities they serve with safe drinking water. While it has been demonstrated at the laboratory–scale that RPM can significantly improve the hydraulic disinfection efficiency of a contact basin in terms of baffling factor (BF) there was a lack of fundamental understanding of why RPM is so effective. Conceptually, the RPM slows and spreads the jet flow from a sharp inlet. Yet the mechanics of a jet flow through a highly porous material such as RPM is not well understood. Insight into the dynamics of such a flow is important in order to be able to use RPM in a manner that maximizes the benefits and minimizes the (unintended) drawbacks. The main aim of this dissertation is to use laboratory-scale experiments to study the mechanics of a turbulent jet flow from a long pipe through RPM and the impact on the hydraulic disinfection efficiency and final water quality for a disinfection contactor. There are three main objectives in this work: (1) To gain fundamental insights regarding turbulent jet flow through a highly porous media (such as RPM); (2) To address practical concerns for the application of the use of RPM in disinfection contactors; and (3) To provide guidance in terms of best practice for the innovative use of RPM to enhance hydraulic disinfection efficiency in SWTSs. The first part of this dissertation focuses on the resulting flow fields of a turbulent jet flow (5-20 gpm) through a wall of RPM of various thicknesses (L). An experiment was conducted in a flume using a Particle Image Velocimetry (PIV) system to map the flow fields downstream of the jet up to x⁄dj ≈ 30 (where dj is the diameter of the jet, i.e. inlet pipe). Once the PIV data were verified using a Laser-Doppler Anemometry (LDA) system and validated for a jet into an ambient (provided as a baseline), the velocity fields of the jet flow downstream of the walls of RPM were analyzed. A second order relationship was observed between the thickness of RPM and the spread of the flow. It was also observed that the jet velocities decay exponentially through RPM. With respect to flow rate, the spreading rate increased slightly, but there was a slight decrease in the decay of the jet as the flow rate increased. While the maximum velocities were reduced by over 90% after L ≈ 5dj, it was only after L ≈ 15dj that the flow downstream of the RPM was nearly uniform. Furthermore, the coefficients of drag showed a non-monotonic relationship with respect to the particle Reynolds number (Redp) that followed the well-established trend of a uniform flow around an infinitely long cylinder. This relationship provides valuable insight into the different regimes of the highly complex flow within and/or downstream of a highly porous material. Next, the potential improvement in the hydraulic disinfection efficiency and the possible energy loss as a result of the presence of random packing material in a laboratory-scale chlorine contactor were investigated. Tracer tests were conducted on a 55-gal drum tank filled with RPM in varying amounts in different configurations to measure the efficiency of each setup in terms of baffling factor. The bulk pressure drop was measured to determine the energy loss for each configuration. The results of this study show that securing RPM near the inlet, in any amount, improves the BF by 300% to more than 900%. The amount of RPM begins to have an impact at or above an inlet jet Reynolds number of 27,700. Also, changes in head loss due to the presence of RPM (in any amount, configuration, and/or flow rate) were generally considered to be negligible. Finally, a concern surrounding the potential for excessive biofilm growth is addressed through a long-term study. The inflow, outflow, and RPM were monitored for heterotrophic bacteria (via heterotrophic plate counts) and Pseudomonas aeruginosa as indicators of bacteriological water quality and the presence of biofilm. The results of this study show that there was no substantial biofilm growth in a lab-scale chlorine contactor and no substantial increase in bacterial counts for the bulk outflow over a 10-week period. Thus, the potential for excessive biofilm growth should not be considered a barrier concerning the use of RPM to improve the hydraulic disinfection efficiency of chlorine contactors in small drinking water treatment systems. Overall, this dissertation work aims to contribute a foundational understanding of turbulent jet flow through a highly porous material such as RPM as well as address some practical concerns for the innovative application of RPM to improve the hydraulic disinfection efficiency. From the results of the studies conducted, best practice guidelines have been developed to maximize the potential benefit of using RPM in disinfection contactors. Ultimately, the hope of this work is to promote the use of RPM to help SWTSs that are struggling to meet SDWA standards and to provide the communities they serve with safe drinking water.Item Open Access Toward an improved understanding of the synoptic and mesoscale dynamics governing nocturnal heavy-rain-producing mesocale convective systems(Colorado State University. Libraries, 2015) Peters, John M., author; Schumacher, Russ S., advisor; van den Heever, Sue, committee member; Johnson, Richard, committee member; Niemann, Jeffrey D., committee member; Weisman, Morris, committee memberIn the first stage of this research, rotated principal component analysis was applied to the atmospheric fields associated with a large sample of heavy-rain-producing mesoscale convective systems (MCSs) that exhibited the training-line adjoining stratiform (TL/AS) morphology. Cluster analysis in the subspace defined by the leading two resulting principal components revealed two sub-types with distinct synoptic and mesoscale characteristics, which are referred to as warm-season type and synoptic type events respectively. Synoptic type events, which tended to exhibit greater horizontal extent than warm-season type events, typically occurred downstream of a progressive upper-level trough, along a low-level potential temperature gradient with the warmest air to the south and southeast. Warm-season type events on the other hand occurred within the right entrance region of a minimally-to-anticyclonically curved upper level jet streak, along a low-level potential temperature gradient with the warmest low-level air to the southwest. Synoptic-scale forcing for ascent was stronger in synoptic type events, while low-level moisture was greater in warm-season type events. Warm-season type events were frequently preceded by the passage of a trailing stratiform (TS) type MCS, while synoptic type events often occurred prior to the passage of a TS type system. An idealized modeling framework was developed to simulate a quasi-stationary heavy-rain-producing MCSs. A composite progression of atmospheric fields from warm season TL/AS MCSs was used as initial and lateral boundary conditions for a numerical simulation of this MCS archetype. A realistic TL/AS MCS initiated and evolved within a simulated mesoscale environment that featured a low-level jet terminus, maximized low-level warm air advection, and elevated maximum in convective available potential energy. The first stage of MCS evolution featured an eastward moving trailing-stratiform type MCS that generated a surface cold pool. The initial system was followed by rearward off-boundary development (ROD), where a new line of convective cells simultaneously re-developed north of the surface cold pool boundary. Backbuilding persisted on the western end of the new line, with individual convective cells training over a fixed geographic region. The final stage was characterized by a deepening and southward surge of the cold pool, resulting in the weakening and slow southward movement of the training line. The dynamics of warm season TL/AS MCSs are elucidated through the analysis of the idealized simulation, along with a simulation of an observed case. The environmental conditions external to the MCS contributed to the development of a new convective line west of the initial MCS, and displaced northward of the southwestern flank of the surface OFB. Southwesterly low-level flow was thermodynamically stabilized as it lifted over the southwestern OFB from a pattern of adiabatic cooling below latent heating. This flow traveled 80-100 km northeastward beyond the surface OFB to the point where large-scale lifting sufficiently destabilized the flow for deep convection. These factors explain the geographic offset of the second convective line from the surface OFB left by the forward-propagating MCS. Eventually the surface cold pool became sufficiently deep so that gradual ascent of parcels with moisture and instability over the feature began triggering new convection close to the OFB (rather than 80-100 km away from it), which eventually drove the system southward. These results suggest that large-scale environmental factors were predominantly responsible for the quasi-stationary behavior of the simulated MCS, though upscale convective feedbacks played an important role in the complexity of the convective evolution.