Browsing by Author "Venayagamoorthy, Subhas Karan, committee member"
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Item Open Access A simplified approach to understanding boundary layer structure impacts on tropical cyclone intensity(Colorado State University. Libraries, 2018) Delap, Eleanor G., author; Bell, Michael M., advisor; Maloney, Eric D., committee member; Venayagamoorthy, Subhas Karan, committee memberThe relationship between tropical cyclone boundary layer (TCBL) structure and tropical cyclone (TC) intensity change is difficult to understand due to limited observations of the complex, non-linear interactions at both the top and bottom boundaries of the TCBL. Consequently, there are debates on how the TCBL interacts with surface friction and how these interactions affect TC intensity change. To begin to address these questions, a conceptual framework of how axisymmetric dynamics within the TCBL can impact TC intensity change is developed from first principles in the form of a new, simple logistic growth equation (LGE). Although this LGE bears some similarities to the operational LGE Model (LGEM; DeMaria 2009), the difference is that our growth-limiting term incorporates TCBL structure and surface drag. The carrying capacity of the LGE—termed the instantaneous logistic potential intensity (ILPI) in this study—is used to explore the relationship between TCBL structure and TC intensity. The LGE is also further solved for the drag coefficient (CD) to explore the relationships between it and both TCBL structure and TC intensity. The validity of this new LGE framework is then explored in idealized numerical modeling using the axisymmetric version of Cloud Model 1 (CM1; Bryan and Fritsch 2002). Results show that CM1 exhibits changes to TCBL structure and TC intensity that are consistent with the LGE framework. Sensitivity of these results to the turbulent mixing lengths, Lh and Lv, are also explored, and general LGE relation- ships still hold as CD is increased. Finally, the LGE framework is applied to observations, and initial CD retrievals indicate that while this new method is low compared to Bell et al. (2012), they are still plausible estimates.Item Open Access Characterization of direct injected propane and iso-octane at engine-like conditions in a high-pressure spray chamber(Colorado State University. Libraries, 2022) Windell, Brye Thomas, author; Windom, Bret C., advisor; Olsen, Daniel, committee member; Venayagamoorthy, Subhas Karan, committee memberThis thesis focuses on the recommission, modification, and testing of a high-pressure spray chamber (HPSC) and its role in aiding the experimental and numerical examination of direct injection (DI) propane at various engine-like conditions to address fundamental limitations of achieving near diesel efficiencies in heavy duty on-road liquified petroleum gas (LPG) engines. The HPSC was reconstructed and is capable of incorporating optical diagnostic techniques including high-speed Schlieren and planar Mie scattering imaging. High-speed Schlieren was used to characterize the global spray morphology and vapor phase regions while planar Mie scattering allowed for individual plume resolution providing insights into the liquid regions of the spray. These optical imaging techniques unveiled propane's spray propagation was fed by flash boiling effects, spray collapse, and high degree of vaporization, unlike iso-octane. This resulted in a direct proportionality of propane's penetration length to temperature, an inversely proportional relationship to ambient pressure, and a direct proportionality to injection pressure. Contrary to propane, iso-octane's spray morphology exhibited minor changes as temperatures and pressures were varied. Due to these unique effects, flash boiling, spray collapse, and high degree of vaporization, propane is classified as an unconventional spray, dissimilar to iso-octane's spray morphology. Experimental testing provided corrections to numerical models that were developed to reproduce the under-expanded jet dynamics. The numerical modeling results were found to be sensitive to cone and inclusion angles. The current work serves as preliminary results for an experimental validation campaign which aid in the numerical model development for future heavy duty on-road LPG engines.Item Open Access Effect of large-scale anisotropy on the small-scale structure of turbulence(Colorado State University. Libraries, 2014) Morshed, Khandakar Niaz, author; Dasi, Lakshmi Prasad, advisor; Kirkpatrick, Allan, committee member; Gao, Xinfeng, committee member; Venayagamoorthy, Subhas Karan, committee memberEven though the small-scale structure of turbulence has been hypothesized to be locally isotropic with universal properties, numerous studies document the departure from local isotropy and universality in the presence of strong mean shear (or large-scale anisotropy). The goal of this work is to elucidate the effects of strong shear on the small-scale structure with emphasis on the physical mechanism through which mean shear deviates local structure from isotropy. Two dimensional time-resolved particle image velocimetry (PIV) experiments were performed in a stationary turbulent flow past a backward facing step at Reynolds numbers 13600 and 5500 based on the maximum velocity and step height. Large-scale anisotropic properties of the flow along with local turbulence characteristics were quantified in detail. Special points of interest distributed within the measurement domain for varying large-scale anisotropic characteristics were probed to analyze small-scale structure. Results show that velocity structure functions and their scaling exponents systematically align with the principal directions of deformation of the mean flow field. Furthermore, the probability density function (PDF) of the instantaneous dissipative scales indicate a potentially universal mechanism of how mean shear affects the distribution of dissipative scales captured through a local Reynolds number based on mean shear and dissipation rate. PDFs of the instantaneous dissipative scales in all directions demonstrate that mean shear strength and local principal axis directions dictate the behavior of structure functions, correlation functions, thereby influencing the dissipative scale PDFs in a directionally dependent manner.Item Open Access Investigation of relationships between tropical cyclone structure and intensity change(Colorado State University. Libraries, 2022) Casas, Eleanor G., author; Bell, Michael M., advisor; Randall, David A., committee member; Maloney, Eric D., committee member; Venayagamoorthy, Subhas Karan, committee memberRapid intensification (RI) of a tropical cyclone (TC) remains one of the largest sources of intensity forecast error, due in part to internal dynamics that are complex and less well understood. Part of the difficulty in improving understanding of RI is due to complex interactions across a wide range of TC intensities, shapes, and sizes. In this doctoral study, I investigate these interactions by first simplifying the complexity and reducing the dimensionality of the intensity and structure parameter space to distill the key aspects of variability from observations, and then re-introducing physical complexity back into the experimental design through idealized modeling. In Chapter 2, an Empirical Orthogonal Function (EOF) analysis is used to develop the intensity-size framework that lays the foundation for the rest of this doctoral study. In addition to commonly-used TC metrics, a new structural parameter is introduced that describes the decay of tangential wind outside the radius of maximum wind (RMW). The utility of this framework is demonstrated for describing key TC evolutionary features with observations of Hurricanes Rita (2005) and Charley (2004) and numerical simulations of Rita. In Chapter 3, simplified TC analytic profiles are used to construct physically realistic wind fields that can explore the intensity-size phase space. Results suggest that while there are systematic differences between the details of the reconstructed wind fields using different methods, they all are representative of observed variability in TC structure despite being derived from a relatively small set of parameters derived from the EOFs. In Chapter 4, these simplified TC wind profiles are used to investigate the tropical cyclone boundary layer (TCBL) response across our intensity-size phase space using both height-averaged (slab) and height-resolved TCBL numerical models. The results suggest that while there are some different dynamical ramifications of the specific analytic profiles used, the response depends more on the location in the intensity and size phase space than on the differences between analytic wind formulations. The results indicate that (1) strong, big TC profiles produce the strongest supergradient wind within the TCBL; (2) weak, big TCs have the largest RMW contraction as the TCBL adjusts; and (3) weak TCs regardless of size have TCBL responses that are less conducive for intensification. Finally, in Chapter 5, full-physics, axisymmetric models are used to test whether the one-way TCBL responses found in Chapter \ref{c4_results} are consistent with two-way TCBL interactions with influences from convection, and explore the dependencies of intensification rates on TC internal structure. The results suggest that small, strong TCs can achieve the highest rapid intensification rates. The findings suggest that while intensification rates do not systematically vary with contraction rates of the RMW, both intensification and contraction rates do have some dependence on different aspects of TC intensity and size across the phase space. When visualized in the phase space, there is a relatively smooth transition between a "initially large mode" and "initially small mode" of RI. The findings of this doctoral study provide new insights into the role of TC intensity and size in the RI process.Item Open Access On the observed and simulated responses of the extratropical atmosphere to surface thermal forcing(Colorado State University. Libraries, 2019) Wills, Samantha M., author; Thompson, David W. J., advisor; Alexander, Michael, committee member; Barnes, Elizabeth, committee member; Maloney, Eric, committee member; Venayagamoorthy, Subhas Karan, committee memberThe ocean is an integral part of the climate system, and its closely-coupled interactions with the atmosphere system have wide-ranging impacts on the large-scale and local patterns of climate and weather variability from one region of the globe to another. Improvements in the resolution of satellite observations and numerical models over the past decade have led to a series of advances in understanding the role of the ocean in extratropical air-sea interaction. While the influence of the extratropical ocean can be relatively subtle and difficult to detect, recent studies have provided a growing body of evidence suggesting that the extratropical ocean has a potentially important influence on the atmospheric circulation on a wide variety of timescales. The aim of this thesis is to improve the current understanding on the role of the extratropical ocean in climate by 1) presenting new observational analyses on the relationships between midlatitude SST anomalies and the atmospheric circulation on subseasonal timescales and 2) providing a new, simplistic framework for interpreting the atmospheric response to surface thermal forcing across the globe in an idealized global climate model. In the first theme of this thesis, observational analyses of daily-mean data are exploited to re-examine the evidence for midlatitude air-sea interaction over the Kuroshio-Oyashio Extension region, and important comparisons are drawn to a previous companion study over the Gulf Stream Extension region. The results indicate that during the boreal winter season, SST anomalies in both the Gulf Stream and Kuroshio-Oyashio Extension regions are associated with distinct spatial and temporal patterns of atmospheric variability that precede and follow peak amplitude in the SST field on daily-mean timescales. In particular, a very similar pattern of low pressure anomalies that develops over the warm SST anomalies is viewed as the most robust common aspect of the atmospheric "response" over both ocean basins. The least common aspect of the "response" is characterized by robust high pressure anomalies that develop over the North Atlantic and have a seemingly unique relationship to positive lower-tropospheric temperature anomalies generated over the Gulf Stream Extension region. These results suggest that extratropical SST anomalies on subseasonal timescales are capable of forcing significant changes in the large-scale atmospheric circulation through the transfer of heat from the ocean to the atmosphere. Partially motivated by the results from the observational analyses, the second theme of this thesis presents a simplified model framework to critically assess the one-way influence of the ocean on the atmosphere at different locations across the globe. A series of steady-state and transient numerical experiments are designed to explore the atmospheric response to surface thermal forcing in an idealized "aquaplanet" configuration of the NCAR Community Atmosphere Model, Version 5.3. The results indicate that in each of the extratropical SST perturbation experiments, there is a consistent and robust steady-state atmospheric response (of similar sign and amplitude) to surface thermal forcing. The response is characterized by a hemispheric-scale, equivalent-barotropic pattern of atmospheric circulation anomalies reminiscent of the model's leading mode of internal variability and is seemingly independent of the latitudinal placement of the heat source. This result is explored further, and a possible explanation of the consistent steady-state atmospheric circulation response is discussed.Item Open Access On the observed relationships between variability in sea surface temperatures and the atmospheric circulation in the Northern Hemisphere(Colorado State University. Libraries, 2015) Wills, Samantha M., author; Thompson, David W. J., advisor; Barnes, Elizabeth, committee member; Venayagamoorthy, Subhas Karan, committee memberThe advent of increasingly high-resolution satellite observations and numerical models has led to a series of advances in our understanding of the role of midlatitude sea surface temperature (SST) in climate variability, especially near western boundary currents (WBC). For example, recent observational analyses suggest that ocean dynamics play a central role in driving interannual SST variability over the Kuroshio-Oyashio and Gulf Stream Extension regions, and recent numerical experiments suggest that variations in the SST field in the Kuroshio-Oyashio Extension region may have a much more pronounced influence on the atmospheric circulation than previously thought. We assess the observational support for (or against) a robust atmospheric response to midlatitude ocean variability in the Kuroshio-Oyashio and Gulf Stream Extension regions. We apply lead/lag analysis based on daily data to assess relationships between SST anomalies and the atmospheric circulation on transient timescales, building off of previous studies that have applied a similar methodology to weekly data. In addition, we employ a novel approach to separate the regressions into an "atmospheric forcing" pattern and an "atmospheric response" pattern through spatial linear decomposition. The analysis reveals two distinct patterns associated with midlatitude atmosphere/ocean interaction in the vicinity of the major Northern Hemisphere WBCs: 1) a pattern that peaks 2-3 weeks before the SST anomalies (the "atmospheric forcing") and 2) a pattern that peaks after the SST anomalies (the "atmospheric response"). The latter pattern is independent of the former, and is interpreted as the signature of SST variability in the atmospheric circulation. Further analysis is required to understand if the "atmospheric response" pattern truly reflects the response to the SST anomalies within the WBC regions.Item Open Access The role of moisture-convection feedbacks in simulating the intraseasonal oscillation(Colorado State University. Libraries, 2009) Hannah, Walter, author; Maloney, Eric D., advisor; Randall, David A. (David Allan), 1948-, committee member; Venayagamoorthy, Subhas Karan, committee memberThe sensitivity of the intraseasonal oscillation (ISO) in the National Center for Atmospheric Research's (NCAR) Community Atmosphere Model (CAM) version 3.1 with Relaxed Arakawa-Schubert (RAS) convection modified with the moisture trigger of Tokioka et al. (1988) is analyzed with respect to changes to the specified minimum entrainment rate. Implementation of the Tokioka moisture trigger results in a drier and cooler troposphere due to the suppression of deep convection. A higher minimum entrainment threshold leads to more suppressed deep convection and improves the sensitivity of convection to free tropospheric humidity. This is accompanied by enhanced intraseasonal variability in the tropics. The simulated ISO which results from a non-zero minimum entrainment rate resembles a moisture mode. Variance of the column integrated saturation fraction is increased when minimum entrainment rate is increased, and precipitation becomes an increasingly non-linear function of saturation fraction which indicates that moisture-convection feedbacks are enhanced in the model. A reduction in the mean column moist static energy export by divergent motions indicates that the simulations with non-zero minimum entrainment thresholds are able to achieve negative gross moist stability which has been suggested as a necessary condition to be able to produce a moisture mode. This decrease in gross moist stability with increased minimum entrainment rate is accompanied by a lowering of the mean diabatic heating profile maximum. Additional simulations are analyzed to investigate the impact of a rain re-evaporation fraction parameter on the simulated ISO. A higher rain re-evaporation fraction leads a stronger ISO signal in the model. However, In contrast to the effect of increased minimum entrainment rate, increased rain re-evaporation fraction yields a mean state which is warmer and moister. This discrepancy in mean state humidity change indicates that intraseasonal variability has no unique dependence on basic state humidity, in contradiction to that suggested in previous studies.