Browsing by Author "Giammanco, Ian Matthew"
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Item An observational study of the South Plains nocturnal low-level jet(2005-08) Giammanco, Ian Matthew; Peterson, Richard E.; Schroeder, John L.; Chang, Chia-Bo; Swift, Andrew H. P.The presence of nocturnal low-level jet streams across the Great Plains of the United States has been well documented over the last fifty years. These features are the main source of moisture transport across this region of the country. The evolution of these features has become a significant area of interest as more structures come in contact with the layer occupied by the low-level jet. The major wind resource in the United States lies in the Great Plains, the same region, which experiences the highest frequency of low-level jet streams. Low-level jets occur throughout the year but exhibit the highest frequency during the summer months as shown by Whiteman et al. [1]. The development of wind turbines that extend up to heights near two hundred meters has resulted in these structures coming into contact with the layer just beneath the jet maximum. This layer is characterized by stable, stratified flows and intense wind shear values. The stratified layers are also responsible for the formation of coherent turbulent structures, such as Kelvin-Helmholtz waves. The turbine itself is now subjected to potentially damaging turbulence. The goal of this work is to present three low-level jet cases, utilizing data from Texas Tech University’s 200 meter instrumented tower and radar wind profiler. This experiment examines the turbulent structure of the three low-level jet features in order to gain an understanding of the motions in the nocturnal boundary layer. This experiment focused on warm season jets, specifically May through July. Three low-level jet events were examined in detail. All three events exhibited a jet maximum greater than 20 ms-1 below a height of one kilometer. The shear generated beneath the jet maximum greatly exceeded the International Electrotechnical Commission’s standard shear exponent value of 1/7. Two of the three low-level jet events produced substantial turbulence. The magnitude of turbulent kinetic energy during the established jet increased with height during the periods of significant turbulence. The Richardson number for each case, for the layer between 46 and 158 meters, fell below the critical value of 0.25 during the lifetime of the low-level jet event. The 2 June 2004 low-level jet exhibited a jet maximum below 200 meters in altitude. This would place any wind energy system in direct contact with the layer just beneath the jet maximum in a region of high shear. The 25 May 2004 low-level jet event illustrates the generation of significant turbulence due to the interaction between the low-level jet and a thunderstorm outflow boundary. Prolonged turbulence was generated lasting over two hours as the boundary passed the instrumented tower. This experiment focused on describing all aspects of the nocturnal low-level jet and the ability of the Texas Tech 200 meter tower to provide high resolution observations of the nocturnal boundary layer. The driving force behind this work is to examine the motions associated with low-level jets in the layer occupied by wind energy systemsItem An observational study of tropical cyclone low-level wind maxima(2010-08) Giammanco, Ian Matthew; Schroeder, John L.; Powell, Mark D.; Smith, Douglas A.Over the last decade substantial improvements have been made in one’s ability to observe the tropical cyclone boundary layer. Mean wind profiles computed from GPS dropwindsonde data have shown a “jet-like” wind speed maximum located near 500 m above ground level however measurements from individual GPS dropwindsondes (GPS sondes) exhibit variability. Tropical cyclone low-level wind maxima represent a source of momentum available for vertical transport; however little is known regarding their characteristics over open ocean conditions or at landfall. In order to thoroughly characterize low-level wind maxima, over 1080 GPS sondes were employed. Given the lack of GPS sonde data at landfall the National Weather Service’s network of Doppler radar systems was used to mitigate the data void. Over 380 velocity azimuth display wind profiles were derived in order to evaluate the structure and evolution of the boundary layer wind profile at landfall. These data were processed to investigate low-level wind maxima as well to separate the influence of turbulence from quasi-steady low-level jet features described in previous studies. Analysis of the GPS sonde dataset revealed a decrease in the height of the wind maximum with radius and mean boundary layer wind speed. An azimuthal dependence was also observed as the left-front storm-relative sector contained the lowest mean wind maximum. Low-level jet features were observed within more than half of all GPS sondes their mean and variance mirrored the statistics associated with low-level wind maxima. The characteristics of the jet features were in good agreement with previous numerical studies. Logarithmic and power law profiles were also found to perform quite well for composite vertical wind profiles; however when used for individual GPS sondes they were somewhat less effective. The use of velocity azimuth display (VAD) wind profiles proved to be effective in resolving the boundary layer wind vertical wind profile. The height of the wind maximum was found to be radially and azimuthally dependent. Persistent low-level jet features were identified primarily within the off-shore flow regime. The passage of rainbands was also found to influence the vertical wind profile. Log and power law profiles also performed well for VAD derived wind profiles.