Three Representative Case Studies of the South Plains Nocturnal Low Level Jet

Abstract eng:
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 paper is to present three low-level jet cases, utilizing data from Texas Tech University’s 200m instrumented tower and radar wind profiler. This paper examines the turbulent structure of the three lowlevel jet features in order to gain an understanding of the motions in the nocturnal boundary layer. This experiment focuses 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 increased with height during the periods of significant turbulence as the jet feature had become established. The Richardson number for each case, for the layer between 46 and 158 meters, fell below the critical value of 0.25 at some point during the lifetime of the low-level jet feature. 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.

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