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Yansen Wang, Cheryl L. Klipp, Dennis M. Garvey, David A. Ligon, Chatt C. Williamson, Sam S. Chang, Rob K. Newsom, and Ronald Calhoun

1. Introduction One of the characteristics of lower-troposphere winds over the Great Plains of the central United States is the low-level jet (LLJ). The LLJ is a thin stream of fast-moving air, usually more than 10 m s −1 , elevated about 200–500 m above the ground ( Hoecker 1963 ; Bonner 1968 ). The LLJ can appear in the daytime as a result of baroclinic forcing over sloping terrain ( Holton 1967 ) or because of the dynamical pressure differences caused by localized convection ( Bowen 1996

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P. Ramamurthy, E. R. Pardyjak, and J. C. Klewicki

positive x direction ( u velocity; denoted “along canyon”). As indicated in Table 2 , the mean winds at a height of 50 m measured upstream of the city were slightly out of the southwest and were relatively consistent. The average wind speeds at 50 m ranged between 6.4 and 8.3 m s −1 with the largest wind speeds occurring during the stable period. The higher wind speeds at the 50-m level during the night are consistent with the regular observation of a nocturnal jet upstream of Oklahoma City during

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M. A. Nelson, E. R. Pardyjak, M. J. Brown, and J. C. Klewicki

vertical covariances normalized by local TKE for southeasterly winds are presented in Fig. 8 . The data indicate that a large region of shear is generated when a wall-jet-like flow (hereinafter called a wall jet), produced by the impinging downdraft, interacts with the flow channeling from the eastern end of the canyon. In contrast to the increasing correlation of u ′ w ′ with height at the base of the canyon for southwesterly winds, the correlation is somewhat small and variable in both u ′ w

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M. A. Nelson, E. R. Pardyjak, J. C. Klewicki, S. U. Pol, and M. J. Brown

downdraft is occurring elsewhere through conservation of mass. Regions with strong downdrafts are observed in and near the intersections, which suggests that the local maximum in M may be evidence of a wall jet radiating outward from the impinging of a downdraft of higher-momentum fluid on the street surface near the intersection. Similar to what was found in the vertical velocities for southerly flow, the vertical velocities in the canyon interior have small positive mean values for southwesterly

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Stevens T. Chan and Martin J. Leach

data (dashed lines) for the two IOPs is shown in Fig. 4 , indicating that the constructed profiles are a reasonable representation of the observations. Based on the sodar and sonic anemometer data we examined, directional shear and vertical motion do not appear to be significant for the simulated releases. However, for IOP 8, because of the presence of a nocturnal low-level jet ( Lundquist and Mirocha 2006 ), directional shear and vertical motion may be significant and may have to be considered in

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Donald A. Burrows, Eric A. Hendricks, Steve R. Diehl, and Robert Keith

, examining the vertical wind speed profiles for the nighttime cases ( Figs. 11e and 11f ) reveals that they are distinctly different from the daytime profiles ( Figs. 11a–c ). Whereas the daytime profiles are reasonably close to a logarithmic profile, the nighttime profiles are nearly linear. The linear profile likely is the result of the formation of the low-level jet (LLJ) at night in the Oklahoma City area as studied by Lundquist and Mirocha (2006) . The peak velocity of this jet was generally

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