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Ronald B. Smith, Bryan K. Woods, Jorgen Jensen, William A. Cooper, James D. Doyle, Qingfang Jiang, and Vanda Grubišić

1. Introduction The sharp boundary between the troposphere and stratosphere has been largely explained as the result of convective cloud-top entrainment (e.g., Staley 1960 ; Reid and Gage 1981 ; Held 1982 ; Sherwood and Dessler 2003 ). The tropopause is defined not only by the jump in temperature lapse rate but also by strong gradients in water vapor and ozone concentrations. In midlatitudes, the jet stream often has its maximum speed at the tropopause level. It is suspected that these

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Vanda Grubišić and Brian J. Billings

be associated with 1) an upper-level pressure trough along the Pacific coast with strong westerly flow across the Sierra, and 2) a cold or occluded front approaching California from the northwest, placing Owens Valley in the prefrontal environment. In addition, the jet stream typically was found to cross Oregon or northern California during strong wave events, though some events did occur with a westerly jet stream crossing the Sierra. It was also observed that cold air damming occurred to some

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Patrick A. Reinecke and Dale R. Durran

( Fig. 3a ), a relatively sharp, negatively tilted trough is situated directly over the northern California coastline. A 45 m s −1 jet maximum is located at the base of the trough with strong southwesterly flow extending northeastwards to the northern Sierra Nevada. Six hours later, at 0000 UTC 26 March 2006 ( Fig. 3b ), the trough has progressed eastward such that the strongest winds are directly upstream of the Owens Valley and oriented nearly perpendicular to the barrier. The spatial extent of

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James D. Doyle and Dale R. Durran

the subject of two of the first modern U.S. multiagency field programs in meteorology, the Sierra Wave Project (SWP) and its follow-on, the Jet Stream Project (JSP), both of which took place in the early 1950s ( Holmboe and Klieforth 1957 ; Grubišić and Lewis 2004 ). With the exception of research aircraft observations of several rotor events in the lee of the Rocky Mountains ( Lester and Fingerhut 1974 ) and occasional serendipitous remote sensing lidar measurements of rotors ( Banta et al. 1990

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Vanda Grubišić and Ivana Stiperski

orography. The vertical profile of the barrier-normal component of wind in this sounding is characterized by a four-layer structure consisting of a nearly stagnant layer near the ground, a deep layer of positive shear in the midtroposphere extending to a “nose” of a pronounced jet, and a decrease of wind speed in the stratosphere. Such vertical atmospheric structure is commonly observed in many strong lee-wave events upstream of the Sierra Nevada. As illustrated in Fig. 2 , the selected sounding shares

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Lukas Strauss, Stefano Serafin, and Vanda Grubišić

mechanisms of turbulence generation in Owens Valley are presented in more detail in section 3b . a. Synoptic conditions during T-REX IOPs The Sierra Nevada is well known for launching large-amplitude mountain waves. The generation of mountain waves in its lee is frequently associated with the passage of frontal systems ( Holmboe and Klieforth 1957 ). From October to May, with the shift of the polar jet to the south, midlatitude synoptic systems tend to travel past the mountain range, directing strong

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Bowen Zhou and Fotini Katopodes Chow

origin. Cold (warm) colors indicate flow (m s −1 ) toward (away) from the lidar. Plot is centered at the lidar location. As shown in section 4a , westerly flow in the bottom layer is a result of the downslope flow from the western sidewall. The drainage flow extends east of the lidar, to nearly the bottom of the eastern sidewall (see Fig. 3a ). The jet-like structure of the drainage flow can be seen on the bottom panel of Fig. 4 . The westerly flow in the top layer is a component of the synoptic

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Michael Hill, Ron Calhoun, H. J. S. Fernando, Andreas Wieser, Andreas Dörnbrack, Martin Weissmann, Georg Mayr, and Robert Newsom

mountains. During the early 1950s, the Sierra Wave Project and the Jet Stream Project used sailplane measurements with ground tracking to categorize characteristic modes of mountain-wave and rotor phenomena (see, e.g., Holmboe and Klieforth 1957 ; Grubišić and Lewis 2004 ). Lidar radial velocity data acquired near Boulder, Colorado, in February of 1987 allowed Ralph et al. (1997) to calculate streamlines and velocity vectors that exhibit full-scale rotor behavior. An important difference between

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James D. Doyle, Vanda Grubišić, William O. J. Brown, Stephan F. J. De Wekker, Andreas Dörnbrack, Qingfang Jiang, Shane D. Mayor, and Martin Weissmann

Jet Stream Project, both of which took place in the early 1950s ( Holmboe and Klieforth 1957 ; Grubišić and Lewis 2004 ), were the first coordinated research efforts focused on observing topographically forced phenomena and documented several research aircraft penetrations of rotors and associated turbulence ( Holmboe and Klieforth 1957 ). The relatively rare in situ research aircraft encounters with rotors ( Lester and Fingerhut 1974 ), along with occasional lidar observations of downslope

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