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Stephen A. Cohn, Vanda Grubiššićć, and William O. J. Brown

the wave characteristics is not a simple unconstrained fit. First, measurements from the three profilers are interpolated to a common time and height grid. With only three closely spaced sites, the fit is sensitive to small changes in vertical velocity. Such changes could result from variance in low-signal-strength conditions or a variety of other effects. For this IOP, data were averaged over 10 min in time and 150 m in height (although for clarity we interpolated onto a much finer grid of 5 min

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Juerg Schmidli, Gregory S. Poulos, Megan H. Daniels, and Fotini K. Chow

surrounding peaks of the Sierra Nevada mountain range to the west and the White Mountains to the northeast reach more than 4000 m, the peaks of the Inyo Mountains in the southeast are about 3000 m high. Because of the semiarid climate a large fraction of the net radiation is converted to sensible heat, which can lead to strong thermally induced flows. The dataset obtained during the EOPs includes radio soundings, remote sensing measurements from wind profilers, radio acoustic sounding systems (RASS

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

(e.g., Salathé and Smith 1992 ). The displacement estimates provide one method for testing the linearity of the waves. We define the nondimensional nonlinearity parameter A = Nη M / U . A typical low stratosphere value is A = (0.02 s −1 )(1000 m)/ (35 m s −1 ) ≈ 0.6, so that nonlinear effects are important but not dominant at the observed altitudes. Further aloft, as the wind speed decreases, A exceeds unity and the wave will become more nonlinear. 4. Conserved variable diagrams If the

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James D. Doyle, Qingfang Jiang, Ronald B. Smith, and Vanda Grubišić

flow response is governed by ĥ and the mountain Rossby number, Ro = U / fL , where f is the Coriolis parameter and L is the mountain half-width (e.g., Gill 1982 ; Pierrehumbert and Wyman 1985 ; Thorsteinsson 1988 ). For large Ro (>10), rotational effects are weak and the response is primarily governed by vertically propagating gravity waves ( Gill 1982 ). Inertia–gravity waves dominate when Ro ∼ 1, which are characterized by quasi-horizontal energy propagation and relatively small

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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

results from very high-resolution numerical simulations presented in Doyle and Durran (2007) reveal the presence of turbulent, small-scale vortices that they refer to as subrotors , which may pose a significant aviation hazard because of their intensity. However, the existence of subrotor vortices has never been observationally confirmed as sufficient quantitative measurements to document even the basic characteristics of subrotors have been lacking. The Sierra Wave Project and its follow-on, the

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Vanda Grubišić and Brian J. Billings

are too small in spatial scale to be routinely sampled by conventional observing networks. Complex terrain settings in which they occur, high levels of turbulence, and intermittent internal structure make rotors difficult to sample using in situ aircraft measurements. A brief historical account of the early twentieth-century studies of rotors is given in Hertenstein and Kuettner (2005) . The term rotor was first applied by Küttner (1938 , 1939 ) following his observational studies using

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Bowen Zhou and Fotini Katopodes Chow

-propagating gravity waves ( Sun et al. 2004 ), turbulence and mean shear interactions ( Nakamura and Mahrt 2005 ), and slope and valley flow transitions ( WHP09 ). Note that except for the last reference, all mechanisms are derived from the Cooperative Atmospheric-Surface Exchange Study -1999 (CASES-99) over nearly flat terrain. In reality, SBL flows are usually affected by the complex land surface. Large-scale topographic features such as mountains and valleys have pronounced effects in stratified flows ( Baines

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James D. Doyle and Dale R. Durran

low-level rotor, resulted in the loss of an engine on a commercial United Airline Boeing 747–100 at 600-m AGL near Anchorage, Alaska ( Kahn et al. 1997 ). In spite of their clear significance to the meteorology and aviation communities, the dynamics and structure of rotors are poorly understood and forecasted, in part because of infrequent and insufficient observational measurements, and inadequate sophistication and fidelity of numerical weather prediction models. Mountain waves and rotors were

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