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Bowen Zhou and Fotini Katopodes Chow

valley atmosphere at the same elevation. Furthermore, it has a nonzero initial momentum ( u s ~ 3 m s −1 , see Fig. 9a ) as a result of the synoptic forcing. As the parcel plunges down-slope, it is accelerated by greater buoyancy forcing as a result of its initial temperature deficit. When it reaches the elevation of its neutral buoyancy, the excess momentum gained from the downslope acceleration, together with its initial momentum, allows the parcel to continue its down-slope motion into a colder

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Vanda Grubišić and Ivana Stiperski

-REX) in 2006 ( Grubišić et al. 2008 ). It was noted already in SWP that trapped lee waves of longer wavelengths, in particular wavelengths that were close to the ridge separation distance, were more likely to achieve large amplitudes ( Holmboe and Klieforth 1957 ), suggesting a form of resonant response of the flow to the double-ridge forcing. Together with its pilot Sierra Rotors Project (SRP) in 2004, T-REX has once again drawn attention to the phenomenon of trapped lee waves and rotors over Owens

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James D. Doyle and Dale R. Durran

equations expressed using tensor notation are where c p is the specific heat at constant pressure, R d is the dry gas constant, g is the acceleration due to gravity, ρ is the density, c is the speed of sound, ν is the hyperdiffusion coefficient, and f the Coriolis force. In this study, the Coriolis force is specified as 10 −4 s −1 . The overbar variables correspond to the reference state, which is horizontally homogeneous and in hydrostatic balance. The turbulent subgrid-scale fluxes for

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Lukas Strauss, Stefano Serafin, and Vanda Grubišić

is absent; instead, channeled up-valley flow prevails. Note that the idealized diagrams of scenarios A and B are somewhat similar to two of those by Jiang and Doyle (2008 , their Fig. 12). The latter, however, refer to a weak westerly wind event during the Sierra Rotors Project (SRP) IOP 12, primarily determined by thermal forcing in the valley. A schematic of daytime turbulent interaction of in-valley westerlies with channeled up-valley flow (scenario C) is shown in Fig. 16c . The wave

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Patrick A. Reinecke and Dale R. Durran

circulations in the initial conditions. Anthes et al. (1985) argued that the predictability of many mesoscale phenomena can substantially exceed that suggested by Lorenz if they are sufficiently organized by the large-scale flow (e.g., fronts) or controlled by well-known external forcing such as orography, thermal contrasts, or other land-use characteristics. Klemp and Lilly (1975) presented some of the earliest results suggesting that topography may enhance mesoscale predictability. They predicted

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

difficulty maintaining an accurate pressure altitude. In Fig. 7 , we compare the observed wind speed, averaged for the two legs, with the geostrophic speed for all the racetracks in the six flights. For slower winds, the flow is slightly supergeostrophic, while for fast winds aloft, the flow is significantly subgeostrophic. We suggest that the strong subgeostrophy aloft is caused by streamline curvature and southward centripetal force in the upper-level troughs above the Sierras. To evaluate this

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

. , 1959 : The rotor in the lee of mountains. Geophysics Research Directorate Note 6, AD-208862, Air Force Cambridge Research Laboratories, 20 pp . Lhermitte , R. , and L. Miller , 1970 : Doppler radar methodology for the observation of convective storms. Preprints, 14th Radar Meteorology Conf., Tucson, AZ, Amer. Meteor. Soc., 133–138 . Lugt , H. , 1979 : The dilemma of defining a vortex. Recent Developments in Theoretical and Experimental Fluid Mechanics, U. Müller, K. G. Roesner

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Bryan K. Woods and Ronald B. Smith

both convective and shear instabilities ( Liu et al. 1999 ; Satomura and Sato 1999 ). Even in cases for which we believe the aircraft sampled in situ wave breaking, the horizontal transects do not provide any vertical information of either the buoyancy or shear forcing terms to support either mechanism. More work will be required to reveal the generation mechanism for our T-REX short waves. Acknowledgments Wavelet software was provided by C. Torrence and G. P. Compo and is available online ( http

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

instability exhibits scale preferences and generates distinctive energy maxima at different scales. The energy cascade associated with steepening or breaking mountain waves may have similar characteristics ( Weinstock 1986 ; Jiang and Doyle 2004 ; Doyle et al. 2005 ). In contrast to classical phenomenological turbulence theory, which assumes steady forcing (i.e., energy source) in the large- or small-scale limit, the energy maxima associated with KH- or wave breaking–generated large eddies may serve as

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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

in two spatial dimensions. J. Atmos. Sci. , 42 , 1321 – 1339 . Klemp , J. , and R. Wilhelmson , 1978 : The simulation of three-dimensional convective storm dynamics. J. Atmos. Sci. , 35 , 1070 – 1096 . Klemp , J. , and D. R. Durran , 1983 : An upper boundary condition permitting internal gravity wave radiation in numerical mesoscale models. Mon. Wea. Rev. , 111 , 430 – 444 . Kuettner , J. , 1959 : The rotor flow in the lee of mountains. GRD Research Note 6, Air Force

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