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Juerg Schmidli, Brian Billings, Fotini K. Chow, Stephan F. J. de Wekker, James Doyle, Vanda Grubišić, Teddy Holt, Qiangfang Jiang, Katherine A. Lundquist, Peter Sheridan, Simon Vosper, C. David Whiteman, Andrzej A. Wyszogrodzki, and Günther Zängl

and turbulence, and (iii) the uncertainties associated with the parameterization of radiation transfer and surface–atmosphere interactions. Thus apart from an idealized topography, the setup of the simulations is as close as possible to real-case simulations. The models are run with comprehensive model physics including a radiation transfer scheme, land surface scheme, and turbulence parameterization. A large computational domain and periodic lateral boundary conditions are used in order to

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James D. Doyle, Saša Gaberšek, Qingfang Jiang, Ligia Bernardet, John M. Brown, Andreas Dörnbrack, Elmar Filaus, Vanda Grubišić, Daniel J. Kirshbaum, Oswald Knoth, Steven Koch, Juerg Schmidli, Ivana Stiperski, Simon Vosper, and Shiyuan Zhong

increase as the horizontal scale of the phenomenon decreases, effectively limiting the intrinsic predictability. The notion that mesoscale phenomena forced by the lower boundary attain enhanced predictability has been hypothesized (e.g., Anthes et al. 1985 ). However, this perspective is likely overly optimistic because of lateral boundary conditions, numerical dissipation, and adjustment issues ( Errico and Baumhefner 1987 ; Vukicevic and Errico 1990 ), as well as nonlinearities introduced by the

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THE TERRAIN-INDUCED ROTOR EXPERIMENT

A Field Campaign Overview Including Observational Highlights

Vanda Grubišić, James D. Doyle, Joachim Kuettner, Stephen Mobbs, Ronald B. Smith, C. David Whiteman, Richard Dirks, Stanley Czyzyk, Stephen A. Cohn, Simon Vosper, Martin Weissmann, Samuel Haimov, Stephan F. J. De Wekker, Laura L. Pan, and Fotini Katopodes Chow

The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotemporal characteristics and internal structure of a tightly coupled system consisting of an atmospheric rotor, terrain-induced internal gravity waves, and a complex terrain boundary layer. In addition, T-REX had several ancillary objectives including the studies of UTLS chemical distribution in the presence of mountain waves and complex-terrain boundary layer in the absence of waves and rotors. This overview provides a background of the project including the information on its science objectives, experimental design, and observational systems, along with highlights of key observations obtained during the field campaign.

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Stephan F. J. De Wekker and Shane D. Mayor

1. Introduction Observations of atmospheric flow over complex terrain remain a research priority for several reasons. First, observations are needed to evaluate numerical simulations that are challenged by terrain because of the difficulty of accurately representing subgrid-scale processes and boundary conditions. Flow over terrain induces flow separation ( Jiang et al. 2007 ), rotors ( Doyle and Durran 2002 , 2007 ), turbulence, and mountain waves ( Smith et al. 2007 ) at a variety of

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Ivana Stiperski and Vanda Grubišić

is not the case for the perturbation pressure and surface wind speed profiles that display symmetry in the nS runs and asymmetry under the fS conditions. The symmetric shape of the pressure perturbation, present also in the fnS runs, appears to result from the alteration of the upstream profile due to the boundary layer effects (not shown). The symmetry of the surface wind speed, on the other hand, results from the direct influence of surface friction on the low-level flow. Fig . 9. Distribution

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Bowen Zhou and Fotini Katopodes Chow

1. Introduction The classical description of the nighttime atmospheric boundary layer ( Nieuwstadt 1984 ) is set in a horizontally homogeneous environment over flat terrain, under quasi-steady-state conditions. Turbulence is generated at the surface and transported upward. Within the stable boundary layer (SBL), the production of turbulence by mean shear dominates over the destruction by buoyancy, such that turbulence is temporally continuous. The continuously turbulent SBL usually occurs under

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Lukas Strauss, Stefano Serafin, and Vanda Grubišić

of large-amplitude waves and rotorlike structures, horizontal pressure gradients at the valley floor were often relatively weak. It is hypothesized that wave-induced pressure perturbations are subject to critical-level absorption and/or turbulent diffusion of wave energy in the boundary layer, preventing them from extending to the valley bottom. Under certain conditions, the resistance of the stably stratified valley atmosphere to the incoming dynamically forced flow seems to be important for

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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

each boundary ( Pearson 1974 ; Durran et al. 1993 ). The mitigation of reflected waves from the upper boundary is accomplished through a radiation condition formulated following the Durran (1999) approximation to the Klemp and Durran (1983) and Bougeault (1983) techniques. The simulations are initialized using a reference state based on the conditions upstream of the Sierra Nevada during T-REX IOP 13 through the use of a radiosonde launched west of the Sierra at 2100 UTC 16 April, shown in

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Patrick A. Reinecke and Dale R. Durran

. The lateral boundary conditions on the 27-km domain are specified from operational forecasts of the Naval Operational Global Atmospheric Prediction System (NOGAPS) and perturbed for each ensemble member using the fixed-covariance-perturbation method described in Torn et al. (2006) . The perturbations are Gaussian with zero mean and a covariance that is balanced for synoptic-scale motions. The covariance relations are obtained from the Weather Research and Forecasting-Variational (WRF-VAR) data

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James D. Doyle and Dale R. Durran

. Durran , D. R. , 1999 : Numerical Methods for Wave Equations in Geophysical Fluid Dynamics . Springer-Verlag, 465 pp . Durran , D. R. , M. Yang , D. N. Slinn , and R. G. Brown , 1993 : Toward more accurate wave permeable boundary conditions. Mon. Wea. Rev. , 121 , 604 – 620 . Epifanio , C. C. , and D. R. Durran , 2001 : Three-dimensional effects in high-drag-state flows over long ridges. J. Atmos. Sci. , 58 , 1051 – 1065 . Epifanio , C. C. , and D. R. Durran , 2002

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