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J. Vanneste

centrifugally stable stratified Taylor–Couette flow. Phys. Rev. Lett. , 86 , 5270 – 5273 . Molemaker , M. J. , J. C. McWilliams , and I. Yavneh , 2005 : Baroclinic instability and loss of balance. J. Phys. Oceanogr. , 35 , 1505 – 1517 . Muraki , D. J. , 2003 : Revisiting Queney’s flow over a mesoscale ridge. Preprints. Ninth Conf. on Mountain Meteorology , Aspen, CO, Amer. Meteor. Soc., 5.3 . Ólafsdóttir , E. I. , 2006 : Atmospheric-wave generation: An exponential

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Vladimir Zeitlin

1. Introduction The question of “spontaneous” emission of inertia–gravity waves (IGWs), which is being actively discussed in the literature, starting from the pioneering paper ( O’Sullivan and Dunkerton 1995 ), in the context of realistic (e.g., Plougonven and Snyder 2007 ) or idealized (e.g., Dritschel and Viúdez 2007 ) numerical simulations, is a question of the limits of decoupling of fast (waves) and slow (vortices) motions in geophysical fluid dynamics and, therefore, a question of the

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Shuguang Wang, Fuqing Zhang, and Chris Snyder

1. Introduction Gravity waves propagating vertically from the lower atmosphere are widely recognized to play important roles in a variety of atmospheric phenomena. Known sources of these gravity waves include mountains, moist convection, fronts, upper-level jets, geostrophic adjustment, and spontaneous generation ( Fritts and Alexander 2003 , and references therein). Among these, jets are often responsible for generating low-frequency inertia–gravity waves with characteristic horizontal

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Y. D. Afanasyev, P. B. Rhines, and E. G. Lindahl

1. Introduction A balanced flow is the vortical predominantly nondivergent flow that can be described by an appropriate balance relation, of which semigeostrophy and quasigeostrophy are examples. The divergent flow in the form of inertia–gravity waves is therefore filtered out and assumed to be negligible. However, under certain circumstances, the balanced flow can undergo further adjustment toward a new (balanced) state (e.g., Ford et al. 2000 ; McIntyre and Norton 2000 ). In the process of

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Chris Snyder, David J. Muraki, Riwal Plougonven, and Fuqing Zhang

as the components of velocity. Emitted waves in this case have spatial scales that are large compared to the characteristic scale of the balanced flow and phase speeds that are large compared to advective velocities. Localized atmospheric jets (and our simulated vortex dipoles), on the other hand, are characterized by small R and are continuously stratified. These flows obey QG dynamics to a first approximation and have an aspect ratio H / L of the order of f / N , where N is the buoyancy

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Paul D. Williams, Thomas W. N. Haine, and Peter L. Read

1. Introduction Inertia–gravity waves are observed ubiquitously throughout the stratified parts of the atmosphere (e.g., Eckermann and Vincent 1993 ; Sato et al. 1997 ; Dalin et al. 2004 ) and ocean (e.g., Thorpe 2005 ). Orthodox mechanisms for inertia–gravity wave generation include dynamical instability (e.g., Kelvin–Helmholtz shear instability; Chandrasekhar 1961 ), which is a known source of atmospheric gravity waves ( Fritts 1982 , 1984 ). Another possible mechanism is the

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Kaoru Sato and Motoyoshi Yoshiki

1. Introduction Gravity waves are atmospheric waves with a restoring force of buoyancy, which are characterized by their small spatial scales and short periods. Gravity waves have the ability to transport momentum, mostly in the vertical, over a long distance and deposit it in the mean field through dissipation and breaking processes. Since the importance of this ability of gravity waves in the middle atmosphere was recognized in early 1980s, many observational, numerical, and theoretical

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Yonghui Lin and Fuqing Zhang

trace gravity waves through different gridded numerical representations of the atmosphere (e.g., Guest et al. 2000 ; Broutman et al. 2001 ; Gerrard et al. 2004 ). For the Wentzel–Kramers–Brillouin (WKB) assumptions used in deriving the ray-tracing equations to be valid, the spatial derivatives of the background atmospheric parameters must vary smoothly during numerical integration. The following parameter (hereafter referred to as WKB index), is introduced to ensure the validity of the WKB

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Norihiko Sugimoto, Keiichi Ishioka, and Katsuya Ishii

1. Introduction Gravity wave radiation from unsteady rotational flows is one of the most fascinating topics in atmospheric science, from the theoretical ( Zeitlin et al. 2003 ; Vanneste and Yavneh 2004 ), observational, experimental ( Williams et al. 2005 ), as well as numerical ( Schecter and Montgomery 2004 ; Dritschel and Vanneste 2006 ) viewpoints. It is well known that gravity waves play an important role in the middle atmosphere by driving general circulation ( Holton et al. 1995

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John A. Knox, Donald W. McCann, and Paul D. Williams

1938 ; Cahn 1945 ) and from boundary condition perturbations leading to topographically forced gravity waves ( Smith 1979 ). Instead, it is fundamentally rooted in the “universal ‘internal’ . . . nonlinearity of atmospheric motions,” as demonstrated by Medvedev and Gavrilov (1995) in their independent extension of Lighthill’s theory. While the weakness of spontaneous emission in Lighthill–Ford theory is sometimes emphasized, in this paper we stress the fact that the theory does indeed predict

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