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Shuguang Wang, Fuqing Zhang, and Chris Snyder

. Ford , R. , M. E. McIntyre , and W. A. Norton , 2000 : Balance and the slow quasimanifold: Some explicit results. J. Atmos. Sci. , 57 , 1236 – 1254 . Fritts , D. C. , and M. J. Alexander , 2003 : Gravity wave dynamics and effects in the middle atmosphere. Rev. Geophys. , 41 , 1003 – 1063 . Grell , G. A. , J. Dudhia , and D. R. Stauffer , 1994 : A description of the fifth-generation Penn State/NCAR mesoscale model (MM5). NCAR Tech. Note, NCAR/TN-398+STR, 122 pp

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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

into the upper atmosphere (defined here as the 13-km level). Termination of all these rays is due to encountering critical layers. The rays from the region of strongest shorter-wave activity (ray 2 to ray 6 from left to right in Fig. 5 ) terminate in the low-to-middle troposphere while the rays near the right edge of the strong shorter-wave activity are somewhat similar to the reverse rays (A, B, and C) from WP1 discussed earlier. This again suggests that WP1 may originate from near

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Dong L. Wu and Stephen D. Eckermann

current GW parameterizations is their poorly constrained specifications of lower atmospheric sources (see, e.g., McLandress and Scinocca 2005 ). Although it is recognized that GWs can be excited by flow across mountains, convection, and imbalance/instability within rapidly evolving baroclinic jet/frontal systems (e.g., Fritts et al. 2006 ), the relative contributions of these sources to the GW spectrum encountered in the middle atmosphere remains highly uncertain, particularly with respect to GWs

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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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James J. Riley and Erik Lindborg

stratified flows and also found inertial range behavior, with coefficients consistent with those found by Lindborg. Horizontal inertial range spectral behavior has often been observed in the middle atmosphere. In Fig. 3 the horizontal wavenumber spectra from the upper troposphere and lower stratosphere are reproduced from Nastrom and Gage (1985) . It is a remarkable feature of these spectra that they exhibit k −5/3 h spectra in the wavenumber range corresponding to wavelengths between 1 and 500 km

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

. Earth , 24B , 455 – 460 . Lovegrove , A. F. , P. L. Read , and C. J. Richards , 2000 : Generation of inertia–gravity waves in a baroclinically unstable fluid. Quart. J. Roy. Meteor. Soc. , 126 , 3233 – 3254 . McIntyre , M. E. , 2001 : Global effects of gravity waves in the middle atmosphere: A theoretical perspective. Adv. Space Res. , 27 , 1723 – 1736 . O’Sullivan , D. , and T. J. Dunkerton , 1995 : Generation of inertia–gravity waves in a simulated life cycle of

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

becoming more circular and the cyclone less so. A quadrupole in w arises at the level of the QG approximation. 2 To reveal more precisely how w differs from that predicted by QG theory, we have computed w QG , the QG vertical velocity, given θ and the geostrophic velocities v g from each simulation. Solving where Q = − g / θ 0 (∂ v g /∂ x · ∇ θ , ∂ v g /∂ y · ∇ θ ), yields w QG ( Gill 1982 , his section 12.10). Results for w QG are shown in the middle panels of Fig. 11 . As

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K. Ngan, P. Bartello, and D. N. Straub

, random straining by the geostrophic base flow may be a generic mechanism for the generation of imbalance via a nominally balanced flow (i.e., spontaneous imbalance). This mechanism could be important in the middle atmosphere, where there is horizontal random straining (e.g., Shepherd et al. 2000 ). In the geophysical literature, the hyperbolic instability, which underlies the pressureless growth mechanism, has received less attention than other instability mechanisms; however, an analysis of

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David A. Schecter

temperature) of the atmosphere increases monotonically with altitude and that the axisymmetric PV distribution q ( r , θ ) of the unperturbed cyclone decreases monotonically with radius on a surface of constant θ . With suitable boundary conditions, such a vortex is stable in the context of balanced dynamics ( Montgomery and Shapiro 1995 ; Ren 1999 ). On the other hand, stability is not guaranteed when IG waves are allowed to interact with DVRWs—in which case SI can occur. a. PV and angular

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