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Kaoru Sato and Motoyoshi Yoshiki

planetary waves and baroclinic waves are more important in this region (e.g., Plumb 2002 ). It was shown theoretically ( Tanaka and Yamanaka 1985 ; Palmer et al. 1986 ; McFarlane 1987 ) and observationally ( Lilly and Kennedy 1973 ; Sato 1994 ) that the weak wind layer in the lower stratosphere was partly maintained by topographically forced gravity waves. In the equatorial middle atmosphere, gravity wave–induced forces do not cause the meridional circulation because the Coriolis force balancing

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

a source of gravity waves via the local change of the horizontal advection of horizontal divergence; and term 1C is the time derivative of the familiar Jacobian term found in both the divergence equation and in its approximated form, the nonlinear balance equation (NBE; Zhang et al. 2000 ). Term 2 is expressible as a combination of the horizontal divergence and the vertical component of relative vorticity ζ : The product of divergence, planetary vorticity, and relative vorticity is found in

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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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Michael E. McIntyre

, Lighthill showed that practically any unsteady vortical flow will spontaneously emit sound for any value of M , however small. He also showed that through destructive interference the sound emission is surprisingly weak when M ≪ 1, far weaker than one would estimate from naive order-of-magnitude analyses. The ideas are well known and are reviewed, for instance, in Ford et al. (2000 , section 2). From an atmosphere–ocean dynamics perspective, the emission of sound may be viewed as the simplest

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