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Bruno Deremble, Guillaume Lapeyre, and Michael Ghil

jet, called the polar-front jet (PFJ) (e.g., Son and Lee 2005 ; Walker and Schneider 2006 ). This double-jet structure has been studied in idealized quasigeostrophic (QG) models ( Koo and Ghil 2002 ; Kravtsov et al. 2005 ), which helped explain it by the presence of multiple equilibria in a β -channel configuration. The predictions of these models were evaluated in the Southern Hemisphere (SH) by Koo et al. (2002) and in the Northern Hemisphere (NH) by Kravtsov et al. (2006) . As mentioned

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David J. Muraki and Chris Snyder

Tuyl and Young 1982 ; Hakim 2000 ; Cunningham and Keyser 2000 ). Applications to tropopause disturbances have been limited by the fact that, except for Berestov (1979) , the available dipole solutions all come from barotropic or layered models. For these applications, the surface quasigeostrophic equations (sQG; Juckes 1994 ; Held et al. 1995 ) offer a most natural and simple description of the flow, in which potential vorticity is uniform away from the strong gradients at the tropopause, and

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Rebecca E. Morss, Chris Snyder, and Richard Rotunno

examines spectral slopes, error growth at different spatial scales, and predictability behavior in a model a step up in complexity and atmospheric realism from homogeneous, isotropic turbulence: a multilevel quasigeostrophic (QG) channel model with an atmospheric jet and evolving temperature perturbations at the upper and lower surfaces. By investigating spectral slopes and predictability in an intermediate context, the study aims to serve as a bridge between predictability paradigms from homogeneous

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Kyle Swanson

models, specifically those containing a vigorous spectra of synoptic transients. Along these lines, Itoh and Kimoto (1996) show that multiple chaotic attractors can occur in a multilayer quasigeostrophic system in the presence of baroclinic instability, although increased supercriticality breaks the barriers between those attractors leading to itinerant behavior. However, in models with fully developed baroclinic turbulence, multiple attractors have not been reported. For example, Sempf et al

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Riccardo Farneti

with a discussion of the results in section 6 . 2. Model description We use the Quasigeostrophic Coupled Model (Q-GCM) ( Hogg et al. 2003a , b ). The model is symmetric around the ocean–atmosphere interface, and the mixed layer is embedded in the first layer for both the atmosphere and ocean. For a flat-bottomed three-layer configuration, the QG potential vorticity (QGPV) equation is where q i and ψ i are the layer potential vorticities and streamfunctions respectively, J ( a , b ) = a x b

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Chiung-Yin Chang and Isaac M. Held

1. Introduction Ever since Phillips (1956) ’s pioneering work, the quasigeostrophic two-layer model has repeatedly proven itself to be useful for understanding the baroclinic eddy dynamics in the extratropical troposphere. To better study how eddy statistics depend on mean flows, the model can be further simplified with a fixed background zonal flow with vertical shear that is independent of latitude and that assumes the deviations from this flow to be horizontally homogeneous. This is known

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Alexandre Laîné, Guillaume Lapeyre, and Gwendal Rivière

MM93 model) based on the quasigeostrophic (QG) equations of fluid motion. The model shows interesting properties in terms of high- and low-frequency variability in midlatitudes, similar to the observed ones ( MM93 ; D’Andrea and Vautard 2001 ). This model therefore served as a basis for analyses of the midlatitudes (e.g., Corti et al. 1997 ; D’Andrea and Vautard 2001 ; Rivière 2009 ). Most of the studies of the atmospheric storm track have examined its behavior in a dry context, but several

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Pablo Zurita-Gotor and Geoffrey K. Vallis

quasigeostrophic (QG) models. Although linear baroclinic instability theory is well established, the relevance, or even the existence, of modal development or exponential growth for the eddy–mean flow interaction problem is unclear, and the equilibration of baroclinic eddies at finite amplitude remains a complex problem. An early attempt to address the nonlinear problem was that of Green (1970) , who attempted to build a theory of the general circulation based on the diffusion of potential vorticity (PV) by

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Shu-Chih Yang, Matteo Corazza, Alberto Carrassi, Eugenia Kalnay, and Takemasa Miyoshi

remains to be clearly assessed. This work compares variational and ensemble-based DA schemes based on the quality of their analyses as well as their computational costs. One ensemble-based and two variational schemes are applied to a quasigeostrophic channel model using the same “noisy” observations. The two variational schemes are 3DVAR (developed by Morss 1999 and Morss et al. 2001 , following Parrish and Derber 1992 ) and 4DVAR (newly implemented for this study). The ensemble scheme is the

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Ross Tulloch and K. Shafer Smith

-difference model approaches a representation of the Eady model. This representation, however, is only accurate up to a horizontal wavenumber that depends on the vertical resolution of the model, for the following reasons. Temperature θ in the quasigeostrophic approximation is related to the vertical derivative of the streamfunction as gθ / θ 0 = fψ z ≃ fδψ / δz , where δψ / δz is the finite-difference approximation to ψ z and g is gravitational acceleration. Because in geostrophic flow vertical

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