On the Equilibrium Spectrum of Transient Waves in the Atmosphere
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چکیده
The spectrum of transient waves in the mid-latitude atmosphere is explored. There are two maxima, one at planetary and one at synoptic scale. The very long wave maximum seems to be Rhines' wavenumber of cascade arrest, i.e. the scale at which a barotropic reverse energy cascade stops due to the prominence of linear Rossby wave dynamics. The synoptic peak is explained here by the theory of nonlinear baroclinic adjustment: a balance between quasi-linear energy extraction from the mean ow due to baroclinic instability and a resulting up-scale cascade. The mechanisms for the two energy maxima appear to operate independently of each other. Computations with a quasi-geostrophic two-level model in a beta-plane channel corroborate the nonlinear baroclinic adjustment mechanism. It is demonstrated that the model and hence the theory, although simple, can simulate the dynamics of the real atmosphere both in the terms of the transient mid-latitude spectrum and the meridional temperature gradients achieved in summer and winter. Nonlinear baroclinic adjustment is shown to agree with Salmon's theory of wave-wave equilibration from quasi-geostrophic turbulence. The former goes further, however; it is capable of explaining not only the equilibrium dynamics but also the mechanistic approach thereto, for di erent channel geometries and at di erent levels of forcing. The mechanism is shown to be di erent from both stochastically forced transient growth and neutralization theories of the atmosphere.
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تاریخ انتشار 1997