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We consider the global horizontal kinetic energy (KE) spectrum simulated with a mechanistic General Circulation Model run at high spatial resolution. In particular, we diagnose all contributions to the spectral budget of the KE.
This reveals a strong influence of the horizontal advection terms in the upper troposphere, which decrease KE at synoptic scales and increase it at mesoscales, implying a net energy cascade. The irrotational part of the mesoscales thus energized represents extratropical gravity-wave sources that become relevant in the middle atmosphere. The spectral horizontal advection terms also show a high temporal variability, including events with a reversed energy cascade (backscattering). Hence, a net energy cascade in the mesoscales must be considered as a residuum of strong downscaling and backscattering. An equilibrated KE budget with the net cascade balanced by frictional dissipation shows up only for averages at least longer than 10 model days. These results are qualitatively robust when using different resolutions or different formulations of the horizontal diffusion. |
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