The Horizontal Kinetic Energy Spectrum and Spectral Budget Simulated by a High-Resolution Troposphere-Stratosphere-Mesosphere GCM.
Horizontal kinetic energy spectra simulated by high resolution versions of the Geophysical Fluid Dynamics Laboratory SKYHI middle atmosphere general circulation model are examined. The model versions considered resolve heights between the ground and ~80 km and the horizontal grid spacing of the highest resolution version is about 35 km. Tropospheric kinetic energy spectra show the familiar ~-3 power-law dependence on horizontal wavenumber for wavelengths between about 5000 and 500 km and have a slope of ~-5/3 at smaller wavelengths. Qualitatively similar behavior is seen in the stratosphere and mesosphere, but the wavelength marking the transition to the shallow regime increases with height, taking a value of ~2000 km in the stratosphere and ~4000 km in the mesosphere.
The global spectral kinetic energy budget for various height ranges is computed as a function of total horizontal wavenumber. Contributions to the kinetic energy tendency from nonlinear advective processes, from conversion of available potential energy, from mechanical fluxes through the horizontal boundaries of the region, and from parameterized subgrid-scale dissipation are all examined. In the troposphere, advective contributions are negative at large scales and positive over the rest of the spectrum. This is consistent with a predominantly downscale nonlinear cascade of kinetic energy into the mesoscale. The global kinetic energy budget in the middle atmosphere differs significantly from that in the troposphere, with the positive contributions at most scales coming predominantly from vertical energy fluxes.
The kinetic energy spectra calculated from two model versions with different horizontal resolution are compared. Differences between the spectra over the resolved range of the lower resolution version are smallest in the troposphere and increase with height, owing mainly to large differences in the divergent components. The result suggests that the parameterization of dynamical subgrid--scale processes in middle atmosphere general circulation models, as well as in high-resolution tropospheric general circulation models may need to be critically re-evaluated.