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| Titel |
The mesoscale kinetic energy spectrum of a baroclinic life cycle |
| VerfasserIn |
M. Waite, C. Snyder |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250030622
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| Zusammenfassung |
| The observed atmospheric kinetic-energy spectrum exhibits a transition between k-3 and
k-5-3 dependence at a scale of about 500 km. Despite two decades of research, the dynamics
that lead to this transition remain uncertain. Proposed explanations for the -5/3 spectrum
below 500 km range from an inverse cascade in nearly two-dimensional turbulence to a
forward cascade produced by weakly nonlinear inertia-gravity waves or three-dimensional
but highly anisotropic stratified turbulence.
We investigate the mesoscale energy spectrum within a high-resolution simulation of an
idealized baroclinic wave, with horizontal and vertical resolution Îx - 10km and
Îz - 60m. Because it exhibits many realistic mesoscale features such as fronts, jets, and
inertia–gravity waves (IGWs), this simulation allows us to study the fundamental mesoscale
dynamics without the complicating effects of moist convection and topography. The natural
question is whether such dynamics are sufficient to yield a kinetic energy spectrum similar to
the observed.
The simulation exhibits a shallowing of the mesoscale spectrum with respect to the large
scales, in qualitative agreement with observations, but this shallowing is restricted to the
lower stratosphere. No transition in the power law occurs in the upper troposphere, despite
the simulation having sufficient vertical resolution to capture the expected vertical
scales of anisotropic stratified turbulence. At both levels, the mesoscale divergent
kinetic energy spectrum—a proxy for the IGW energy spectrum—resembles a
-5/3 power law in the mature stage. Divergent kinetic energy dominates the lower
stratospheric mesoscale spectrum, accounting for its shallowing, but rotational
kinetic energy dominates the upper tropospheric spectrum. The tendency equation
for the kinetic energy spectrum shows that the lower stratospheric spectrum is not
governed solely by a downscale energy cascade, but rather is influenced by the vertical
pressure flux divergence associated with vertically propagating IGWs. Thus, the
shallowing in the lower stratosphere has its source in the spontaneous excitation
of IGWs by the larger-scale motions in the troposphere and near the tropopause. |
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