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Titel |
The interaction between synoptic-scale balanced flow and a finite-amplitude mesoscale wave field throughout all atmospheric layers: Weak and moderately strong stratification |
VerfasserIn |
Ulrich Achatz, Bruno Ribstein, Fabian Senf, Rupert Klein |
Konferenz |
EGU General Assembly 2017
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 19 (2017) |
Datensatznummer |
250139429
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Publikation (Nr.) |
EGU/EGU2017-2667.pdf |
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Zusammenfassung |
The interaction between locally monochromatic finite-amplitude mesoscale waves, their
nonlinearly induced higher harmonics, and a synoptic-scale flow is reconsidered, both in the
tropospheric regime of weak stratification and in the stratospheric regime of moderately
strong stratification. A review of the basic assumptions of quasi-geostrophic theory
on an f-plane yields all synoptic scales in terms of a minimal number of natural
variables, i.e. two out of the speed of sound, gravitational acceleration and Coriolis
parameter. The wave scaling is defined so that all spatial and temporal scales are shorter
by one order in the Rossby number, and by assuming their buoyancy field to be
close to static instability. WKB theory is applied, with the Rossby number as scale
separation parameter, combined with a systematic Rossby-number expansion of all
fields. Classic results for synoptic-scale-flow balances and inertia-gravity wave
(IGW) dynamics are recovered. These are supplemented by explicit expressions for
the interaction between mesoscale geostrophic modes (GM), a possibly somewhat
overlooked agent of horizontal coupling in the atmosphere, and the synoptic-scale flow. It
is shown that IGW higher harmonics are slaved to the basic IGW, and that their
amplitude is one order of magnitude smaller than the basic-wave amplitude. GM higher
harmonics are not that weak and they are in intense nonlinear interaction between
themselves and the basic GM. Compressible dynamics plays a significant role in the
stratospheric stratification regime, where anelastic theory would yield insufficient results.
Supplementing classic derivations, it is moreover shown that in the absence of mesoscale
waves quasi-geostrophic theory holds also in the stratospheric stratification regime. |
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