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| Titel |
Educing the emission mechanism of internal gravity waves in the differentially heat rotating annulus |
| VerfasserIn |
Joran Rolland, Steffen Hien, Ulrich Achatz, Sebastian Borchert, Mark Fruman |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250133854
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| Publikation (Nr.) |
 EGU/EGU2016-14511.pdf |
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| Zusammenfassung |
| Understanding the lifecycle of gravity waves is fundamental to a good comprehension of
the dynamics of the atmosphere. In this lifecycle, the emission mechanisms may
be the most elusive. Indeed, while the emission of gravity waves by orography or
convection is well understood, the so-called spontaneous emission is still a quite open
topic of investigation [1]. This type of emission usually occur very near jet-front
systems in the troposphere. In this abstract, we announce our numerical study of the
question.
Model systems of the atmosphere which can be easily simulated or built in a laboratory
have always been an important part of the study of atmospheric dynamics, alongside global
simulations, in situ measurements and theory. In the case of the study of the spontaneous
emission of gravity waves near jet-front systems, the differentially heated rotating annulus set
up has been proposed and extensively used. It comprises of an annular tank containing water:
the inner cylinder is kept at a cold temperature while the outer cylinder is kept at a
warm temperature. The whole system is rotating. Provided the values of the control
parameters (temperature, rotation rate, gap between the cylinders, height of water) are
well chosen, the resulting flow mimics the troposphere at midlatitudes: it has a jet
stream, and a baroclinic lifecycle develops on top of it. A very reasonable ratio of
Brunt-Väisälä frequency over rotation rate of the system can be obtained, so as to
be as close to the atmosphere as possible. Recent experiments as well as earlier
numerical simulations in our research group have shown that gravity waves are indeed
emitted in this set up, in particular near the jet front system of the baroclinic wave
[2].
After a first experimental stage of characterising the emitted wavepacket, we focused our
work on testing hypotheses on the gravity wave emission mechanism: we have tested and
validated the hypothesis of spontaneous imbalance generated by the flow in geostrophic
balance. For the first stage of this investigation, we separated the flow between a balance and
an imbalanced part at first order in Rossby number: the balanced pressure field was computed
through an inversion of the potential vorticity equation [3]. The balanced horizontal velocity
field and buoyancy were then computed using the geostrophic and hydrostatic balance
conditions. We first checked that this decomposition gave on the one hand a large
scaled balanced flow, comprising mostly of the baroclinic wave, and on the other
hand a small scale flow comprising mostly of the gravity wave signal. We then
proceeded with the central stage of the validation: we simulated the tangent linear
dynamics of the imbalanced part of the flow [4]. The equations are linearised about
the balanced part, and any imbalances forces the modeled imbalanced part. The
output of this simulation compares very well with the actual imbalanced part, thus
confirming that the observed gravity waves are indeed generated through spontaneous
imbalance. To our knowledge, this is the first demonstration of emission by this
mechanism in a flow which is not idealised: a flow which can be obtained as a result of a
numerical simulation of primitive equations or actually observed in a laboratory
experiment.
References
[1] R. Plougonven, F. Zhang, Internal gravity waves from atmospheric jets and fronts,
Rev. Geophys. 52, 33-76 (2014).
[2] S. Borchert, U. Achatz, M.D. Fruman, Spontaneous Gravity wave emission in the
differentially heated annulus, J. Fluid Mech. 758, 287–311 (2014).
[3] F. Zhang, S.E . Koch, C. A. Davis, M. L. Kaplan, A Survey of unbalanced flow
diagnostics and their application, Adv. Atmo. Sci. 17, 165–183 (2000).
[4] S. Wang, F. Zhang, Source of gravity waves within a vortex dipole jet revealed by a
linear model, J. Atmo. Sci. 67, 1438–1455 (2010). |
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