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| Titel |
Case studies of gravity wave generation from cyclogenesis over the Southern Ocean |
| VerfasserIn |
Riwal Plougonven, Albert Hertzog |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250083627
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| Zusammenfassung |
| Better understanding of the generation of gravity waves from jets and fronts is necessary in
order to improve the parameterization of these waves in climate models. The way in which
energy may be transferred from motions that are primarily balanced (jets and fronts) to
gravity waves also constitutes a fundamental problem of geophysical fluid dynamics, with
applications also in oceanography. Several mechanisms have been highlighted theoretically
(Lighthill radiation, unbalanced instabilities, sheared disturbances, emission from
dipole-like flows) but their relevance to real flows remains to be proven. The emission in
idealized baroclinic life cycles is analogous to the emission from dipoles, but the
gap between idealized simulations and real flows and observations remains to be
overcome.
We present the investigation of several case studies bases on mesoscale numerical
simulations and superpressure balloon observations. The simulations were carried out with
the Weather Research and Forecast Model, with a horizontal resolution of dx = 20Â km and
sometimes dx = 10Â km. They cover two months during the Vorcore campaign
which took place in 2005 over Antarctica and the Southern Ocean. The overall
comparison has shown that the simulations somewhat underestimate momentum
fluxes, but provide a reliable description of their spatial and temporal variability. The
probability distribution function from both observations and simulations, when
taken in regions away from topography, is very well approximated by a lognormal
distribution.
Episodes of intense momentum fluxes over the Southern Ocean, far from islands are
identified for case studies, some of which include balloon observations. The emitted gravity
waves appear over intense and rapid cyclogenesis events. It appears that, despite
the high latitude, moist effects contribute to the emission of the gravity waves.
This effect may result from the more rapid development of cyclogenesis in a moist
atmosphere, but appears to also be tied to the intensification of localized updrafts
ahead of the surface front. Consistent with some theoretical studies, the presence of
intense shear above the fronts, and transverse to the fronts, appears to enhance
significantly the emission. Sensitivity to moisture and resolution is addressed using dry
simulations and double resolution simulations. Momentum fluxes are decreased typically
by a factor 2 in dry simulations, and increased by a factor 2 when doubling the
resolution.
Finally, the relevance of the different mechanisms that have been highlighted theoretically
is discussed in the light of these case studies. The importance of moist effects, although
expected and hinted at in certain studies, has been overlooked by studies which focused, for a
start, on dry dynamics. |
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