Subsynoptic Vortices in the Mediterranean Sea Region'

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Sub-synoptic Vortices in the Mediterranean Sea
Region.

• By Jeroen Derksen
• 04-22-02

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Satellite Picture of the Vortex of 9 October
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Situation

• A relative warm Mediterranean sea.
• Alps (up to 4800m) northwest of the
  Mediterranean.
• Relative cold air coming from the Atlantic over
  the Alps.

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Question In What Category of Systems Does It
Belongs?

• Baroclinic, like mid-latitude cyclones?
• Barotropic, like tropical hurricanes?
• Is it something, which has features of both?

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Baroclinic systems

• Horizontal temperature gradient.
• Vertical wind shear (via thermal wind eqn.).
• If upper and lower minima aligns in a low
  pressure system, it will dissipate.

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Barotropic systems

• No horizontal temperature gradient.
• And thus no vertical wind shear
• Horizontal wind shear.
• A barotropic system depends on surface heat and
  water vapor fluxes for its fuel.

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Ive Looked at Three Case Studies.

• F. Meneguzzo, M. Pasqui and M. Rossi looking at
  the vortex of 26-28 march 1999.
• R. Atlas and O. Reale looking at the vortex of
  3-5 October 1996.
• R. Atlas and O. Reale looking at the vortex of
  6-10 October 1996.

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Initiation

• Before the sub-synoptic vortex has developed, a
  baroclinic lee cyclone (BLC) has started to
  develop.
• The BLC weakens again, leaving a trough nearby
  Italy.
• This results in a baroclinic environment

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Evolution

• A part of the trough starts to intensify again.
• Tilt between lower and upper minima disappears.
• The upper level jet is moving to the south of the
  vortex.
• Symmetry increases.
• An eye-like feature develops.

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Vertical Structure

• No vertical wind shear
• Strong horizontal wind shear
• Cylindrical wind speed structure
• Baroclinic environment

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Convective Instability of a Second Kind (CISK)

• Air flows to the center of a low.
• Convergence occurs
• Air is forced up
• Latent heat is released
• Air rises more, making the pressure to drop
  underneath.
• Air is flowing even faster into the center.
• Etc.

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Dissipating

• The vortex makes landfall.
• It looses its eye-like feature.
• It looses its symmetric structure.
• Probably due to land friction and lack of an
  energy source.

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Conclusions

• F. Meneguzzo, M. Pasqui and M. Rossi conclude
  that the vortex possessed features of a BLC, but
  during its mature stage the role of latent and
  sensible heat fluxes increases.
• R. Atlas and O. Reale conclude that in spite of
  the marginal role played by baroclinity during
  the development of these vortices, the
  cyclogenesis takes place in an environment in
  which baroclinity might have been present in an
  earlier stage.

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Conclusions of Other Authors.

• Rasmussen en Zick (1987) conclude that it is
  unlikely that baroclinic instability plays a role
  for formation and continued existence of these
  sub-synoptic vortices.
• Buzzi and Tibaldi (1978) conclude that latent
  heat plays an unimportant role in the initiation
  of these vortices, but it may play a significant
  role in subsequent stages.

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The January 1995 Event On http//www.mindspring.c
om/jbeven/intr0008.htmis a great collection of
satellite images, following an event in January
1995. There are some remarkable features.
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My Own Conclusions

• The event starts with a baroclinic environment.
• Latent and sensible heat fluxes become dominant.
• The system develops in a barotropic system.

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References

• Buzzi, A. And Tibaldi, S., 1978 Cyclogenesis in
  the lee of the alps A case study. Quart. J.
  Meteor. Soc., 104, 271-287.
• Holton, J., 1992 an introduction to dynamical
  meteorology. 3d ed. Academic press, 511 pp.
• Meneguzzo, F., Pasqui, M., Messeri, G. And Rossi,
  M. High-resolution simulation of a deep
  Mediterranean cyclone using RAMS model.
• References (Contd)
• Rasmussen, E. And Zick, C., 1987 A subsynoptic
  vortex over the Mediterranean with some
  resemblance to polar lows. Tellus, 39A, 408-425.
• Reale, O. And atlas, R., 2001 tropical
  cyclonelike vortices in the Extratropics
  observational evidence and synoptic analysis.
  Weather and forecasting vol. 16, no. 1, pp.
  734.
• Teeuwen, H., 2000 latente warmte is meer dan
  barocline processen. Meer dus he. De laatste is
  dus minder. De Kruik 34-52.

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