Polar low

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Polar low over the Sea of ​​Japan in December 2009

Polar lows are similar in behavior, size, and appearance to tropical cyclones , but are generally much more short-lived.

The diameter of polar lows is usually several hundred kilometers. They are often associated with strong winds, which, however, rarely reach hurricane strength. Their lifespan is 1 to 2 days on average. In contrast to most tropical storms, they develop very quickly and reach their maximum within 24 hours. They usually form in areas with deep arctic cold air flowing over relatively warm water.

On satellite images , polar lows look very similar to hurricanes . Spiral convection bands wrap around the depth and in certain cases even a cloud-free eye can develop. Aircraft research suggests that these low pressure systems may have a warm core, as is the case with hurricanes. Polar lows are difficult to predict due to a lack of measurement data in the polar region , which should be included in the weather models, and their small horizontal extent.

Mechanisms of origin

Sensible and latent warmth

When a cold air mass moves over a water surface, sensible heat is transported from the water to the air. This reduces the stability of the air mass in the air layer close to the ground. The cold air mass has a low pseudo- potential temperature , which is why moisture is transported rapidly in cold air, while it is modified by the input of sensible heat. Clouds form shortly after the cold air mass moves over water, indicating that latent heat is being released. This deep convection, concentrated in a narrow area, is from time to time associated with the development of a polar low.

Baroclinic instability

Baroclinic instability (see also Baroclinicity ) is related to the vertical shear of the main flow. Baroclinic instabilities are amplified by the conversion of potential energy, which is related to the mean horizontal temperature gradient . (Holton 1992).

Pronounced baroclinic zones near the ground can arise due to various conditions.

  • Ground winds can flow parallel or almost parallel along the edges of the pack ice , which creates the possibility of sharp baroclinic zones.
  • The convergence of different wind currents close to the ground and geographical conditions can also favor the formation of baroclinic zones close to the ground. This can mean that polar lows can develop at a great distance from the edges of the pack ice and the origin of the cold air mass.

It used to be assumed that polar lows were the result of thermal instability. That changed when Harrold and Browning (1969) used radar images to investigate a polar depression that crossed south-west England in December 1967 . They found that much of the precipitation comes from uniformly large-scale uplift , not from the merging of smaller convective cells (showers). In their study, the convection was on the back of the system.

Rasmussen et al. (1994) claim that deep polar developments that can be attributed exclusively to baroclinic or convective processes are rare. Rasmussen and Aakjær (1989), however, reported two polar lows which reached Denmark and which, it seemed, were exclusively baroclinic throughout their lifespan. One system formed near the main frontal zone, while the other was associated with a drop of cold air , which was the end product of an occlusion . In the previous work, they claimed that such events are quite common in the North Sea region .

Barotropic instability

Barotropic instability (see barotropy ) is a wave instability , which is associated with the horizontal shear in a jet stream- like flow . Barotropic instabilities grow by extracting kinetic energy from the main flow field. (Holton 1992)

Barotropic instability can lead to the formation of shear vortices close to the ground. If this development is supported by higher air layers, these eddies can develop into polar lows. Rasmussen et al. (1994 and 1996) identified this as a possible formation mechanism for polar lows, which they studied in the Labrador Sea .

Cold mountain troughs and drops of cold air

When a baroclinic wave close to the ground, barotropic shear vortices, or regions with increased convection have formed in a cold air mass, when will it develop further, if at all?

Rasmussen (1992) asserts, "In the case of a straight upstream flow with little vorticity advection, polar lows do not develop, even in cases where the temperature is very low at high altitudes." With that in mind, we need to look for reinforcing mechanisms. An obvious mechanism in the northern latitudes are cold troughs and / or closed high troughs with a cold core (so-called cold air drops ).

Rasmussen (1996) claims that all polar eddies he studied in the Labrador Sea were triggered by an altitude trough or a drop of cold air. In all cases examined by Parker and Hudson (1991) and Parker (1992), a cold trough and / or a closed eddy at the 500 hPa level was also involved. In the Pacific , a “comma cloud” (comma-shaped cloud formation) is often accompanied by an altitude trough. This can help for the early detection of reinforcing mechanisms.

A study by Noer et al. (2003) on the formation of polar lows in the Norwegian marine region shows that in all cases a cold high trough or eddy was associated with the deep development. Indeed, it can often be shown that the movement and strength of these systems at high altitudes give a good idea of ​​how the further development and movement of the polar lows is going.

Conditional Instability of the Second Kind (CISK)

The similarity between some polar lows and tropical cyclones has led some researchers to speculate that similar processes may be involved. Conditional instability of the second kind , or CISK for short, is one of these processes.

Charney and Eliassen (1964) defined CISK as a mutual interaction between small-scale convection and large-scale perturbation whereby:

So CISK is a positive feedback mechanism.

A number of researchers previously suspected that CISK was one of the driving forces behind the formation of polar lows. It is now believed that while CISK is involved in its creation, it is not the only relevant mechanism.

Instability due to the interaction of air and water

Emanuel (1986) rejected the CISK idea for tropical cyclones. He suggested that tropical cyclones result from instabilities caused by ocean-atmosphere interactions. Abnormal flows of sensible and latent heat from the sea surface induced by strong ground winds and falling pressure lead to stronger temperature anomalies and thereby to a further intensification of the ground winds and the pressure drop.

Emanuel and Rotunno (1989) tested Emanuel's theory of atmosphere-ocean interaction for polar lows. For their case study, they used a simple non-linear analytical model and an axisymmetric numerical model . The results showed that their hypothesis agreed with observed polar depressions. However, your model required an existing disturbance as a triggering mechanism before the instability resulting from the atmosphere-ocean interaction could take effect.

literature

  • Agee, Ernest M. and Steven R. Gilbert (1989): An aircraft investigation of mesoscale convection over Lake Michigan during the January 10, 1984 cold air outbreak , J. Atmos. Sci., 46, no. 13, 1877-1897.
  • Bader, MJ, GS Forbes, JR Grant, RBE Lilley, and AJ Waters (1995): Images in Weather Forecasting , Cambridge University Press. ISBN 978-0-521-45111-6
  • Charney, J. and A. Eliassen (1964): On the growth of the hurricane depression , J. Atmos. Sci., 21, 68-75.
  • Emanuel, Kerry, A. (1986): A two stage air sea interaction theory for polar lows , Proc., The Third International Conference on Polar Lows, Norway.
  • Emanuel, Kerry A. and Richard Rotunno (1989): Polar lows as arctic hurricanes , Tellus, 41A, 1-17.
  • Holton, James R. , 1992: Introduction to Dynamic Meteorology . 3d ed. Academic Press Ltd., London, UK ISBN 978-0-12-354015-7
  • Harrold, TW, and KA Browning (1969): The polar low as a baroclinic disturbance , Quart. J. Roy. Meteor. Soc., 95, 710-723.
  • Mullen, S. (1983): Explosive cyclogenesis associated with cyclones in polar air streams , Mon. Wea. Rev., 111, 1537-1553.
  • Noer, G. and M. Ovhed (2003): Forecasting of polar lows in the Norwegian and the Barents Sea , Proc. of the 9th meeting of the EGS Polar Lows Working Group, Cambridge, UK.
  • Parker, MN and Edward Hudson (1991): Polar Low Handbook for Canadian Meteorologists , Environment Canada, Atmospheric Environment Service.
  • Parker, MN (1992): Polar lows in the Beaufort Sea and Davis Strait , Proc. Applications of New Forms of Satellite Data in Polar Low Research, Polar Low Workshop, Hvanneyri, Iceland.
  • Parker, Neil (1997): Cold Air Vortices and Polar Low Handbook for Canadian Meteorlogists , Environment Canada.
  • Rasmussen, Erik A. and PD Aakjær (1989): Two polar lows affecting Denmark , Vejret, Special Issue in English, The Danish Meteorological Society.
  • Rasmussen, Erik A., John Turner, and Paul Twitchell (1992): Applications of new forms of satellite data in polar low research , Bulletin of the American Meteorological Society , 74, 1057-1073.
  • Rasmussen, Erik A. and Anette Cederskov (1994): Polar lows: A critical analysis. The life cycles of extratropical cyclones , Vol 111. Proc. of an International Symposium, Bergan, Norway.
  • Rasmussen, Erik A., Chantal Claud, and James F. Purdom (1996): Labrador Sea polar lows , The Global Atmosphere and Ocean System, Vol. 4, 275-333.
  • Shapiro, MA, T. Hampel, and LS Fedor (1989): Research aircraft observations of an arctic front over the Barents Seas , Polar and Arctic Lows, Polar and Arctic Lows. Twitchell, P., E. Rasmussen, K. Davidson, Eds., A. Deepak Publishing.

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