Polar low
However, polar lows are similar to tropical cyclones in behavior, size and appearance, generally much shorter-lived.
The diameter of polar lows is usually several hundred kilometers. They are often associated with strong winds, but rarely reach hurricane strength. Their lifespan is on average 1 to 2 days. Unlike most tropical storms develop very rapidly, reaching a maximum within 24 hours. Usually they form in areas with deep arctic cold air passing over relatively warm water.
Polar lows look very similar to satellite images like hurricanes. Spiral Konvektionsbänder wrap around the low and in some cases even a cloud- free eye can form. Studies with aircraft indicate that this low- pressure systems may have a warm core, as is the case with hurricanes. Polar lows are difficult to predict due to lack of data in the polar region, which should be incorporated into the weather models, and their small horizontal extent.
Pathogenesis of
Sensible and latent heat
When a cold air mass moves over a water surface is a transport of sensible heat from the water instead of air. This reduces the stability of the air mass in the surface air layer. The cold air mass has a low potential pseudo- temperature, and therefore, a rapid transport of moisture takes place in cold air, as it is modified by the input of sensible heat. Clouds are formed shortly after the cold air mass moves over water, indicating that latent heat is released. These high -reaching, focused on a narrowly limited area convection is connected from time to time with the development of polar lows.
Baroclinic instability
Baroclinic instability ( see also baroclinicity ) is related to the vertical shear of the main flow. Baroclinic instability is aggravated by conversion of potential energy, which is related to the mean horizontal temperature gradient. ( Holton 1992).
Strong baroclinic zones near the ground can arise due to various conditions.
- Surface winds can flow in parallel or almost parallel along the ice edges, which gives the possibility for the emergence of sharp baroclinic zones.
- The convergence of various ground-level wind currents and geographical counter units can also favor the development of baroclinic zones near the ground. This can lead to the fact that polar lows can develop at large distances to the pack ice edge and the origin of the cold air mass.
Previously it was thought that polar lows would be the result of thermal instability. That changed as Harrold and Browning (1969 ) radar images used to investigate in December 1967 a polar low, which crossed the South West of England. They found that a large part of the precipitate by uniformly large-scale uplift is not created through the merging of smaller convective cells ( showers ). In their study, the convection was located on the back of the system.
Rasmussen et al. (1994 ) claim that polar low developments, which are solely due to baroclinic or convective processes are rare. Rasmussen and Aakjær (1989 ), however, reported two polar lows, which reached Denmark and as it seemed, throughout their lifetime were exclusively baroclinically. One system was formed near the main frontal zone, while the other was associated with a cold air drops, which was the end product of an occlusion. In the previous paper, they claimed that such events in the North Sea region are quite common.
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 ground- Scherungswirbel. If this development supported by higher layers of the atmosphere, these vortices can develop into polar lows. Rasmussen et al. (1994 and 1996) identified this as a possible formation mechanism for polar lows, which they have studied in the Labrador Sea.
Cold Upper Troughs and cold air drops
If a ground-level baroclinic wave barotropic shear vortices or regions have formed with forced convection in a cold air mass when it comes to further development, if at all?
Rasmussen (1992 ) asserts, " In the case of a rectilinear flow with low level vorticity advection are no polar lows develop, even in cases where the temperature is very low at high altitude. " With that in mind, we must look for reinforcing mechanisms out. One obvious mechanism in the northern latitudes are cold Upper Troughs and / or completed level lows with cold core (so-called cold air drops).
Rasmussen ( 1996) claims that all the polar vortex, which he has studied in the Labrador Sea, were triggered by an upper level trough or a cold air drops. Were in all cases which Parker and Hudson ( 1991) and Parker ( 1992) investigated a cold trough and / or a closed vortex on the 500 - hPa level was also involved. In the Pacific, is a "comma cloud" ( comma-shaped cloud formation ) is often accompanied by an upper level trough. This can help in the early detection of reinforcing mechanisms.
A study by Noer et al. ( 2003) on the formation of polar lows in the Norwegian Sea region shows that a cold upper level trough or vertebrae was associated in all cases with the deep development. In fact you can often be shown that the movement and strength of these systems provide high altitude a good impression, as the further development and movement of polar lows expires.
Conditional Instability of the Second Child ( CISK )
The similarity between some polar lows and tropical cyclones has led some researchers to conjecture that similar processes may be involved. Conditional instability of the second kind, short CISK, is one of these processes.
Charney and Eliassen (1964 ) defined thereby CISK as a mutual interaction between small-scale convection and larger scale disturbance:
- The large-scale convergence organizes the Cumuluskonvektion
- Released condensation heat the large-scale system feeds energy into the clouds
CISK is therefore a positive -feedback mechanism.
A number of researchers suspected earlier that CISK is one of the driving forces for the development of polar lows. Meanwhile, it is believed that CISK is indeed involved in the development, but it is not the only relevant mechanism.
Instability caused by the interaction of air and water
Emanuel (1986 ) rejected the idea of CISK for tropical cyclones. He suspected that tropical cyclones from instabilities result by ocean - atmosphere interactions. Abnormal fluxes of sensible and latent heat induced by the sea surface by strong surface winds and falling pressure lead to stronger temperature anomalies and thus to a further strengthening of the surface winds and the pressure case.
Emanuel and Rotunno (1989 ) tested Emanuel's theory of the 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 was consistent with observed polar low developments. But your model needed already an existing fault will be the trigger mechanism before the force resulting from air-sea interaction instability could have an effect.