Jetstream ( English jet stream , originally from the translation of the German word beam current ) dynamically displacing strong wind bands mentioned, which usually occur on the upper troposphere to the stratosphere occur, where the weather activity in the below seamlessly adjacent tropopause ends.
Jet streams are formed as a result of global compensatory movements between different temperature regions or high and low pressure areas and represent the strongest naturally occurring winds , whereby they are very reliable and stable in their occurrence over several days compared to other weather phenomena. In the short term, they separate warm from cold air masses, but ultimately they swirl them through vertical movements in certain areas. The warm air masses are deflected by the Earth's rotation on their way to the North Pole, while maintaining their high orbital speed.
More generally defined, these are atmospheric wind bands with an almost horizontal flow axis ("jet axis") and wind speeds of up to 150 m / s (540 km / h), the wind speed - both vertically and horizontally - increasing rapidly with increasing distance from the center of the flow falls off. They roughly belong to the group of geostrophic winds , in which there is a balance between pressure gradient and Coriolis force .
Occurrence and types
There are four main jet streams, whereby one has to differentiate between two different types and their respective hemispheres . Since they occur at high altitudes, they are displayed or evaluated in isobaric altitude weather maps (mostly in relation to the 200 hPa pressure area ).
- The polar jet stream (PFJ - Polarfrontjetstream ) extends depending on weather situation between 40 ° and 60 ° of latitude in the range of 249- to 300 hPa - isobar in combination with the often up to the floor- polar front . In the core flow, i.e. its center, it reaches speeds of 200 to 500 km / h (known maximum ~ 1970 in Japan 650 km / h) and represents the most important jet flow, whereby it is also of decisive importance for European weather. Since a comparatively strong horizontal temperature gradient develops in the middle latitudes due to the clash of cold polar air and moderate warmer air masses , the PFJ occurs all year round. However, its maximum speed and the lowest altitudes are reached in winter, as the temperature differences between the pole and the equator are then usually greater than in summer and the tropopause is usually much lower. Due to dynamic effects in the earth's atmosphere , the polar front jet stream only occurs in relatively short bands of a few thousand kilometers in length.
- The sub-tropical jet stream (STJ - Subtropenjetstream ) is also a West Wind jet stream in the vicinity of the tropics or sub-tropics , that is in the range of 20 ° to 30 ° of geographical latitude and in the range of 150 to 200 hPa isobars. It occurs at the upper limit of the sloping branch of the Hadley cell , i.e. above the subtropical high pressure belt , and develops out of the Antipassat . The STJ is weaker than the PFJ and often only comes to training in the respective winter months (in the hemispheres). Like the PFJ, it is closely linked to a large horizontal temperature gradient, the so-called subtropical front, which, however, in contrast to the polar front, generally does not extend to the ground. Although it is weaker than the polar front jet stream, it shows a much greater consistency in position and intensity and also stretches closed around the entire globe.
In addition to the well-known large jet streams, there are also
- the Tropical Easterly Jet (TEJ): It extends from the Tibetan plateau to the intertropical convergence zone (ITC) and is particularly effective here as a high-east wind up to northern Africa . In particular, it is not a westerly wind as in the PFJ or STJ, but an easterly wind. It experiences its greatest expression in northern summer, i.e. during the Indian summer monsoon .
- The Low Altitude Jets : They occur in the vicinity of cyclones (low altitude).
- the Nocturnal Jet : A low-altitude jet stream at night .
- stratospheric or mesospheric jet streams.
- A superstorm is a jet stream turbulence consisting of a polar jet stream (PFJ - polar front jet stream) and a subtropical jet stream (STJ - subtropical jet stream).
Causes of emergence
The comparatively strong solar radiation at the equator warms the air masses close to the ground and creates a positive energy balance , while this is negative at the poles due to the latitude dependence of the radiation energy caused by the sun . Consequently, in the area of the equator close to the ground, it is a question of relatively warm air masses, which have a lower density compared to the colder air masses of the poles . The air in the troposphere is therefore more loosely packed along the entire globe-spanning Inertropical Convergence Zone (ITC) than at the poles, which means that the vertical pressure gradient is much lower than at low temperatures and the air pressure therefore falls more slowly with altitude than south or north of the ITC. For this reason, among other things, the troposphere can reach a height of about 18 km along the equator and in the temperate latitudes up to a height of about 12 km, while at the poles it only reaches an average thickness of 8 km. This decrease in air density at the equator is associated with a relative decrease in pressure and thus a stable low pressure belt, precisely the already mentioned intra-tropical convergence zone, whereby a distinction between ITC and equator is necessary. On the other hand, due to the low pressure gradient, there is a high pressure area at high altitudes , which is why a distinction is made between ground- low and high-altitude at the equator .
Over the poles, however, the air masses are much more densely packed. Due to the low level of solar radiation, the air is cold here and due to the higher density it is more difficult to store on the earth's surface. The pressure gradient is consequently much more pronounced here and there are stable high pressure areas on the ground. One speaks therefore of a floor level and accordingly also of a height low .
The air pressure and temperature differences between the equator and the poles are therefore thermally conditioned. They result from the latitude dependence of the solar radiation , which results purely geometrically from the different large angles of incidence of the solar radiation . The drive motor of the emerging dynamic weather and wind system and thus also the jet stream can therefore be found in the sun , despite all other influencing factors .
Pressure gradient force
Between high and low pressure areas there is a compensating force, which is called gradient force or pressure gradient force. In an effort to equalize the pressure or temperature differences, the altitude air, following the gradient force, moves across the latitudes from the high altitude of the equator in the direction of the low altitude of the poles, i.e. from the location of the higher to the location of the lower pressure. The stronger these pressure and temperature differences are, the stronger the gradient force and the resulting wind . These differences are only seldom, for example in tropical cyclones , large enough to accelerate the air near the ground sufficiently, and usually only lead to rotational movements, which are very unstable and due to the lack of a horizontal flow axis, despite sometimes high rotational speeds , do not represent jet streams. These themselves can only form with the pressure differences increasing with altitude and without friction influences ( free atmosphere ). However, the pressure differences also decrease sharply near the tropopause or in the stratosphere. This explains why the very strong jet streams develop above all at sharp air mass boundaries and are also limited vertically to a certain height, i.e. ultimately have the appearance of a wind tube. However, this idealized representation must be expanded to include the so-called Coriolis effect .
Due to the rotation of the earth, the Coriolis force acts on the air flowing towards the pole . This apparent force has the effect that moving air masses are always deflected to the right in the northern hemisphere and always to the left in the southern hemisphere. For air masses flowing towards the pole, this means a deflection to the east on both hemispheres. As a result, the gradient winds flowing towards the pole become the jet streams flowing eastwards.
The (horizontal) Coriolis force described above increases from the equator to the poles. It disappears at the equator. The adjacent diagram illustrates the eastward deflection of the high-altitude winds flowing towards the pole.
Incidentally, the Coriolis force also has a vertical component that influences ascending or descending air masses. They divert ascending air masses to the west and descending air masses to the east. The vertical component decreases from the equator towards the poles. It is zero at the poles.
In the late 19th century , observing high-altitude cloud formations, it was concluded that there must be strong high-altitude winds in their vicinity . However, these could only be observed at very irregular intervals, so that their regularity and comparatively constant strength were not yet recognized. In 1924 the Japanese meteorologist Oishi Wasaburo researched this high wind very carefully. Independently of Oishi, Johannes Georgi discovered strong winds at heights of 10 to 15 km that could not be directly explained by the ground pressure field when he carried out balloon soundings on the northern tip of Iceland in 1926 and 1927.
In the 1930s, internationally coordinated vertical soundings were carried out for the first time. This prompted Richard Scherhag to regularly draw up altitude weather maps from 1935 onwards. In 1937 Scherhag investigated a storm depression over the Labrador Peninsula . He calculated a gradient wind of 275 km / h for an altitude of 5000 m and came to the conclusion that wind speeds of over 300 km / h must be expected in the area of origin of the Atlantic storm cyclones at the height of the tropopause. German weather planes flew from Frankfurt / M on February 20, 1937. into the jet stream and her Heinkel He 46 flew backwards above 5500 m from Mainz to Frankfurt, with an average flow speed of 280 km / h being measured. Heinrich Seilkopf used the term “jet flow” in 1939 for a layer of maximum wind speed near the tropopause in the transition area between high and low altitude. Hermann Flohn mentions in his memoirs that the Belarusian meteorologist Mironovitch also published an article on high wind speeds in the upper troposphere in the French journal La Météorologie before 1939 .
However, these publications were only made in German and French, which severely restricted the exchange of knowledge with British and American meteorologists. The translation of Seilkopf's publication into other languages was even explicitly forbidden. With the outbreak of the Second World War , the exchange of experiences between Germany and the other nations was then completely prevented. The further history of discovery is therefore very inhomogeneous and strongly shaped by the experiences and conditions in the respective countries.
Another reason for the later discovery in the USA, according to Hermann Flohn , is that research there initially concentrated on other analysis methods with which the high altitude winds that are important for aviation could not be derived directly. Therefore, when the Second World War entered the USA, the procedures for creating altitude weather maps and the training of weather advisors had to be changed. Here Carl-Gustaf Rossby did pioneering work by initiating a large training program for weather consultants, in which around 8,000 weather officers were trained, who also worked closely with the British.
In 1942, the Norwegian meteorologist Sverre Petterssen also demonstrated the existence of the jet stream and investigated the mechanisms behind its formation. The Norwegian meteorologist Jacob Bjerknes mentioned the term jetstream in 1943 during a lecture in England. Although there have already been reports of problems with the flight crews with high wind speeds in the upper troposphere, this issue was not initially investigated systematically. In 1944, the B-29 was the first bomber to be completed, which was designed to transport a high bomb load at great heights. In preparation for the air strikes against Japan, the meteorologists of the US Air Force now regularly encountered strong wind fields at great heights. At first, some had problems making this clear to their superiors. As a result of this experience, Rossby began to work intensively on researching the jet stream and predicting its displacement, and the term began to establish itself in the English-speaking world.
Weather and climate
Jet streams are decisive for the air pressure distribution and thus for the formation of the wind and air pressure belts on earth. They represent a major cause of the weather development and an important element for the global heat transfer between the tropics and the poles: If the temperature differences between the air masses from the subtropics (e.g. deserts ) and the poles are sufficiently large , the wind flow on the polar front is higher due to the higher Dynamics of the polar front strongly deflected. Obstacles like the high mountains of the Himalayas and the Rocky Mountains amplify this. This creates the Rossby waves shown in blue in the upper figure . The representation is idealized because the folding of the jet stream is inconsistent and the polar front jet stream does not wind itself around the entire earth. The jet stream is located between warm air of middle latitudes and cold air of higher latitudes. A more realistic picture of the meandering ribbons of the PFJ can be seen in the web links .
The jet stream carries with it layers of air underneath, with dynamic low pressure areas ( cyclones ) in the direction of the pole (twisted counterclockwise over the 'wave troughs', so-called troughs ) and in the direction of the equator high pressure areas (twisted clockwise under the 'wave crests' ) , corresponding to the turbulence of the Rossby wave so-called backs ). Rossby waves are much more pronounced in the northern hemisphere than in the southern hemisphere because of some very large mountains that act as a barrier .
A typical feature of the polar jet stream is the stabilization of its Rosby waves in summer: How far south they penetrate and in what number and form they manifest themselves, then largely determines the weather situation in Central Europe. This experience is also reflected in the farmer's rule about the dormouse day .
A “resonance mechanism that holds waves in the middle latitudes and significantly amplifies them” was brought into relation in 2014 as a cause, among other things, for the number of extreme weather events in summer that began in 2003. These include the record heat wave in Eastern Europe in 2010, which was accompanied by crop losses and devastating forest fires around Moscow.
Climate models establish a connection between cold snaps in the USA, among other things, at the beginning of 2019 and long periods of heat in Europe in 2003, 2006, 2015, 2018 and 2019 due to the jet stream weakening and turbulence caused by man-made climate change . This is one of the consequences of global warming in the Arctic . - "quasi as a resonance reinforcement". Added to this is the man-made influence on the ozone layer.
The effect of the jet stream is particularly noticeable on scheduled flights over longer distances, for example between North America and Europe . Since it is a strong and fairly reliable high-altitude wind, aircraft can use it to achieve higher speeds and also lower fuel consumption. Both flight altitudes and travel routes are therefore adapted to the course of the jet stream so that it can be used as a tailwind or avoided as a headwind. Among other things, he is responsible for ensuring that altitudes of ten to twelve kilometers, depending on the height of the jet stream and the travel route, are favored far beyond a direct “beeline”. On a flight across the Atlantic to Europe, for example, the route runs away from the orthodromes (great circles), which can save several hours of time. However, this also has negative effects on navigation and air traffic control .
Another interesting application of jet streams is for balloon flights. At the end of the Second World War, Japan was able to attack the American mainland with explosive-carrying balloons purely by exploiting these streams (but without any major success). The first balloon circumnavigation of the world by Bertrand Piccard with co-pilot Brian Jones was only possible by using the speed of the jet stream. It took place a little way off the equator and with navigation support from ground stations based on weather data.
Jet streams are accompanied by areas of increased turbulence. This fact must be taken into account for flights.
In astronomy , seeing plays an important role in visual observation and astrophotography . One of the main causes of seeing is the jet stream, as turbulence occurs in the transition layer to deeper air layers due to differences in speed. These turbulences cause rapid changes in the optical refractive index of the air and thus a reduced image quality.
Production of electrical energy
The economic use of high-altitude winds or jet streams to generate electricity by means of aircraft wind power plants is currently still in the research and development stage.
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