Tropopause

The tropopause is the boundary in the Earth's atmosphere between the troposphere and the stratosphere. It is a thermodynamic gradient stratification layer, marking the end of troposphere. It lies, on average, at 17 kilometres (11 mi) above equatorial regions, and above 9 kilometres (5.6 mi) over the polar regions.

Definition

Earth Atmosphere
Schematic showing the different layers of the atmosphere (not to scale). The tropopause is located between the troposphere and the stratosphere.
Jetcrosssection
The tropopause lies higher in the tropics than at the poles.

Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry. More formally, the tropopause is the region of the atmosphere where the environmental lapse rate changes from positive, as it behaves in the troposphere, to the stratospheric negative one. Following is the exact definition used by the World Meteorological Organization:

The boundary between the troposphere and the stratosphere, where an abrupt change in lapse rate usually occurs. It is defined as the lowest level at which the lapse rate decreases to 2 °C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 °C/km.[1]

The tropopause as defined above renders as a first-order discontinuity surface, that is, temperature as a function of height varies continuously through the atmosphere but the temperature gradient does not.[2]

Location

The troposphere is the lowest layer of the Earth's atmosphere; it is located right above the planetary boundary layer, and is the layer in which most weather phenomena take place. The troposphere contains the boundary layer, and ranges in height from an average of 9 km (5.6 mi; 30,000 ft) at the poles, to 17 km (11 mi; 56,000 ft) at the Equator.[3][4] In the absence of inversions and not considering moisture, the temperature lapse rate for this layer is 6.5 °C per kilometer, on average, according to the U.S. Standard Atmosphere.[5] A measurement of both the tropospheric and the stratospheric lapse rates helps identifying the location of the tropopause, since temperature increases with height in the stratosphere, and hence the lapse rate becomes negative. The tropopause location coincides with the lowest point at which the lapse rate falls below a prescribed threshold.

Since the tropopause responds to the average temperature of the entire layer that lies underneath it, it is at its peak levels over the Equator, and reaches minimum heights over the poles. On account of this, the coolest layer in the atmosphere lies at about 17 km over the equator. Due to the variation in starting height, the tropopause extremes are referred to as the equatorial tropopause and the polar tropopause.

Given that the lapse rate is not a conservative quantity when the tropopause is considered for stratosphere-troposphere exchanges studies, there exists an alternative definition named dynamic tropopause.[6] It is formed with the aid of potential vorticity, which is defined as the product of the isentropic density, i.e. the density that arises from using potential temperature as the vertical coordinate, and the absolute vorticity, given that this quantity attains quite different values for the troposphere and the stratosphere.[7] Instead of using the vertical temperature gradient as the defining variable, the dynamic tropopause surface is expressed in potential vorticity units (PVU).[nb 1] Given that the absolute vorticity is positive in the Northern Hemisphere and negative in the Southern Hemisphere, the threshold value should be taken as positive north of the Equator and negative south of it.[9] Theoretically, to define a global tropopause in this way, the two surfaces arising from the positive and negative thresholds need to be matched near the equator using another type of surface such as a constant potential temperature surface. Nevertheless, the dynamic tropopause is useless at equatorial latitudes because the isentropes are almost vertical.[8] For the extratropical tropopause in the Northern Hemisphere the WMO established a value of 1.5 PVU,[8] but greater values ranging between 2 and 3.5 PVU have been traditionally used.[10]

It is also possible to define the tropopause in terms of chemical composition.[11] For example, the lower stratosphere has much higher ozone concentrations than the upper troposphere, but much lower water vapor concentrations, so appropriate cutoffs can be used.

Phenomena

The tropopause is not a "hard" boundary. Vigorous thunderstorms, for example, particularly those of tropical origin, will overshoot into the lower stratosphere and undergo a brief (hour-order or less) low-frequency vertical oscillation.[12] Such oscillation sets up a low-frequency atmospheric gravity wave capable of affecting both atmospheric and oceanic currents in the region.

Most commercial aircraft are flown in the lower stratosphere, just above the tropopause, where clouds are usually absent, as are significant weather perturbations.[13]

See also

Notes

  1. ^ 1 PVU = 10-6 K m2 kg-1 s-1[8]

References

  1. ^ International Meteorological Vocabulary (2nd ed.). Geneva: Secretariat of the World Meteorological Organization. 1992. p. 636. ISBN 978-92-63-02182-3.
  2. ^ Panchev 1985, p. 129.
  3. ^ Hoinka, K. P. (1999). "Temperature, Humidity, and Wind at the Global Tropopause". Monthly Weather Review. 127 (10): 2248–2265. Bibcode:1999MWRv..127.2248H. doi:10.1175/1520-0493(1999)127<2248:THAWAT>2.0.CO;2.
  4. ^ Gettelman, A.; Salby, M. L.; Sassi, F. (2002). "Distribution and influence of convection in the tropical tropopause region". Journal of Geophysical Research. 107 (D10): ACL 6–1–ACL 6–12. Bibcode:2002JGRD..107.4080G. CiteSeerX 10.1.1.469.189. doi:10.1029/2001JD001048.
  5. ^ Petty 2008, p. 112.
  6. ^ Andrews, Holton & Leovy 1987, p. 371.
  7. ^ Hoskins, B. J.; McIntyre, M. E.; Robertson, A. W. (1985). "On the use and significance of isentropic potential vorticity maps". Quarterly Journal of the Royal Meteorological Society. 111 (470): 877–946. Bibcode:1985QJRMS.111..877H. doi:10.1002/qj.49711147002.
  8. ^ a b c Tuck, A. F.; Browell, E. V.; Danielsen, E. F.; Holton, J. R.; Hoskins, B. J.; Johnson, D. R.; Kley, D.; Krueger, A. J.; Megie, G.; Newell, R. E.; Vaughan, G. (1985). "Strat-trop exchange". Atmospheric Ozone 1985 – WMO Global Ozone Research and Monitoring Project Report No. 16. World Meteorological Organization. 1: 151–240.
  9. ^ Hoinka, Klaus P. (December 1998). "Statistics of the Global Tropopause Pressure". Journal of Climate. American Meteorological Society (126): 3303–3325.
  10. ^ Zängl, Günther; Hoinka, Klaus P. (15 July 2001). "The Tropopause in the Polar Regions". Journal of Climate. 14 (14): 3117&nbsp, –&#32, 3139. Bibcode:2001JCli...14.3117Z. doi:10.1175/1520-0442(2001)014<3117:ttitpr>2.0.co;2.
  11. ^ L. L. Pan; W. J. Randel; B. L. Gary; M. J. Mahoney; E. J. Hintsa (2004). "Definitions and sharpness of the extratropical tropopause: A trace gas perspective". Journal of Geophysical Research. 109 (D23): D23103. Bibcode:2004JGRD..10923103P. doi:10.1029/2004JD004982.
  12. ^ Shenk, W. E. (1974). "Cloud top height variability of strong convective cells". Journal of Applied Meteorology. 13 (8): 918–922. Bibcode:1974JApMe..13..917S. doi:10.1175/1520-0450(1974)013<0917:cthvos>2.0.co;2.
  13. ^ Petty 2008, p. 21.

Bibliography

  • Andrews, D. G.; Holton, J. R.; Leovy, C. B. (1987). R., Dmowska; Holton, J. R., eds. Middle Atmosphere Dynamics. Academic Press. p. 371. ISBN 978-0-12-058576-2.
  • Panchev, Stoǐcho (1985) [1981]. Dynamic meteorology. D. Reidel Publishing Company. ISBN 978-90-277-1744-3.
  • Petty, Grant W. (2008). A First Course in Atmospheric Thermodynamics. Madison, WI: Sundog Publishing. ISBN 978-0-9729033-2-5.

External links

Aft pressure bulkhead

The aft pressure bulkhead or rear pressure bulkhead is the rear component of the pressure seal in all aircraft that cruise in a tropopause zone in the earth's atmosphere. It helps maintain pressure when stratocruising and protects the aircraft from bursting due to the higher internal pressure.

Aft pressure bulkheads can either be curved, which reduces the amount of metal needed at the cost of reducing the usable space in the airliner, or flat, which gives more internal space but also more weight.

Atmosphere of Earth

The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth's surface, absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.

The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

The study of Earth's atmosphere and its processes is called atmospheric science (aerology). Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann.

Atmospheric Research

Atmospheric Research is a scientific journal dealing with the part of the atmosphere where meteorological events occur; intended for atmospheric scientists (such as meteorologists and climatologists), aerosol scientists, and hydrologists. It is a highly international journal with attention given to all processes extending from the earth surface to the tropopause, but special emphasis continues to be devoted to the physics of clouds and precipitation, i.e. atmospheric aerosols; microphysical processes; cloud dynamics and thermodynamics; numerical simulation of cloud processes; clouds and radiation; meso- and macrostructure of clouds and cloud systems, and weather modification.

Clear-air turbulence

Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues, such as clouds, and is caused when bodies of air moving at widely different speeds meet.The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000–12,000 metres (23,000–39,000 ft) as it meets the tropopause. Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus clouds can also indicate high probability of CAT.

CAT can be hazardous to the comfort, but rarely the safety, of air travelers.

CAT in the jet stream is expected to become stronger and more frequent because of climate change, with transatlantic wintertime CAT increasing by 59% (light), 94% (moderate), and 149% (severe) by the time of CO2 doubling.

Cumulonimbus calvus

Cumulonimbus calvus is a moderately tall cumulonimbus cloud which is capable of precipitation, but has not yet reached the tropopause, which is the height of stratospheric stability where it forms into a cumulonimbus capillatus (fibrous-top) or cumulonimbus incus (anvil-top). Cumulonimbus calvus clouds develop from cumulus congestus, and its further development under auspicious conditions will result in cumulonimbus incus.

This cloud consists mainly of water droplets. By definition of cumulonimbus cloud, at its top water droplets are transformed into ice crystals. But for cumulonimbus calvus, content of ice crystals are meager and polar are in early stage, so cloud tops still look round and puffy.

Cumulonimbus calvus is characterized by distinctive (between other types of cumulonimbus cloud) rounded shape and relatively sharp edges of its top area, unlike cumulonimbus incus or cumulonimbus capillatus, which have cirriform tops. Developing cumulonimbus calvus lose sharp outlines of the top as more water droplets transform into ice crystals. Strong updrafts may form pileus or thin vertical stripes protruding upwards out of the cloud. When upper parts of the cloud freeze to greater extent and clearly visible cirriforms appears, cumulonimbus calvus metamorphose another species of cumulonimbus.

Cumulonimbus calvus arcus is a sub-form of cumulonimbus calvus, which has arcus cloud ahead of cloud's front.

Equilibrium level

In meteorology, the equilibrium level (EL), or level of neutral buoyancy (LNB), or limit of convection (LOC), is the height at which a rising parcel of air is at the same temperature as its environment.

This means that unstable air is now stable when it reaches the equilibrium level and convection stops. This level is often near the tropopause and can be indicated as near where the anvil of a thunderstorm because it is where the thunderstorm updraft is finally cut off, except in the case of overshooting tops where it continues rising to the maximum parcel level (MPL) due to momentum. More precisely, the cumulonimbus will stop rising around a few kilometres prior to reaching the level of neutral buoyancy and on average anvil glaciation occurs at a higher altitude over land than over sea (despite little difference in LNB from land to sea).

High-Altitude Long Endurance

High-Altitude Long Endurance (HALE) is the description of an air-borne vehicle which functions optimally at high-altitude (as high as 60,000 feet) and is capable of flights which last for considerable periods of time without recourse to landing.

The Tropopause represents high-altitude.

Hot tower

A hot tower is a tropical cumulonimbus cloud that reaches out of the lowest layer of the atmosphere, the troposphere, and into the stratosphere. In the tropics, the border between the troposphere and stratosphere, the tropopause, typically lies at least 15 kilometres (9.3 mi) above sea level. These formations are called "hot" because of the large amount of latent heat released as water vapor condenses into liquid and freezes into ice. The presence of hot towers within the eyewall of a tropical cyclone can indicate possible future strengthening.

Jacques Vanneste

Jacques Vanneste is a professor of mathematics at the University of Edinburgh, whose main research area is fluid dynamics.

His particular research interest is in analytic methods for handling systems with dynamics on two distinct time or length scales. This is relevant, for example, for the interaction between weather and ocean circulation, where fast inertial waves can be generated by slow underlying flows; see for example his work on the tropopause, and his most-cited paper. He is also interested in the dynamics of stirring.

Jet standard atmosphere

Jet Standard Atmosphere is often used by jet manufactures. It assumes a mean sea level temperature of +15 C. The temperature then lapses 2 C per 1000 ft to infinity. There is no Tropopause in the jet standard atmosphere. In the standard atmosphere, the Tropopause is the height at which the temperature stops decreasing. It is also the end of the Troposphere and start of the Stratosphere.

Jet stream

Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds (flowing west to east). Their paths typically have a meandering shape. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet.

The strongest jet streams are the polar jets, at nine–twelve km (30,000–39,000 ft) above sea level, and the higher altitude and somewhat weaker subtropical jets at 10–16 km (33,000–52,000 ft). The Northern Hemisphere and the Southern Hemisphere each have a polar jet and a subtropical jet. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round.

Jet streams are the product of two factors: the atmospheric heating by solar radiation that produces the large-scale Polar, Ferrel, and Hadley circulation cells, and the action of the Coriolis force acting on those moving masses. The Coriolis force is caused by the planet's rotation on its axis. On other planets, internal heat rather than solar heating drives their jet streams. The Polar jet stream forms near the interface of the Polar and Ferrel circulation cells; the subtropical jet forms near the boundary of the Ferrel and Hadley circulation cells.Other jet streams also exist. During the Northern Hemisphere summer, easterly jets can form in tropical regions, typically where dry air encounters more humid air at high altitudes. Low-level jets also are typical of various regions such as the central United States. There are also jet streams in the thermosphere.

Meteorologists use the location of some of the jet streams as an aid in weather forecasting. The main commercial relevance of the jet streams is in air travel, as flight time can be dramatically affected by either flying with the flow or against, which results in significant fuel and time cost savings for airlines. Often, the airlines work to fly 'with' the jet stream for this reason. Dynamic North Atlantic Tracks are one example of how airlines and air traffic control work together to accommodate the jet stream and winds aloft that results in the maximum benefit for airlines and other users. Clear-air turbulence, a potential hazard to aircraft passenger safety, is often found in a jet stream's vicinity, but it does not create a substantial alteration on flight times.

Léon Teisserenc de Bort

Léon Philippe Teisserenc de Bort (5 November 1855 in Paris, France – 2 January 1913 in Cannes, France) was a French meteorologist and a pioneer in the field of aerology. Together with Richard Assmann (1845-1918), he is credited as co-discoverer of the stratosphere, as both men announced their discovery during the same time period in 1902.

Teisserenc de Bort pioneered the use of unmanned instrumented balloons and was the first to identify the region in the atmosphere around 8-17 kilometers of height where the lapse rate reaches zero, known today as the tropopause.

Maximum parcel level

The maximum parcel level (MPL) is the highest level in the atmosphere that a moist convectively rising air parcel will reach after ascending from the level of free convection (LFC) through the free convective layer (FCL) and reaching the equilibrium level (EL), near the tropopause. As the parcel rises through the FCL it expands adiabatically causing its temperature to drop, often below the temperature of its surroundings, and eventually lose buoyancy. Because of this, the EL is approximately the region where the distinct flat tops (called anvil clouds), often observed around the upper portions of cumulonimbus clouds. If the air parcel ascended quickly enough then it retains momentum after it has cooled and continues rising past the EL, ceasing at the MPL (visually represented by the overshooting top, above the anvil).Dynamic processes within and between convective cells, such as updraft merging and cloud base areal size, factor into the actual ultimate cloud top height, in addition to atmospheric thermodynamics of the MPL. Updraft merging can lead to higher cloud tops thus an implication is that organized convection can be taller convection.

Overshooting top

An overshooting top (or penetrating top) is a dome-like protrusion shooting out of the top of the anvil of a thunderstorm. When an overshooting top is present for 10 minutes or longer, it's a strong indication the storm is severe.

Quasi-biennial oscillation

The quasi-biennial oscillation (QBO) is a quasiperiodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at about 1 km (0.6 mi) per month until they are dissipated at the tropical tropopause. Downward motion of the easterlies is usually more irregular than that of the westerlies. The amplitude of the easterly phase is about twice as strong as that of the westerly phase. At the top of the vertical QBO domain, easterlies dominate, while at the bottom, westerlies are more likely to be found. At the 30mb level, with regards to monthly mean zonal winds, the strongest recorded easterly was 29.55 m/s in November 2005, while the strongest recorded westerly was only 15.62 m/s in June 1995.

Stratosphere

The stratosphere () is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. The stratosphere is stratified (layered) in temperature, with warmer layers higher and cooler layers closer to the Earth; this increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet radiation by the ozone layer. This is in contrast to the troposphere, near the Earth's surface, where temperature decreases with altitude. The border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins. Near the equator, the stratosphere starts at as high as 20 km (66,000 ft; 12 mi), around 10 km (33,000 ft; 6.2 mi) at midlatitudes, and at about 7 km (23,000 ft; 4.3 mi) at the poles. Temperatures range from an average of −51 °C (−60 °F; 220 K) near the tropopause to an average of −15 °C (5.0 °F; 260 K) near the mesosphere. Stratospheric temperatures also vary within the stratosphere as the seasons change, reaching particularly low temperatures in the polar night (winter). Winds in the stratosphere can far exceed those in the troposphere, reaching near 60 m/s (220 km/h; 130 mph) in the Southern polar vortex.

Tropical upper tropospheric trough

A tropical upper tropospheric trough (TUTT), also known as the mid-oceanic trough,or commonly called as Western Hemisphere or "upper cold low" is a trough situated in upper-level (at about 200 hPa) tropics. Its formation is usually caused by the intrusion of energy and wind from the mid-latitudes into the tropics. It can also develop from the inverted trough adjacent to an upper level anticyclone. TUTTs are different from mid-latitude troughs in the sense that they are maintained by subsidence warming near the tropopause which balances radiational cooling. When strong, they can present a significant vertical wind shear to the tropics and subdue tropical cyclogenesis. When upper cold lows break off from their base, they tend to retrograde and force the development, or enhance, surface troughs and tropical waves to their east. Under special circumstances, they can induce thunderstorm activity and lead to the formation of tropical cyclones.

Troposphere

The troposphere is the lowest layer of Earth's atmosphere, and is also where nearly all weather conditions take place. It contains approximately 75% of the atmosphere's mass and 99% of the total mass of water vapor and aerosols. The average height of the troposphere is 18 km (11 mi; 59,000 ft) in the tropics, 17 km (11 mi; 56,000 ft) in the middle latitudes, and 6 km (3.7 mi; 20,000 ft) in the polar regions in winter.

The total average height of the troposphere is 13 km.

The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km (1.2 mi; 6,600 ft) deep depending on the landform and time of day. Atop the troposphere is the tropopause, which is the border between the troposphere and stratosphere. The tropopause is an inversion layer, where the air temperature ceases to decrease with height and remains constant through its thickness.The word troposphere is derived from the Greek tropos (meaning "turn, turn toward, change") and sphere (as in the Earth), reflecting the fact that rotational turbulent mixing plays an important role in the troposphere's structure and behaviour. Most of the phenomena associated with day-to-day weather occur in the troposphere.

Tropospheric ozone

Ozone (O3) is a trace gas of the troposphere, with an average concentration of 20-30 parts per billion by volume (ppbv), with close to 100 ppbv in polluted areas. Ozone is also an important constituent of the stratosphere, where the ozone layer exists. The troposphere is the lowest layer of the Earth's atmosphere. It extends from the ground up to a variable height of approximately 14 kilometers above sea level. Ozone is least concentrated in the ground layer (or planetary boundary layer) of the troposphere. It's concentration increases as height above sea level increases, with a maximum concentration at the tropopause. About 90% of total ozone in the atmosphere is in the stratosphere, and 10% is in the troposphere. Although tropospheric ozone is less concentrated than stratospheric ozone, it is of concern because of its health effects. Ozone in the troposphere is considered a greenhouse gas, and may contribute to global warming.Photo-chemical and chemical reactions involving ozone drive many of the chemical processes that occur in the troposphere by day and by night. At abnormally high concentrations (the largest source being emissions from combustion of fossil fuels), it is a pollutant, and a constituent of smog.Photolysis of ozone occurs at wavelengths below approximately 310-320 nano-meters. This reaction initiates the chain of chemical reactions that remove carbon monoxide, methane, and other hydrocarbons from the atmosphere via oxidation. Therefore, the concentration of tropospheric ozone affects how long these compounds remain in the air. If the oxidation of carbon monoxide or methane occur in the presence of nitrogen monoxide (NO), this chain of reactions has a net product of ozone added to the system.

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