Tornadogenesis

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A sequence of images showing the birth of a supercellular tornado. First, the rain-free cloud base lowers as a rotating wall cloud. This lowering concentrates into a funnel cloud, which continues descending simultaneously as a circulation builds near the surface, kicking up dust and other debris. Finally, the visible funnel extends to the ground, and the tornado begins causing major damage. Dimmit Sequence.jpg
A sequence of images showing the birth of a supercellular tornado. First, the rain-free cloud base lowers as a rotating wall cloud. This lowering concentrates into a funnel cloud, which continues descending simultaneously as a circulation builds near the surface, kicking up dust and other debris. Finally, the visible funnel extends to the ground, and the tornado begins causing major damage.
Composite of eight images shot in sequence as a tornado formed in Kansas in 2016 Evolution of a Tornado.jpg
Composite of eight images shot in sequence as a tornado formed in Kansas in 2016

Tornadogenesis is the process by which a tornado forms. There are many types of tornadoes and these vary in methods of formation. Despite ongoing scientific study and high-profile research projects such as VORTEX, tornadogenesis is a volatile process and the intricacies of many of the mechanisms of tornado formation are still poorly understood. [1] [2] [3]

Tornado Violently rotating column of air that is in contact with both the earths surface and a cumulonimbus cloud in the air

A tornado is a rapidly rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. The windstorm is often referred to as a twister, whirlwind or cyclone, although the word cyclone is used in meteorology to name a weather system with a low-pressure area in the center around which, from an observer looking down toward the surface of the earth, winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern. Tornadoes come in many shapes and sizes, and they are often visible in the form of a condensation funnel originating from the base of a cumulonimbus cloud, with a cloud of rotating debris and dust beneath it. Most tornadoes have wind speeds less than 110 miles per hour (180 km/h), are about 250 feet (80 m) across, and travel a few miles before dissipating. The most extreme tornadoes can attain wind speeds of more than 300 miles per hour (480 km/h), are more than two miles (3 km) in diameter, and stay on the ground for dozens of miles.

VORTEX projects

The Verification of the Origins of Rotation in Tornadoes Experiment or VORTEX are field projects that study tornadoes. VORTEX1 was the first time scientists completely researched the entire evolution of a tornado with an array of instrumentation, enabling a greater understanding of the processes involved with tornadogenesis. A violent tornado near Union City, Oklahoma was documented in its entirety by chasers of the Tornado Intercept Project (TIP) in 1973 and visual observations led to advancement in understanding of tornado structure and life cycles. VORTEX2 utilized enhanced technology allowing scientists to improve forecasting capabilities to improve advanced warnings to residents. VORTEX2 sought to elucidate how tornadoes form, how long they last and why they last that long, and what causes them to dissipate.

Contents

A tornado is a violently rotating column of air in contact with the surface and a cumuliform cloud base. Tornado formation is caused by the stretching of environmental and/or storm-induced vorticity that tightens it into an intense vortex. There are various ways this may come about and thus various forms and sub-forms of tornadoes. Although each tornado is unique, most kinds of tornadoes go through a life cycle of formation, maturation, and dissipation. [4] The process by which a tornado dissipates or decays, occasionally conjured as tornadolysis, is of particular interest for study as is tornadogenesis, longevity, and intensity.

Cloud base

A cloud base is the lowest altitude of the visible portion of a cloud. It is traditionally expressed either in metres or feet above mean sea level or above a planetary surface, or as the pressure level corresponding to this altitude in hectopascals.

In continuum mechanics, the vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point, as would be seen by an observer located at that point and traveling along with the flow.

Vortex term in fluid dynamics

In fluid dynamics, a vortex is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in the wake of a boat, and the winds surrounding a tropical cyclone, tornado or dust devil.

Mesocyclones

Classical tornadoes are supercellular tornadoes, which have a recognizable pattern of formation. [5] The cycle begins when a strong thunderstorm develops a rotating mesocyclone a few miles up in the atmosphere. As rainfall in the storm increases, it drags with it an area of quickly descending air known as the rear flank downdraft (RFD). This downdraft accelerates as it approaches the ground, and drags the rotating mesocyclone towards the ground with it. Storm relative helicity (SRH) has been shown to play a role in tornado development and strength. SRH is horizontal vorticity that is parallel to the Inflow of the storm and is tilted upwards when it is taken up by the updraft, thus creating vertical vorticity.

Supercell thunderstorm that is characterized by the presence of a mesocyclone

A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms, supercells are the overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local weather up to 32 kilometres (20 mi) away. They tend to last 2-4 hours.

Thunderstorm type of weather

A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds, and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.

Mesocyclone

A mesocyclone is a vortex of air within a convective storm. It is air that rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a severe thunderstorm. Such thunderstorms can feature strong surface winds and severe hail. Mesocyclones often occur together with updrafts in supercells, within which tornadoes may form at the interchange with certain downdrafts.

As the mesocyclone lowers below the cloud base, it begins to take in cool, moist air from the downdraft region of the storm. This convergence of warm air in the updraft, and this cool air, causes a rotating wall cloud to form. The RFD also focuses the mesocyclone's base, causing it to siphon air from a smaller and smaller area on the ground. As the updraft intensifies, it creates an area of low pressure at the surface. This pulls the focused mesocyclone down, in the form of a visible condensation funnel. As the funnel descends, the RFD also reaches the ground, creating a gust front that can cause severe damage a good distance from the tornado. Usually, the funnel cloud begins causing damage on the ground (becoming a tornado) within a few minutes of the RFD reaching the ground.[ citation needed ]

Field studies have shown that in order for a supercell to produce a tornado the RFD needs to be no more than a few Kelvin cooler than the updraft. Also the FFD (forward flank downdraft) seems to be warmer within tornadic supercells than in non-tornadic supercells.[ citation needed ]

Although many envision a top-down process in which a mid-level mesocyclone first forms and couples with a low-level mesocyclone or tornadocyclone and a vortex then forms below the cloud base and becomes a concentrated vortex due to convergence upon reaching the surface, it has long been observed and there is now more rapidly growing evidence that many tornadoes form first near the surface or simultaneously from the surface to low and mid levels aloft. [6] [7]

Misocyclones

Waterspouts

Waterspouts are defined as tornadoes over water. However, while some waterspouts are supercellular (also known as "tornadic waterspouts"), forming in a process similar to that of their land-based counterparts, most are much weaker and caused by different processes of atmospheric dynamics. They normally develop in moisture-laden environments with little vertical wind shear in areas where wind comes together (convergence), such as land breezes, lake effect bands, lines of frictional convergence from nearby landmasses, or surface troughs. Waterspouts normally develop as their parent clouds are in the process of development. It is theorized that they spin upward as they move up the surface boundary from the horizontal shear near the surface, and then stretch upward to the cloud once the low level shear vortex aligns with a developing cumulus or thunderstorm. [8] Their parent cloud can be as innocuous as a moderate cumulus, or as significant as a supercell.

Landspouts

Landspouts are tornadoes that do not form from supercells and are similar in appearance and structure to fair-weather waterspouts with the exception that they form over land instead of water. They are thought to form in a manner similar to that of weaker waterspouts [9] in that they form during the growth stage of convective clouds by the ingestion and tightening of boundary layer vorticity by the cumuliform tower's updraft.

Mesovortices

QLCS

Tornadoes sometimes form with mesovortices within squall lines, most often in middle latitudes regions. Mesocyclonic tornadoes may also form with embedded supercells within squall lines.

Tropical cyclones

Mesovortices or mini-swirls within intense tropical cyclones, particularly within eyewalls, may lead to tornadoes. Embedded supercells may produce mesocyclonic tornadoes in the right front quadrant or particularly in certain situations with outer rainbands.

Fire whirls and pyro-tornadogenesis

Most fire or volcanic eruption induced whirlwinds are not tornadic vortices, however, on rare occasion circulations with large wildfires, conflagrations, or ejecta do reach an ambient cloud base, and in extremely rare cases pyrocumulonimbus with tornadic mesocyclones have been observed.


See also

Related Research Articles

Squall sudden, sharp increase in the sustained winds over a short time interval

A squall is a sudden, sharp increase in wind speed lasting minutes, contrary to a wind gust lasting seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the increase to the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.

Squall line

A squall line or quasi-linear convective system (QLCS) is a line of thunderstorms forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts. Strong straight-line winds can occur where the squall line is in the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes which develop within the summer season are known as derechos, and they move quite fast through large sections of territory. On the back edge of the rainband associated with mature squall lines, a wake low can be present, sometimes associated with a heat burst.

Waterspout weather phenomenon, intense columnar vortex that occurs over a body of water, commonly a non-supercell tornado over water

A waterspout is an intense columnar vortex that occurs over a body of water. Some are connected to a cumulus congestus cloud, some to a cumuliform cloud and some to a cumulonimbus cloud. In the common form, it is a non-supercell tornado over water.

Wall cloud cloud formation

A wall cloud is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which tornadoes sometimes form. It is typically beneath the rain-free base (RFB) portion of a thunderstorm, and indicates the area of the strongest updraft within a storm. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm; most strong tornadoes form from these. Many wall clouds do rotate, however some do not.

Hook echo

A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flow into a mesocyclone resulting in a curved feature of reflectivity. The echo is produced by rain, hail, or even debris being wrapped around the supercell. It is one of the classic hallmarks of tornado-producing supercells. The National Weather Service may consider the presence of a hook echo coinciding with a tornado vortex signature as sufficient to justify issuing a tornado warning.

Cyclogenesis

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.

Gustnado short-lived, shallow surface-based vortex generated by a thunderstorm

A gustnado is a short-lived, shallow surface-based vortex which forms within the downburst emanating from a thunderstorm. The name is a portmanteau by elision of "gust front tornado", as gustnadoes form due to non-tornadic straight-line wind features in the downdraft (outflow), specifically within the gust front of strong thunderstorms. Gustnadoes tend to be noticed when the vortices loft sufficient debris or form condensation cloud to be visible although it is the wind that makes the gustnado, similarly to tornadoes. As these eddies very rarely connect from the surface to the cloud base, they are very rarely considered as tornadoes. The gustnado has little in common with tornadoes structurally or dynamically in regard to vertical development, intensity, longevity, or formative process --as classic tornadoes are associated with mesocyclones within the inflow (updraft) of the storm, not the outflow.

Fire whirl whirlwind induced by a fire and often partially composed of flame or ash

A fire whirl, also commonly known as a fire devil, or, as a fire tornado, firenado, fire swirl, or fire twister, is a whirlwind induced by a fire and often composed of flame or ash. These start with a whirl of wind, often made visible by smoke, and may occur when intense rising heat and turbulent wind conditions combine to form whirling eddies of air. These eddies can contract a tornado-like vortex that sucks in debris and combustible gases.

Anticyclonic tornado

An anticyclonic tornado is a tornado which rotates in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. The term is a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes. Many anticyclonic tornadoes are smaller and weaker than cyclonic tornadoes, forming from a different process, as either companion/satellite tornadoes or nonmesocyclonic tornadoes.

Landspout slang term for a kind of tornado not associated with the mesocyclone of a thunderstorm

A landspout is a term created by atmospheric scientist Howard B. Bluestein in 1985 for a kind of tornado not associated with a mesocyclone. The Glossary of Meteorology defines a landspout as

Rear flank downdraft

The rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornadoes. Large hail within the rear flank downdraft often shows up brightly as a hook on weather radar images, producing the characteristic hook echo, which often indicates the presence of a tornado.

A skipping tornado is a process tornado which has a discontinuous damage path.

Convective storm detection is the meteorological observation, and short-term prediction, of deep moist convection (DMC). DMC describes atmospheric conditions producing single or clusters of large vertical extension clouds ranging from cumulus congestus to cumulonimbus, the latter producing thunderstorms associated with lightning and thunder. Those two types of clouds can produce severe weather at the surface and aloft.

Satellite tornado smaller tornado that orbits around a larger "parent" tornado

A satellite tornado is a tornado that rotates around a larger, primary tornado and interacts with the same mesocyclone. Satellite tornadoes occur apart from the primary tornado and are not considered subvortices; the primary tornado and satellite tornadoes are considered to be separate tornadoes. The cause of satellite tornadoes is not known. Such tornadoes are more often anticyclonic than are typical tornadoes and these pairs may be referred to as tornado couplets. Satellite tornadoes most commonly form in association with very large and intense tornadoes.

Pannus, or scud clouds, is a type of fractus cloud at low height above ground, detached, and of irregular form found beneath nimbostratus or cumulonimbus clouds. These clouds are often ragged or wispy in appearance. When caught in the outflow (downdraft) beneath a thunderstorm, scud clouds will often move faster than the storm clouds themselves. When in an inflow (updraft) area, scud clouds tend to rise and may exhibit lateral movement ranging from very little to substantial.

Mesovortices are small scale rotational features found in convective storms, such as those found in bow echos, supercell thunderstorms, and the eyewall of tropical cyclones. They range in size from tens of miles in diameter to a mile or less, and can be immensely intense.

The following is a glossary of tornado terms. It includes scientific as well as selected informal terminology.

References

  1. Coffer, Brice E.; M. D. Parker (2017). "Volatility of Tornadogenesis: An Ensemble of Simulated Nontornadic and Tornadic Supercells in VORTEX2 Environments". Mon. Wea. Rev. 145: 4605–4625. doi:10.1175/MWR-D-17-0152.1.
  2. Trapp, R. Jeffrey; R. Davies-Jones (1997). "Tornadogenesis with and without a Dynamic Pipe Effect". J. Atmos. Sci. 54: 113–133. doi:10.1175/1520-0469(1997)054<0113:TWAWAD>2.0.CO;2.
  3. Davies-Jones, Robert (28 January 2006). "Tornadogenesis in supercell storms: What We Know and What We Don't Know". Symposium on the Challenges of Severe Convective Storms. Atlanta, GA: American Meteorological Society.
  4. French, Michael M.; D. M. Kingfield (2019). "Dissipation Characteristics of Tornadic Vortex Signatures Associated with Long-Duration Tornadoes". J. Appl. Meteor. Climatol. 58: 317–339. doi:10.1175/JAMC-D-18-0187.1.
  5. Doswell, Moller, Anderson; et al. (2005). "Advanced Spotters' Field Guide" (PDF). US Department of Commerce. Archived from the original (PDF) on 2006-08-23. Retrieved 2006-09-20.External link in |publisher= (help)CS1 maint: Multiple names: authors list (link)
  6. Jana, Houser; H. Bluestein; A. Seimon; J. Snyder; K. Thiem (Dec 2018). "Rapid-Scan Mobile Radar Observations of Tornadogenesis". AGU Fall Meeting. Washington, DC: American Geophysical Union.
  7. Trapp, R. J.; E. D. Mitchell (1999). "Descending and Nondescending Tornadic Vortex Signatures Detected by WSR-88Ds". Wea. Forecasting. 14: 625–639. doi:10.1175/1520-0434(1999)014<0625:DANTVS>2.0.CO;2.
  8. Barry K. Choy and Scott M. Spratt. Using the WSR-88D to Predict East Central Florida Waterspouts. Retrieved on 2006-10-25.
  9. National Weather Service (June 30, 2017). EF-0 Landspout Tornado near Grand Junction, MI on June 30, 2017 https://www.weather.gov/grr/summary20170630 . Retrieved 20 March 2018.Missing or empty |title= (help)

Further reading