Anticyclonic tornado
Encyclopedia
An anticyclonic tornado is a tornado
which rotates in a clockwise
direction in the Northern Hemisphere
and a counterclockwise direction in the Southern Hemisphere
. It is simply a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes. Many, but not all, anticyclonic tornadoes are smaller and weaker than cyclonic tornadoes, forming from a different process.
and updraft area bordering the updraft-downdraft interface (which is also near the mesoscale
"triple point") zone of supercell
thunderstorm
s. The thunderstorm itself is rotating, with a rotating updraft known as a mesocyclone
, and then a smaller area of rotation at lower altitude the tornadocyclone (or low-level mesocyclone) which produces or enables the smaller rotation that is a tornado. All of these may be, but aren't always, quasi-vertically aligned continuing from the ground to the mid-upper levels of the storm. All of these cyclone
s and scaling all the way up to large extratropical
(low-pressure systems) and tropical cyclone
s rotate cyclonically. Rotation in these synoptic scale systems stems partly from the Coriolis effect
, however, thunderstorms and tornadoes are too small to be significantly affected. The common property here is an area of lower pressure, thus surrounding air flows into the area of less dense air forming cyclonic rotation
. The rotation of the thunderstorm itself is induced mostly by vertical wind shear
.
Various processes can produce an anticyclonic tornado. Most often they are satellite tornadoes of larger tornadoes which are directly associated with the tornadocyclone and mesocyclone. Occasionally anticyclonic tornadoes occur as an anticyclonic companion (mesoanticyclone) to a mesocyclone within a single storm. Anticyclonic tornadoes can occur as the primary tornado with a mesocyclone and under a rotating wall cloud. Also, anticyclonic supercells (with mesoanticyclone), which usually are storms that split and move to the left of the parent storm motion, though very rarely spawning tornadoes, spawn anticyclonic tornadoes. There is an increased incidence of anticyclonic tornadoes associated with tropical cyclones, and mesovortices within bow echo
es may spawn anticyclonic tornadoes.
The first anticyclonic tornado associated with a mesoanticyclone was spotted on WSR-88D weather radar in Sunnyvale, California May 4th, 1998. The tornado was an F-2 on the Fujita Scale
.
Tornado
A tornado is a violent, dangerous, 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. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
which rotates in a clockwise
Clockwise
Circular motion can occur in two possible directions. A clockwise motion is one that proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back to the top...
direction in the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
and a counterclockwise direction in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
. It is simply a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes. Many, but not all, anticyclonic tornadoes are smaller and weaker than cyclonic tornadoes, forming from a different process.
Formation
Most strong tornadoes (the classic conception of a tornado) form in the inflowInflow
Inflow may refer to:*Inflow - In hydrology, the source of the water in a body of water*Infiltration/Inflow, inappropriate connection of surface runoff drains into sanitary sewers*Inflow , air that flows inwards to a thunderstorm...
and updraft area bordering the updraft-downdraft interface (which is also near the mesoscale
Mesoscale meteorology
Mesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers...
"triple point") zone of supercell
Supercell
A supercell is a thunderstorm that is characterized by the presence of a mesocyclone: a deep, continuously-rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms...
thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
s. The thunderstorm itself is rotating, with a rotating updraft known as a mesocyclone
Mesocyclone
A mesocyclone is a vortex of air, approximately 2 to 10 miles in diameter , within a convective storm....
, and then a smaller area of rotation at lower altitude the tornadocyclone (or low-level mesocyclone) which produces or enables the smaller rotation that is a tornado. All of these may be, but aren't always, quasi-vertically aligned continuing from the ground to the mid-upper levels of the storm. All of these cyclone
Cyclone
In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...
s and scaling all the way up to large extratropical
Extratropical cyclone
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth having neither tropical nor polar characteristics, and are connected with fronts and...
(low-pressure systems) and tropical cyclone
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
s rotate cyclonically. Rotation in these synoptic scale systems stems partly from the Coriolis effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
, however, thunderstorms and tornadoes are too small to be significantly affected. The common property here is an area of lower pressure, thus surrounding air flows into the area of less dense air forming cyclonic rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
. The rotation of the thunderstorm itself is induced mostly by vertical wind shear
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
.
Various processes can produce an anticyclonic tornado. Most often they are satellite tornadoes of larger tornadoes which are directly associated with the tornadocyclone and mesocyclone. Occasionally anticyclonic tornadoes occur as an anticyclonic companion (mesoanticyclone) to a mesocyclone within a single storm. Anticyclonic tornadoes can occur as the primary tornado with a mesocyclone and under a rotating wall cloud. Also, anticyclonic supercells (with mesoanticyclone), which usually are storms that split and move to the left of the parent storm motion, though very rarely spawning tornadoes, spawn anticyclonic tornadoes. There is an increased incidence of anticyclonic tornadoes associated with tropical cyclones, and mesovortices within bow echo
Bow echo
A bow echo is a term describing the characteristic radar return from a mesoscale convective system that is shaped like an archer’s bow. These systems can produce severe straight-line winds and occasionally tornadoes, causing major damage.-Research:...
es may spawn anticyclonic tornadoes.
The first anticyclonic tornado associated with a mesoanticyclone was spotted on WSR-88D weather radar in Sunnyvale, California May 4th, 1998. The tornado was an F-2 on the Fujita Scale
Fujita scale
The Fujita scale , or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation...
.
Notable Anticyclonic Tornadoes
- West Bend tornadoApril 4, 1981, West Bend tornadoThe April 4, 1981, West Bend tornado was a fatal F4, anticyclonic tornado that struck the Wisconsin city of West Bend in the early morning hours of April 4, 1981. It killed 3 people, and injured another 53. The tornado was largely unexpected and the storm that produced it did not display features...
- Grand Island tornado
See also
- VortexVortexA vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
- Vorticity
- Pressure gradient forcePressure gradient forceThe pressure gradient force is not actually a 'force' but the acceleration of air due to pressure difference . It is usually responsible for accelerating a parcel of air from a high atmospheric pressure region to a low pressure region, resulting in wind...
- TornadoTornadoA tornado is a violent, dangerous, 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. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
- April 4, 1981, West Bend tornadoApril 4, 1981, West Bend tornadoThe April 4, 1981, West Bend tornado was a fatal F4, anticyclonic tornado that struck the Wisconsin city of West Bend in the early morning hours of April 4, 1981. It killed 3 people, and injured another 53. The tornado was largely unexpected and the storm that produced it did not display features...
External links
- Tornadic Meso-anticyclonic supercell in Sunnyvale California (NSSLNational Severe Storms LaboratoryThe National Severe Storms Laboratory is a National Oceanic and Atmospheric Administration weather research laboratory located at the National Weather Center in Norman, Oklahoma....
)