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A supercell over Texas.
A supercell is a thunderstorm characterized by a mesocyclone—a persistent, deep, rotating updraft. For this reason supercells are sometimes called rotating thunderstorms.
Among four commonly described thunderstorm categories—supercell, squall line, multicell, and single-cell storms—supercells are among the rarest, yet they can develop into some of the most severe storms. They are often relatively isolated from other thunderstorms and can influence local weather across an area extending roughly 30 kilometres around them.
Supercells are commonly divided into three types: classic, low-precipitation (LP), and high-precipitation (HP). LP supercells are often observed in drier climates such as the central and northern plains of North America, whereas HP supercells are more common in humid regions. Supercells can occur anywhere in the world when suitable atmospheric conditions exist, but they are particularly common across the broad central plains of the United States, including the region popularly known as Tornado Alley.
They are usually found in the warm sector of a low-pressure system, generally moving northeastward roughly parallel to the associated cold front. Because they can persist for hours, they are sometimes described as quasi-steady storms. A supercell can deviate from the mean wind direction relative to the vertical wind shear; storms deviating to the right or left are termed right-movers and left-movers. In some cases a storm can split into two separate, oppositely rotating supercells. [source]
Supercells occur in many sizes and vertical dimensions. Their prominent hazards can include large hail, torrential rainfall, strong winds, and significant downdrafts. Although supercells provide an environment capable of producing tornadoes within their mesocyclones, the source notes that only about 30 percent or fewer actually produce tornadoes.
The first storm identified as a supercell is described as the Wokingham storm in England, studied by Keith Browning and Frank Ludlam in 1962. Browning's early work was later followed by Lemon and Doswell in developing the modern conceptual model of supercells. Available observations indicate that supercells are especially frequent over the central United States and southern Canada, extending into the southeastern United States and northern Mexico, east-central Argentina and nearby Uruguay, Bangladesh, parts of eastern India, South Africa, and eastern Australia. They also occur at times in other mid-latitude regions, including eastern China and Europe. Regions experiencing many supercells generally also experience many tornadoes.
The article cites the conceptual model presented in Severe Thunderstorm Evolution and Mesocyclone Structure as Related to Tornadogenesis by Leslie R. Lemon and Charles A. Doswell III.
How rotation develops
Supercells acquire rotation through the tilting of horizontal vorticity created by wind shear. Strong updrafts act on air rotating about a horizontal axis and tilt that rotation into the vertical, producing a deep rotating updraft—the mesocyclone.
A capping inversion is commonly involved in the development of powerful updrafts. The cap places a stable inversion layer—warmer air above colder air—over the normal atmospheric boundary layer and prevents warm surface air from rising. This can allow the air beneath the cap to become warmer and/or more humid while air above it cools. The result is an increasingly unstable layer of warm, moist air beneath cooler air. When the cap weakens or moves away, explosive cloud development can occur.
Radar appearance
In North America, Doppler radar commonly shows a supercell with a hook-like feature on the southwestern side of the storm, with the broader precipitation structure extending northeastward. The heaviest precipitation is generally found on the southwestern side and may end abruptly near the main updraft base, which itself may not be visible on radar.
The rear-flank downdraft (RFD) can wrap precipitation counterclockwise toward the northern and northwestern side of the updraft base, producing a hook echo associated with the presence of a mesocyclone.
Features of a supercell
Overshooting top: A dome-shaped protrusion above the storm's anvil, produced when an exceptionally strong updraft penetrates into the upper levels of the troposphere. A ground observer must be at a suitable distance to see it; from too close, the anvil itself can hide the overshooting top.
Anvil: The anvil forms when rising air in the storm core reaches the upper part of the troposphere and is forced to spread horizontally. It is very cold and largely precipitation-free. The source describes anvils forming when rising air reaches roughly 15,200–21,300 metres or higher. A distinctive anvil can spread outward ahead of a storm like a shelf; in some cases it also spreads backward against the shear, forming a back-sheared anvil, another indication of a very strong updraft.
Mesocyclone: A cyclonically rotating vortex, typically about 2–10 kilometres in diameter, within a convective storm.
Cyclone: A low-pressure system rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Downdraft: A small-scale downward current of air in cumulonimbus clouds.
Cumulonimbus: A vertically developed tower-like cloud, often with an anvil-shaped top, commonly associated with heavy rain, lightning, hail, and tornadoes.
Vorticity: A vector measure of local rotation in a fluid flow; mathematically, the curl of the velocity vector.
Capping inversion: A statically stable layer above the atmospheric boundary layer.
Boundary layer: The layer of fluid immediately adjacent to a bounding surface.
Troposphere: The lowest part of the atmosphere, extending from Earth's surface to roughly 10–20 kilometres, where temperature generally decreases with increasing altitude.