Tornadoes and Supercells

The supercell is the most dangerous kind of thunderstorm, a rotating engine whose deep, persistent mesocyclone can concentrate spin into the violent vortex of a tornado.

Definition

A supercell is a thunderstorm with a persistent, deep, rotating updraft called a mesocyclone, and a tornado is a violently rotating column of air extending from the base of a storm to the ground.

Scope

This topic covers the structure and dynamics of supercell thunderstorms, the origin of their rotating mesocyclone in vertical wind shear, the processes that produce tornadoes, and the rating of tornado intensity by damage.

Core questions

• How does vertical wind shear give a supercell its rotating updraft?
• What is a mesocyclone and how does it form?
• What processes concentrate rotation into a tornado near the ground?
• How is tornado intensity estimated and rated?

Key theories

Mesocyclone formation by shear tilting

Horizontal vorticity associated with vertical wind shear is tilted into the vertical and stretched by the storm's updraft, producing the deep, persistent rotation that defines a supercell's mesocyclone.

Tornadogenesis near the surface

Tornadoes form when near-surface rotation, often involving baroclinic vorticity generated along storm-scale boundaries, is concentrated and stretched beneath the mesocyclone into an intense low-level vortex.

Mechanisms

In an environment of strong vertical wind shear, a thunderstorm's updraft tilts horizontal vorticity into the vertical and stretches it, creating a rotating mesocyclone that gives the supercell its longevity and severe-weather potential. Tornadoes develop when rotation is also generated and concentrated near the surface, often along the storm's rear-flank downdraft boundary, and is stretched beneath the mesocyclone into a small, intense vortex. The resulting tornado is rated after the fact by the damage it causes.

Clinical relevance

Supercells produce the most violent tornadoes as well as giant hail and damaging winds, so understanding their structure underlies the radar detection of mesocyclones, the issuance of tornado warnings, and research aimed at improving warning lead times to protect lives.

History

Keith Browning identified and named the supercell in the 1960s, and Fujita developed the damage-based scale for rating tornado intensity; field campaigns using mobile radar and storm intercepts, described in works such as Bluestein's, have since advanced understanding of mesocyclones and tornadogenesis.

Key figures

• Tetsuya Theodore Fujita
• Keith Browning
• Howard Bluestein

Seminal works

• markowski2010
• bluestein2013

Frequently asked questions

What makes a supercell different from an ordinary thunderstorm?

A supercell has a single, deep, persistently rotating updraft called a mesocyclone, which lets it last for hours and produce the most severe weather, including large hail and strong tornadoes, unlike short-lived ordinary storms.

How is a tornado's strength measured?

Tornado intensity is estimated after the event from the damage it causes, using the Enhanced Fujita scale, because direct wind measurements inside tornadoes are rare; the scale ranges from EF0 for the weakest to EF5 for the most violent.