mesocyclone

A mesocyclone is a rotating updraft within a severe thunderstorm, most often found in a supercell. It is a large, vertically oriented region of cyclonic rotation that can span several kilometers in width and extend through a significant portion of the troposphere. Mesocyclones can persist for tens of minutes to an hour or more and are commonly associated with the most intense storms.

Formation results from strong vertical wind shear, especially a combination of speed and directional changes with height. This shear tilts horizontal vorticity into the vertical and the updraft tilts and stretches it, concentrating rotation within the storm’s core. The result is a robust, rotating updraft that can organize and sustain a powerful storm.

Detection and structure: Doppler radar often reveals a mesocyclone as a persistent cyclonic velocity couplet in the mid- to low troposphere, sometimes accompanied by a hook-shaped reflectivity pattern. The presence of a mesocyclone increases the likelihood of severe weather within the storm, including large hail and damaging winds, and can be a precursor to tornado formation.

Relation to tornadoes and hazards: Not all mesocyclones produce tornadoes, but many strong tornadoes originate within or near mesocyclones. The exact genesis of a tornado depends on subsequent processes within the storm, including the interaction of vorticity with downdrafts and other storm dynamics. Mesocyclones are a key diagnostic feature for forecasters assessing tornado risk.

Geographic occurrence: Mesocyclones are most commonly observed in tornado-prone regions such as the North American Plains but occur worldwide wherever severe thunderstorms develop with sufficient wind shear.