A mesocyclone is a meso-gamma mesoscale (or storm scale) region of rotation (vortex), typically around 2 to 6 mi (3.2 to 9.7 km) in diameter, most often noticed on radar within thunderstorms. In the Northern Hemisphere, it is usually located in the right rear flank (back edge with respect to direction of movement) of a supercell, or often on the eastern, or leading, flank of a high-precipitation variety of supercell. The area overlaid by a mesocyclone’s circulation may be several miles (km) wide, but substantially larger than any tornado that may develop within it, and it is within mesocyclones that intense tornadoes form.
Description Mesocyclones are medium-scale vortices of rising and converging air that circulate around a vertical axis. They are most often associated with a local region of low-pressure. Their rotation is (usually) in the same direction as low pressure systems in a given hemisphere: counter-clockwise in the northern, and clockwise in the southern hemisphere, with the only occasional exceptions being the smallest-scale mesocyclones. Mesoanticyclones that rotate in an opposite direction may accompany mesocyclones within a supercell but these tend to be weaker and often more transient than mesocyclones, which can be sustained for tens of minutes or hours, and also cyclically form in succession within a supercell. Mesoanticyclones are relatively common with left-moving supercells that split from parent supercells in certain vertical wind shear regimes. A mesocyclone is usually a phenomenon that is difficult to observe directly. Visual evidence of rotation – such as curved inflow bands – may suggest the presence of a mesocyclone, but the cylinder of circulating air is often too large to be recognized when viewed from the ground, or may not carry clouds distinct enough from the surrounding calmer air to make the circulating air flow obvious. Mesocyclones are identified by Doppler weather radar observations as a rotation signature which meets specific criteria for magnitude, vertical depth, and duration. On U.S. NEXRAD radar displays, algorithmically identified mesocyclones, such as by the mesocyclone detection algorithm (MDA), are typically highlighted by a yellow solid circle on the Doppler velocity display; other weather services may have other conventions.
Within thunderstorms They are of greatest concern when contained within severe thunderstorms, since mesocyclones often occur together with updrafts in supercells, within which tornadoes may form near the interchange with a downdraft. Mesocyclones are localized, approximately 2 km (1.2 mi) to 10 km (6.2 mi) in diameter within strong thunderstorms. Thunderstorms containing persistent mesocyclones are supercell thunderstorms (although some supercells and even tornadic storms do not produce lightning or thunder and thus are not technically thunderstorms). Doppler weather radar is used to identify mesocyclones. A mesovortex is a similar but typically smaller and weaker rotational feature associated with squall lines.
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