Mesocyclone

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A mesocyclone is a cyclonic vortex of air, between approximately 2 and 10 km diameter within a convective storm. They can often be found in association with a updrafts in supercells, where tornadoes may form. The term refers only to mesoscale cyclones found within convective storms, and does not apply to other cyclones on the mesoscale.^[1] Storms with mesocyclones can feature strong surface winds and severe hail.

1 Definition
2 Identification
3 Formation
4 See also
5 References
6 External links

[edit] Definition

A mesocyclone is an area of vertically oriented atmospheric rotation (usually but not always cyclonic) that is most often associated with a localized low pressure region within a severe thunderstorm. The phenomenon is normally relatively localized in nature lying between the synoptic scale (hundreds of kilometers) and small scale (hundreds of meters).

In layman's terms, the mesocyclone is a rotating updraft within a severe thunderstorm around 5 to 10 miles in diameter, that usually spins in the same direction as low pressure systems for a given hemisphere.

[edit] Identification

The word mesocyclone is associated with weather radar terminology. This is because the presence of a mesocyclone is verified by Doppler weather radar. Mesocyclones are most often identified in the right-rear flank of supercell thunderstorms and squall lines, and may be distinguished by a hook echo or gate-to-gate^{[Confusing — Please clarify]} rotation signature on Doppler weather radar.

Visual cues such as a rotating wall cloud or tornado may also hint at the presence of a mesocyclone. This explains why the term has entered into wider usage in connection with rotating features in severe storms.

[edit] Formation

Wind shear (red) sets air spinning (green).

The updraft (blue) 'tips' the spinning air upright.

The updraft then starts rotating.

Mesocyclones are believed to form when strong changes of wind speed and/or direction with height ("wind shear") sets parts of the lower part of the atmosphere spinning in invisible tube-like rolls. The convective updraft of a thunderstorm is then thought to draw up this spinning air, tilting the rolls' orientation upward (from parallel to the ground to perpendicular) and causing the entire updraft to rotate as a vertical column.

As the updraft rotates, it may form what is known as a wall cloud. The wall cloud is a spinning layer of clouds descending from the mesocyclone. The wall cloud tends to form closer to the center of the mesocyclone. As the wall cloud descends, a funnel-shaped cloud may form at its center. This is the first stage of tornado formation.

The presence of a mesocyclone is believed to be a key factor in the formation of the strong tornadoes associated with severe thunderstorms. Doppler radar, which can detect the rotation of air within the storm, is currently the best means of detecting mesocyclones.

Another way of thinking of a mesocyclone and tornado formation is to imagine a large rotating thunderstorm in progress. Tornadoes typically form at the peak of thunderstorm intensity as the storms begin to weaken. This is because the momentum and vacuum built up as large masses of air rise into the upper atmosphere causes a siphoning effect nearer to the ground. As the updraft is restricted, the entire thunderstorm is fed by smaller pockets of remaining warm air at the ground. The backpressure generated as the warm air runs out sucks the base of the thunderstorm towards the ground (i.e. a wall cloud). Once the warm air at the ground is nearly depleted, the entire top of the thunderstorm and the large wall cloud siphons air from a 1 mile or less diameter region at the ground, forming a tornado. If a moderate supply of warm air is available ahead of the storm, the storm may be tornadic for some time. If the warm air runs out, then the storms essentially chokes itself off and gradually dies.