Editing Mesocyclone (section)

== Formation ==
One of the main ingredients for mesocyclogenesis is the presence of strong changes in wind speed over distance and direction with height, also known as horizontal and vertical [[wind shear]]. This shear classically coincides with the presence of a strong [[Trough (meteorology)|trough]] which may lead to an [[extratropical cyclone]], a type of [[Cyclogenesis#Extratropical cyclones|cyclone]] that forms through the interactions between cold and warm air, known as [[Baroclinity|baroclinicity]]. The pressure and temperature gradients between warm and cold air cause these changes in the wind with height and over distance. The resulting sheared wind field is said to have horizontal [[Vorticity#Examples|vorticity]], or the local tendency of the flowing fluid (here, air) to rotate, which is a property fundamental to any flow where velocity gradients exist.

The associated vorticity is often incorrectly depicted as a horizontally-rolling vortex that is directly tilted into the vertical by a rising updraft. However, in the majority of cases, the environment is horizontally homogenous with horizontal roll vortexes being absent. Horizontal vorticity can instead be thought as an imaginary paddle wheel that is set spinning by the winds that change with height. These winds move the top and bottom of the wheel at different speeds along the horizontal direction, causing it to twist along its axis.<ref>A review for forecasters on the application of hodographs to forecasting severe thunderstorms. 

Charles A. Doswell III

Appeared in 1991 National Weather Digest, 16 (No. 1), 2-16.A REVIEW FOR FORECASTERS ON THE APPLICATION OF HODOGRAPHS TO FORECASTING SEVERE THUNDERSTORMS

National Severe Storms Laboratory Norman, Oklahoma</ref><ref>Principles of Convection III: Shear and Convective Storms

Produced by The COMET® Program

https://www.meted.ucar.edu/mesoprim/shear/print.php
</ref> This local tendency for rotation, or twisting, is what the updraft reorients, rather than a literal tube or vortex of rotating air. When an updraft forms in this environment, ascending [[air parcel|air parcels]] encounter faster sheared air across height, which is entrained and turbulently mixed at the edge of the updraft, exchanging horizontal [[momentum]]. The rising air at the edge speeds up sideways faster then it's moving inward, forcing inner slower air to then also move faster horizontally. Air parcels then begin to curve as they move towards and overshoot the updraft's [[Vertical draft|center of low pressure]], following into a spiral as the process repeats. As the air parcels curve they also rotate about their axis due to the wind shear's twisting motion. This curving, spiraling or rotating motion of the wind can exist without the air necessarily spinning as a vortex.<ref name=":1" />

At this point, the updraft is then said to have differentially advected the momentum of the sheared flow; that is, the differences between the flowing air over a horizontal direction is translated to a vertical direction, resulting in curvature vorticity or the apparent curving and spiraling seen in the rising air (only when horizontal vorticity is [https://www.weather.gov/source/zhu/ZHU_Training_Page/Miscellaneous/vorticity/vorticity.html streamwise], or parallel to the updraft's inflow). However only a segment of a vortex arises; the [[Streamlines, streaklines, and pathlines|streamlines]] are not closed, and there are asymmetries as not all air parcels are rising equally or spiraling at the same rate and direction, so true uniform rotation does not yet exist. In order for organized rotation (an enclosed vortex) to exist, the resulting curvature vorticity must be partially converted to vertical shear vorticity.   

The updraft's pressure field primarily aids in this process, working to reorganize the curvature vorticity so that as the rising and spiraling air moves towards the center of low pressure, adjacent air parcels flowing across the updraft's [[Pressure-gradient force|pressure gradient]] begin rotating at different rates relative to each other. This creates vertical shear vorticity that enhances further rising air motion. The older air now more easily escapes the updraft and the low pressure center strengthens, then contracts in response, tightening the pressure gradient and causing converging air to rise even higher and faster. The effect is that the updraft is "stretched" upward as it more efficiently sucks up air, with rotation then becoming more organized due to [[conservation of angular momentum]]. Since the updraft now pulls in air more strongly, it pulls in more mass and momentum from the surrounding environment, which is conserved in the updraft. In nature this process happens simultaneously with the advection of the wind shear's momentum.

As air parcels continue to converge towards the center of low pressure, parcels closer to the center rotate faster and so are tugged outward due to centrifugal forces, while outer slower moving parcels move inward. The inner faster rotating air exerts a pressure force against the slower moving air, and causes the slower air to speed up. This continues until most air parcels have reached a uniform rate of rotation. Curvature vorticity and vertical shear vorticity are now in balance, and the result is a single coherent vortex that emerges in the updraft. A mesocyclone has formed (spinning counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere) and the incipient supercell storm fully matures.<ref name=":1">{{Cite journal |last=Dahl |first=Johannes M. L. |date=2017-09-01 |title=Tilting of Horizontal Shear Vorticity and the Development of Updraft Rotation in Supercell Thunderstorms |url=https://journals.ametsoc.org/view/journals/atsc/74/9/jas-d-17-0091.1.xml |journal=Journal of the Atmospheric Sciences |language=EN |volume=74 |issue=9 |pages=2997–3020 |doi=10.1175/JAS-D-17-0091.1 |issn=0022-4928|doi-access=free }}</ref>

As the low-level mesocyclone continues to ingest horizontal vorticity, vorticity maximums or vortex patches (areas of slight rotation or transient vortices) may form alongside the boundary where the updraft and its downdrafts – the cool and moist [[forward flank downdraft]] (FFD) and the, often, warmer and more buoyant [[rear flank downdraft]] (RFD) – meet due to the interactions between the warmer and cooler air masses. Surges in the RFD often coincide with the consolidation of these vortex patches, and may lead to tornadogenesis as a result. This is visually indicated by the formation of a [[wall cloud]] or other low cloud structures near the surface as the updraft strengthens from its interactions with the RFD.<ref name=":0">{{Cite journal |last=Fischer |first=Jannick |last2=Dahl |first2=Johannes M. L. |last3=Coffer |first3=Brice E. |last4=Houser |first4=Jana Lesak |last5=Markowski |first5=Paul M. |last6=Parker |first6=Matthew D. |last7=Weiss |first7=Christopher C. |last8=Schueth |first8=Alex |date=2024-07-09 |title=Supercell Tornadogenesis: Recent Progress in Our State of Understanding |url=https://journals.ametsoc.org/view/journals/bams/105/7/BAMS-D-23-0031.1.xml#fig1 |journal=Bulletin of the American Meteorological Society |language=EN |volume=105 |issue=7 |pages=E1084–E1097 |doi=10.1175/BAMS-D-23-0031.1 |issn=0003-0007|doi-access=free }}</ref>

The gallery below shows the three stages of development of a mesocyclone and a view of the storm relative motion on radar of a mesocyclone-producing tornado over [[Greensburg, Kansas]] on 4&nbsp;May 2007. The storm was in the process of producing [[2007 Greensburg tornado|an EF5 tornado]] at the time of the image.
<gallery mode="packed">
File:Meso-1.svg|Wind shear (red) sets air spinning (green).
File:Meso-2.svg|The updraft (blue) 'tips' the spinning air upright.
File:Meso-3.svg|The updraft then starts rotating.
File:Greensburg3 small.gif|Radar view of a mesocyclone. Note that at the time of this image, an [[2007 Greensburg tornado|EF5 tornado]] was on the ground.
</gallery>