Editing Supercell (section)

==Supercell anatomy==
[[File:Supercell02.svg|thumb|upright=1.5|Schematic of a supercell's components]]
The current conceptual model of a supercell was described in ''Severe Thunderstorm Evolution and Mesocyclone Structure as Related to Tornadogenesis'' by Leslie R. Lemon and Charles A. Doswell III (see [[Lemon technique]]). Moisture streams in from the side of the precipitation-free base and merges into a line of warm uplift region where the tower of the [[thundercloud]] is tipped by high-altitude shear winds. The high shear causes horizontal [[vorticity]] which is tilted within the updraft to become vertical vorticity, and the mass of clouds spins as it gains altitude up to the cap, which can be up to {{convert|55000|ft|m}}&ndash;{{convert|70000|ft|m}} above ground for the largest storms, and trailing anvil.

Supercells derive their rotation through the tilting of horizontal [[vorticity]], which is caused by [[wind shear]] imparting rotation upon a rising air parcel by differential forces. Strong updrafts lift the air turning about a horizontal axis and cause this air to turn about a vertical axis. This forms a deep rotating updraft, the [[mesocyclone]].

{| style="margin:auto;"
|-
|[[File:Meso-1.svg|200px|thumb|[[Wind shear]] (red) sets air spinning (green).]]
|[[File:Meso-2.svg|200px|thumb|The [[updraft]] (blue) 'bends' the spinning air upwards.]]
|[[File:Meso-3.svg|200px|thumb|The updraft starts rotating with the spinning column of air.]]
|}

A ''cap'' or [[capping inversion]] is usually required to form an updraft of sufficient strength. The moisture-laden air is then cooled enough to precipitate as it is rotated toward the cooler region, represented by the turbulent air of the [[mammatus cloud]]s where the warm air is spilling over top of the cooler, invading air. The cap is formed where shear winds block further uplift for a time, until a relative weakness allows a breakthrough of the cap (an [[overshooting top]]); cooler air to the right in the image may or may not form a [[shelf cloud]], but the precipitation zone will occur where the [[heat engine]] of the uplift intermingles with the invading, colder air. The cap puts an inverted (warm-above-cold) layer above a normal (cold-above-warm) [[boundary layer]], and by preventing warm surface air from rising, allows one or both of the following:
* Air below the cap warms and/or becomes more moist
* Air above the cap cools

As the cooler but drier air circulates into the warm, moisture laden inflow, the [[cloud base]] will frequently form a wall, and the cloud base often experiences a lowering, which, in extreme cases, are where [[tornado]]es are formed. This creates a warmer, moister layer below a cooler layer, which is increasingly unstable (because warm air is less dense and tends to rise). When the cap weakens or moves, explosive development follows.

In North America, supercells usually show up on [[Weather radar|Doppler weather radar]] as starting at a point or hook shape on the southwestern side, fanning out to the northeast. The heaviest precipitation is usually on the southwest side, ending abruptly short of the ''rain-free updraft base'' or ''main updraft'' (not visible to radar). The ''[[rear flank downdraft]]'', or RFD, carries precipitation counterclockwise around the north and northwest side of the updraft base, producing a "[[hook echo]]" that indicates the presence of a mesocyclone.

===Structure===
[[File:Supercell.svg|thumb|upright=1.5|Structure of a supercell. Northwestward view in the [[Northern Hemisphere]]]]

====Overshooting top====
{{main|Overshooting top}}
This "dome" feature appears above the strongest updraft location on the anvil of the storm. It is a result of an updraft powerful  enough to break through the upper levels of the troposphere into the lower [[stratosphere]].<ref>{{cite journal|last1= Shenk|first1= W. E.|date= 1974|title= Cloud top height variability of strong convective cells|journal= [[Journal of Applied Meteorology]]|volume = 13|issue= 8|pages= 918{{ndash}}922| doi=10.1175/1520-0450(1974)013<0917:cthvos>2.0.co;2 |bibcode = 1974JApMe..13..917S |doi-access= free}}</ref><ref name="Overshooting Tops">{{cite web|url=https://www.eumetsat.int/website/home/Data/Training/TrainingLibrary/DAT_2042700.html|title=Overshooting Tops – Satellite-Based Detection Methods|publisher=[[EUMETSAT]]|date=9 June 2011|access-date=10 May 2019|archive-date=10 May 2019|archive-url=https://web.archive.org/web/20190510090501/https://www.eumetsat.int/website/home/Data/Training/TrainingLibrary/DAT_2042700.html|url-status=dead}}</ref> An observer at ground level and close to the storm may be unable to see the overshooting top because the anvil blocks the sight of this feature. The overshooting is visible from satellite images as a "bubbling" amidst the otherwise smooth upper surface of the anvil cloud.

====Anvil====
An anvil forms when the storm's updraft collides with the upper levels of the lowest layer of the atmosphere, or the tropopause, and has nowhere else to go due to the laws of fluid dynamics- specifically pressure, humidity, and density, in simple terms, the packet of air has lost its buoyancy and cannot rise higher. The anvil is very cold (-30°C) and virtually precipitation-free even though [[virga]] can be seen falling from the forward sheared anvil. Since there is so little moisture in the anvil, winds can move freely. The clouds take on their anvil shape when the rising air reaches {{convert|50000|-|70000|ft|m|sigfig=3|order=flip}} or more. The anvil's distinguishing feature is that it juts out in front of the storm like a shelf. In some cases, it can even shear backwards, called a backsheared anvil, another sign of a very strong updraft.

==== Precipitation-free base ====
This area, typically on the southern side of the storm in North America, is relatively precipitation-free. This is located beneath the main updraft, and is the main area of inflow. While no precipitation may be visible to an observer, large hail may be falling from this area. A region of this area is called the Vault. It is more accurately called the main updraft area.

==== Wall cloud ====
The [[wall cloud]] forms near the downdraft/updraft interface. This "interface" is the area between the ''precipitation area'' and the ''precipitation-free base.'' Wall clouds form when rain-cooled air from the downdraft is pulled into the updraft. This wet, cold air quickly saturates as it is lifted by the updraft, forming a cloud that seems to "descend" from the precipitation-free base. Wall clouds are common and are not exclusive to supercells; only a small percentage actually produce a tornado, but if a storm does produce a tornado, it usually exhibits wall clouds that persist for more than ten minutes. Wall clouds that seem to move violently up or down, and violent movements of cloud fragments (scud or fractus) near the wall cloud, are indications that a tornado could form.

==== Mammatus clouds ====
[[Mammatus cloud|Mammatus]] (Mamma, Mammatocumulus) are bulbous or pillow-like cloud formations extending from beneath the anvil of a thunderstorm. These clouds form as cold air in the anvil region of a storm sinks into warmer air beneath it. Mammatus are most apparent when they are lit from one side or below and are therefore at their most impressive near sunset or shortly after sunrise when the sun is low in the sky. Mammatus are not exclusive to supercells and can be associated with developed thunderstorms and cumulonimbus.

==== Forward flank downdraft (FFD) ====
[[File:Supercell-above.svg|thumb|Diagram of supercell from above. RFD: ''rear flank downdraft'', FFD: ''front flank downdraft'', V: ''V-notch'', U: ''Main Updraft'', I: ''Updraft/Downdraft Interface'', H: ''hook echo'']]

This is generally the area of heaviest and most widespread precipitation. For most supercells, the precipitation core is bounded on its leading edge by a [[shelf cloud]] that results from rain-cooled air within the precipitation core spreading outward and interacting with warmer, moist air from outside of the cell. Between the precipitation-free base and the FFD, a "vaulted" or "cathedral" feature can be observed. In ''high precipitation supercells'' an area of heavy precipitation may occur beneath the main updraft area where the vault would alternately be observed with classic supercells.

==== Rear flank downdraft (RFD) ====
{{Main|Rear flank downdraft}}

The rear flank downdraft of a supercell is a very complex and not yet fully understood feature. RFDs mainly occur within classic and HP supercells although RFDs have been observed within LP supercells. The RFD of a supercell is believed to play a large part in tornadogenesis by tightening existing rotation within the surface mesocyclone. RFDs are caused by mid-level steering winds of a supercell colliding with the updraft tower and moving around it in all directions; specifically, the flow that is redirected downward is referred to as the RFD. This downward surge of relatively cool mid-level air, due to interactions between dew points, humidity, and condensation of the converging of air masses, can reach very high speeds and is known to cause widespread wind damage. The radar signature of an RFD is a hook-like structure where sinking air has brought with it precipitation.

==== Flanking line ====
{{Main|flanking line (meteorology)}}
A flanking line is a line of smaller [[Cumulonimbus cloud|cumulonimbi]] or cumulus that form in the warm rising air pulled in by the main updraft. Due to convergence and lifting along this line, [[landspouts]] sometimes occur on the outflow boundary of this region.

=== Radar features of a supercell ===
[[File:Supercell in Wichita Falls.svg|thumb|300px|Radar reflectivity map]]

;[[Hook echo]] (or pendant): The "hook echo" is the area of confluence between the main updraft and the rear flank downdraft (RFD). This indicates the position of the mesocyclone and probably a tornado.
;[[Bounded weak echo region]] (or BWER): This is a region of low radar reflectivity bounded above by an area of higher radar reflectivity with an [[tilted updraft|untilted updraft]], also called a ''vault''. It is not observed with all supercells but it is at the edge of a very high precipitation echos with a very sharp gradient perpendicular to the RFD. This is evidence of a strong updraft and often the presence of a [[tornado]]. To an observer on the ground, it could be experienced as a zone free of precipitation but usually containing large hail.
;Inflow notch: A "notch" of weak reflectivity on the inflow side of the cell. This is not a ''V-Notch.''
;V Notch: A V-shaped notch on the leading edge of the cell, opening away from the main downdraft. This is an indication of divergent flow around a powerful updraft.
;Hail spike: This [[three body scatter spike]] is a region of weak echoes found radially behind the main reflectivity core at higher elevations when large hail is present.<ref name="TBSS">{{cite web
|url=http://www.nws.noaa.gov/glossary/index.php?word=Hail+spike
|title=Hail spike
|work=Glossary
|publisher=National Oceanic and Atmospheric Administration
|date=June 2009
|access-date=2010-03-03
|archive-date=2010-12-04
|archive-url=https://web.archive.org/web/20101204003917/http://www.nws.noaa.gov/glossary/index.php?word=Hail+spike
|url-status=dead
}}</ref>

==== Descending reflectivity core ====
{{Main article|Descending reflectivity core}}