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Supercell

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File:Noaa1137.jpg
Satellite view of a supercell

A supercell is a thunderstorm with a deep rotating updraft (a mesocyclone) [1]. Supercell thunderstorms are the largest, most severe class of single-cell thunderstorm. It has been argued that there are really only two types of thunderstorm: supercell and ordinary, though some have three classifications: single-cell, multi-cell, and supercell.

Supercells are usually found isolated from other thunderstorms in the warm air in front of a squall line, although they can sometimes be embedded in a squall line. Because they can last for hours, they are known as quasi-steady-state storms. As they usually track to the right of the mean wind, they are said to be right movers.

Supercells can be any size, large or small, low or high topped. Usually they produce copious amounts of hail, torrential rainfall, strong winds, substantial downbursts, and 30% of supercells produce tornadoes within the mesocyclone. [2]

Supercells are usually seen in the Midwestern United States, but they can occur anywhere. The first storm to be identified as such was the Wokingham storm over England, which was studied by Keith Browning and Frank Ludlam in 1962. [3]


Anatomy of a supercell

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).

Supercells begin as a horizontal vortex caused by wind shear. Strong updrafts lift the air turning about a horizontal axis and causes this air to turn about a vertical axis. This forms the deep rotating updraft, the mesocyclone.

A cap or capping inversion is usually required to form an updraft of sufficient strength. 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 too far, allows one or both of the following:

  • Air below the cap warms and/or becomes more moist
  • Air above the cap cools

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 the North America, supercells usually show up on Doppler 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 RFD, or rear flank downdraft carries precipitation counterclockwise around the north and northwest wide of the updraft base, producing a "hook echo" that indicates the presence of a mesocyclone.

Features of a supercell

File:Nssl0032.jpg
Features of a supercell. Note: This is a typical northwestward view in North America
File:Supercella.gif
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
  • Overshooting top

This "dome" feature appears above the anvil of the storm. It is a result of the powerful 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 and rain may be falling from this area. 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 few actually produce a tornado. 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 clouds are bulbous or pillow-like features underneath the anvil.

  • Precipitation area

This is the area of heaviest precipitation. Between the precipitation-free base and the precipitation area, a "vaulted" or "cathedral" feature can be observed. In high precipitation supercells an area of heavy precipitation may occur beneath the main updraft area.

  • Flanking line

A line of smaller cumulonimbii that form in the warm rising air pulled in by the main updraft.

Radar features of a supercell

File:Noaa0677.jpg
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.

  • Bounded weak echo region (or BWER)

This is a region of low reflectivity bounded above by an area of higher reflectivity. This is evidence of a strong updraft.

  • Inflow notch

An "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.

See also: Radar

Supercell variations

Supercell thunderstorms are sometimes classified by meteorologists and storm spotters into three categories. However, not all supercells fit neatly into any one category, and many resemble all three at different times during the lifespan of the storm. The standard definition given above is referred to as the Classic supercell. All types of supercells can produce severe weather.

  • LP

Low Precipitation

LP supercells contain a small precipitation (rain/hail) core separate from the updraft. Although these storms usually produce weak tornadoes, they can produce strong ones. These storms can produce heavy hail. Due to the lack of heavy precipitation, these LP supercells can sometimes show weak radar reflectivity without clear evidence of a hook echo, when in fact they are producing a tornado. On rare occasions, a weak tornado will form midway between the base and the top of the storm, descending from the main Cb (cumulonimbus) cloud. In the United States, these storms almost exclusively form from the Rocky Mountains to the Mississippi River in the spring months.

File:HPsupercell.gif
High precipitation supercell
  • HP

High Precipitation

A supercell with a much heavier precipitation core that actually can wrap all the way around the mesocyclone. These are dangerous storms, because any tornado that forms will generally not be visible, since the mesocyclone is wrapped with rain. These storms cause flooding due to heavy rain, damaging downbursts and weak tornadoes, although they can produce strong to violent tornadoes. They have a lower potential for damaging hail than Classic and LP type, although damaging hail is possible. It has been observed by some spotters that they tend to produce more cloud-to-ground lightning than the other types. Also, unlike the LP and Classic types, severe events usually occur at the front (southeast) of the storm.

Severe weather

Supercells can produce:

  • Large hail
  • Damaging winds
  • Deadly Tornadoes
  • Flooding
  • Deadly Cloud-to-Ground Lightning

Severe events associated with a supercell almost always occur in the area of the updraft/downdraft interface. In North America, this is most often the rear flank (southwest side) of the precipitation area in LP and classic supercells, but sometimes the leading edge (southeast side) of HP supercells.

While tornadoes are perhaps the most dramatic of these severe events, all are dangerous. High winds caused by powerful outflow can reach 130 km/h (80 mph) and downbursts can cause tornado-like damage. Flooding is the leading cause of death associated with severe weather.[4]

Note that none of these severe events are exclusive to supercells, although these events are highly predictable once a supercell has formed.

References

Wikimedia Commons has media related to Supercell.
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