Supercell

A supercell. While many ordinary thunderstorms are similar in appearance, supercells are distinguishable by their large-scale rotation.

A supercell is a severe thunderstorm with a deep, continuously rotating updraft (a mesocyclone).[1] Of the four classifications of thunderstorms (supercell, single-cell, multi-cell, and squall line), supercells are the largest and most severe.

Supercells are usually found isolated from other thunderstorms, although they can sometimes be embedded in a squall line. Because they can last for hours, they are known as quasi-steady-state storms. Supercells have the capability to deviate from the mean wind. If they track to the right of the mean wind (relative to the vertical wind shear), they are said to be "right-movers." Alternatively, if they track to the left of the mean wind (relative to the shear), they are said to be "left-movers." Supercells can sometimes develop two separate updrafts with opposing rotations, which splits the storm into two supercells: one left-mover and one right-mover.

Supercells can be any size, large or small, low or high topped. They usually produce copious amounts of hail, torrential rainfall, strong winds, and substantial downbursts. Supercells are one of the few types of clouds that typically spawn tornadoes within the mesocyclone, although only 30% or less do so.[2]

Supercells can occur anywhere in the world under the right pre-existing weather conditions. According to some, 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]. Supercells are most frequent in the Great Plains of the United States, but occasionally occur in many mid-latitude regions.

Contents

Anatomy of a supercell

Wind shear (red) sets air spinning (green).
The updraft (blue) 'bends' the spinning air upwards.
The updraft starts rotating with the spinning column of air.

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 derive their rotation through tilting of horizontal vorticity (an invisible horizontal vortex) caused by wind shear. Strong updrafts lift the air turning about a horizontal axis and cause 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 a downdraft 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 sinking too far, allows one or both of the following:

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 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 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.

Features of a supercell

Features of a supercell. Note: This is a typical northwestward view in the Northern Hemisphere
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 "dome" feature appears above the anvil of the storm. It is a result of the powerful updraft. If too close it cannot be seen from the ground.

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.

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 (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.

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.

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

Radar features of a supercell

Radar reflectivity map.

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 likely a tornado.

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

A "notch" of weak reflectivity on the inflow side of the cell. This is not a 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.

Low Precipitation (LP)

LP supercells contain a small precipitation (rain/hail) core separate from the updraft. This type of supercell may be easily identifiable with "sculpted" cloud striations in the updraft base or even a "corkscrewed" or "barber pole" appearance on the updraft, and sometimes an almost "anorexic" look compared to classic supercells. This is because they often form along dry lines, thus leaving them with little available moisture despite high upper level wind shear. They usually dissipate rapidly rather than turning into classic or HP supercells, although it is still not unusual for them to do the latter, especially if they happen to collide with a much moister air mass along the way. Although these storms usually produce weak tornadoes, they have been known to produce strong ones. These storms usually produce hail less than 1.00 inch in diameter [4]but can produce large hail even with little or no visible precipitation core, making them hazardous to storm chasers and people and animals caught outside. Due to the lack of a heavy precipitation core, LP supercells can sometimes show weak radar reflectivity without clear evidence of a hook echo, when in fact they are producing a tornado at the time. This is where observations by storm spotter and storm chasers may be of vital importance. Funnel clouds, or more rarely, weak tornadoes will sometimes form midway between the base and the top of the storm, descending from the main Cb (cumulonimbus) cloud. Lightning is rare compared to other supercell types, but it is not unknown and is more likely to occur as intracloud lightning rather than cloud-to-ground lightning. In North America, these storms almost exclusively form from the Rocky Mountains to the Mississippi River in the spring and summer months. They can occur as far north as Montana, North Dakota and even in the provinces of Alberta and Saskatchewan in Canada. They have also been observed by storm chasers in Australia.

LP supercells are quite sought after by storm chasers, because the limited amount of precipitation makes sighting tornadoes at a safe distance much less difficult than with a Classic or HP supercell.

High precipitation supercell

High Precipitation (HP)

The HP supercell has a much heavier precipitation core that actually can wrap all the way around the mesocyclone. These are especially dangerous storms, since the mesocyclone is wrapped with rain and can hide the tornado from view. These storms also cause flooding due to heavy rain, damaging downbursts and weak tornadoes, although they are also known to produce strong to violent tornadoes. They have a lower potential for damaging hail than Classic and LP supercells, although damaging hail is possible. It has been observed by some spotters that they tend to produce more cloud-to-ground and intracloud lightning than the other types. Also, unlike the LP and Classic types, severe events usually occur at the front (southeast) of the storm. The HP supercell is the most common type of supercell in the United States east of the Mississippi River and in the southern parts of the provinces of Ontario and Quebec in Canada.

Severe weather

Satellite view of a supercell

Supercells can produce:

Severe events associated with a supercell almost always occur in the area of the updraft/downdraft interface. In the Northern Hemisphere, 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 over 148 km/h (92 mph)[5][6] and downbursts can cause tornado-like damage. Flooding is the leading cause of death associated with severe weather.[7]

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

Some reports also suggest that the deluge on 26 July 2005 in Mumbai, India was caused by a supercell when there was a cloud formation 15km (9.32 miles) high over the city. On this day 944mm (37.16 inches) of rain fell over the city, of which 700mm (27.56 inches) fell in just four hours.

Notes

  1. ↑ 12B
  2. ↑ NWS Louisville: Supercell Structure and Dynamics
  3. ↑ QJ62Browning.pdf
  4. ↑ Radar Characteristics Of Supercells
  5. ↑ City of Provo, Utah ::
  6. ↑ ksl.com - Storm Damage Estimated at $13 Million in Provo
  7. ↑ Tornadoes Nature's Most Violent Storms

References

See also

External links