A supercell is a severe thunderstorm with a deep, continuously rotating updraft (a
mesocyclone). [ [http://www.stormchasers.au.com/lemon7.htm 12B ] ] 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. [ [http://www.crh.noaa.gov/lmk/soo/docu/supercell.php NWS Louisville: Supercell Structure and Dynamics ] ]
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
Wokinghamstorm over England, which was studied by Keith Browningand Frank Ludlamin 1962. [ [http://www.rmets.org/pdf/qj62browning.pdf QJ62Browning.pdf ] ] . Supercells are most frequent in the Great Plainsof the United States, but occasionally occur in many mid-latitude regions.
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
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 inversionis 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:
*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 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
*Overshooting topThis "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.
*Precipitation-free baseThis 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 cloudforms 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.
*Precipitation areaThis 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 lineA line of smaller cumulonimbi or cumulus that form in the warm rising air pulled in by the main updraft.
Radar features of a supercell
Hook echoor PendantThe "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.
Bounded weak echo region(or BWER)This is a region of low radar reflectivity bounded above by an area of higher radar reflectivity. This is evidence of a strong updraft.
*Inflow notchA "notch" of weak reflectivity on the inflow side of the cell. This is not a "V-Notch".
*V NotchA "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.
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 [ [http://www.theweatherprediction.com/habyhints/237/ Radar Characteristics Of Supercells ] ] 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 Mountainsto the Mississippi Riverin the spring and summer months. They can occur as far north as Montana, North Dakotaand even in the provinces of Albertaand Saskatchewanin 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 (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 Stateseast of the Mississippi Riverand in the southern parts of the provinces of Ontarioand Quebecin Canada.
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) [ [http://www.provo.org/current_issues.082006storm.html City of Provo, Utah :: ] ] [ [http://www.ksl.com/index.php?nid=148&sid=399899 ksl.com - Storm Damage Estimated at $13 Million in Provo ] ] and
downbursts can cause tornado-like damage. Flooding is the leading cause of death associated with severe weather. [ [http://www.nssl.noaa.gov/NWSTornado/ Tornadoes Nature's Most Violent Storms ] ]
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 2005in Mumbai, Indiawas 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.Fact|date=February 2007
* [http://www.crh.noaa.gov/lmk/soo/docu/supercell.php Structure and Dynamics of Supercell Thunderstorms - NWS]
* [http://ww2010.atmos.uiuc.edu University of Illinois World Weather Project]
* [http://www.srh.noaa.gov/oun/severewx/glossary.php Weather Glossary for Storm Spotters - NWS]
*Lemon, Leslie R. (1998): " [http://www.stormchasers.au.com/lemon7.htm On the Mesocyclone "Dry Intrusion" and Tornadogenesis] "
*Lemon, Leslie R., Charles A. Doswell III (1979): [http://www.stormchasers.au.com/107lemon1.htm "Severe Thunderstorm Evolution and Mesocyclone Structure as Related to Tornadogenesis"] . "Monthly Weather Review" Vol. 107, No. 9, pp. 1184-1197.
*Browning, K.A. and Ludlam, F.H. (1962): [http://www.rmets.org/pdf/qj62browning.pdf "Airflow In Convective Storms"] , "Quarterly Journal of the Royal Meteorological Society" 88, 117-135. (PDF)
* [http://www.ejssm.org/ Electronic Journal of Severe Storms Meteorology]
* [http://www.extremeinstability.com/04-5-28.htm Picture sequence of a supercell in Nebraska]
* [http://www.youtube.com/watch?v=kBD8teVUM7w Slideshow of Supercell images]
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