Balls of ice called hailstones are produced during thunderstorms. The strong vertical air currents ina thundercloud force lumps of ice up and down inside the cloud. With each upward movement the hailstones collect another layer of ice. They continue to grow in size until they are too big to be lifted again by the upcurrents. The stronger the upcurrent, the heavier a hailstone can become. A hailstorm is a supercell thunderstorm that is characterized by the presence of a mesocyclone : a deep, continuously-rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms Of the four classifications of thunderstorms (supercell, squall line, multi-cell, and single-cell), supercells are the overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local climate up to 32 kilometres (20 mi) away.
Supercells are often put into two classification types: Low-precipitation (LP) and High-precipitation (HP). LP supercells are usually found in climates that are more arid, such as the high plains of the United States, and HP supercells are most often found in moist climates. Supercells can occur anywhere in the world under the right pre-existing weather conditions, but they are most common in the Great Plains of the United States.
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 or left of the mean wind (relative to the vertical wind shear), they are said to be "right-movers" or "left-movers," respectively. 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.
Supercells can occur anywhere in the world under the right 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. Supercells are most frequent in the Great Plains of the United States and eastern Australia, 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.
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 an updraft of sufficient strength. The cap puts an inverted (cold-above-warm) layer above a normal (warm-above-cold) 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
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 top
This "dome" feature appears above the anvil of the storm. It is a result of the powerful updraft. If an observer at ground level is too close to the storm, they cannot see the overshooting top.
Formed in the uppermost parts of thunderstorm, the anvil is cold and virtually precipitation free. 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 40,000-60,000 or more feet. The anvil's distinguishing feature is that it juts out in front of the storm like a shelf.
- 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 (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 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
Radar features of a supercell
- 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 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 with an untilted updraft. This is evidence of a strong updraft.
- 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.
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 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 in the semi-arid Great Plains during the spring and summer months. Moving east and southeast, they often collide with moist air masses from the Gulf of Mexico, leading to the formation of HP supercells in areas just to the west of Interstate 35 before dissipating further east. LP supercells 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.
High Precipitation (HP)
The HP supercell has a much heavier precipitation core that 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 Interstate 35 and in the southern parts of the provinces of Ontario and Quebec in Canada.
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) and downbursts can cause tornado-like damage. Flooding is the leading cause of death associated with severe weather.
Note that none of these severe events are exclusive to supercells, although these events are highly predictable once a supercell has formed.
The supercell is a global phenomenon, as evidenced by these examples.
Some reports suggest that the deluge on 26 July 2005 in Mumbai, India was caused by a supercell when there was a cloud formation 15 kilometres (49,000 ft) high over the city. On this day 944 mm (37.2 in) of rain fell over the city, of which 700 mm (28 in) fell in just four hours. The rainfall coincided with a high tide, which exacerbated conditions.
On April 14, 1999, a severe storm later classified as a supercell hit the east coast of New South Wales. It is estimated that the storm dropped 500,000 tonnes worth of hailstones during its course. At the time it was the most costly disaster in Australia's insurance history, causing an approximated A$2.3 billion worth of damage, of which A$1.7 billion was covered by insurance.
On February 27, 2007 a supercell hit Canberra, dumping nearly one metre of ice in Civic. The ice was so heavy that a newly built shopping center's roof collapsed, birds were killed in the hail produced from the supercell, and people were stranded. The following day many homes in Canberra were subjected to flash flooding, caused either by storm water infrastructure's inability to cope or through mud slides from cleared land.
In 2010, on 6 March, supercell storms hit Melbourne. The storms caused flash flooding in the center of the city and tennis ball-sized (10 cm/4 in) hailstones hit cars and buildings, causing more than $220 million worth of damage, and sparking 40,000-plus insurance claims. In just 18 minutes, 19 mm (0.75 in) of rain fell, causing havoc as streets were flooded and trains, planes and cars were brought to a standstill.
That same month, on March 22, 2010 a supercell hit Perth. This storm was one of the worst in the city's history, causing six centimeter-sized (2.4 in) hail stones and torrential rain. The city had its average March rainfall in just seven minutes during the storm. Hail stones caused severe property damage, from dented cars to smashed windows. The storm itself caused more than 100 million dollars in damage.
In 2009, on the night of Monday May 25, a supercell formed over Belgium. It was described by Belgian meteorologist Frank Deboosere as "one of the worst storms in recent years" and caused much damage in Belgium - mainly in the provinces of East Flanders (around Ghent), Flemish Brabant (around Brussels) and Antwerp. The storm occurred between about 1:00am and 4:00am local time. An incredible 30,000 lightning flashes were recorded in 2 hours - including 10,000 cloud-to-ground strikes. Hailstones up to 6 centimetres (2.4 in) across were observed in some places and wind gusts over 90 km/h (56 mph); in Melle near Ghent a gust of 101 km/h (63 mph) was reported. Trees were uprooted and blown onto several motorways. In Lillo (east of Antwerp) a loaded goods train was blown from the rail tracks.
On August 18th, 2011, the rock festival Pukkelpop in Kiewit, Hasselt (Belgium) may have been seized by a supercell with mesocyclone around 18:15. Tornado-like winds were reported, trees of over 30 centimetres (12 in) diameter were felled and tents came down. Severe hail scourged the campus. Five people reportedly died and over 140 people were injured. One more died a week later. The event was suspended. Buses and trains were mobilised to bring people home.
Tornado Alley is a region of the United States where severe weather is common. Supercell thunderstorms can sweep through any time between March and November, but are concentrated in the spring. Tornado watches and warnings are frequently necessary in the spring and summer.
Gainesville, Georgia was the site of the fifth deadliest tornado in U.S. history in 1936, where Gainesville was devastated and 203 people were killed.
The 1980 Grand Island tornado outbreak affected the city of Grand Island, Nebraska on June 3, 1980. Seven tornadoes touched down in or near the city that night, killing 5 and injuring 200.
South Africa witnesses several supercell thunderstorms each year with the inclusion of isolated tornadoes. On most occasions these tornadoes occur in open farmlands and rarely does damage to property, as such many of the tornadoes which do occur in South Africa are not reported. The majority of supercells develop in the central, northern and north eastern parts of the country. The Free State, Gauteng and Kwazulu Natal are typically the provinces where these storms are most commonly experienced, though supercell activity is most certainly not limited to these provinces. On occasion hail reaches sizes in excess of golf balls with as mentioned above, the rare formation of tornadoes.
On the 6 May 2009 a well defined hook echo was noticed on local South African radars, along with satellite imagery this supported the presence of a strong supercell storm. Reports from the area indicated heavy rains, winds and large hail.