What is a Supercell?

The largest and most severe types of thunderstorms are referred to as supercells. A supercell can occur anywhere in the world with the right weather conditions but happen most frequently in the Great Plains area of the United States.

A supercell thunderstorm is characterized by a deep rotating updraft or mesocyclone. They are usually isolated from other thunderstorm activity but can be embedded in squalls. Supercells are able to deviate from the mean wind. They can track to the right or left, relative to the shear. Supercells themselves can be of any size but normally produce large amounts of torrential rainfall, strong winds and hail.

Supercells obtain their rotation by the tilting of horizontal voracity, or invisible vortex, that is caused by wind shear. The air that is turning on a horizontal axis is lifted by strong updrafts. This causes the air to turn on a vertical axis. A capping inversion is normally needed to create a downdraft of sufficient strength. The cap places a layer of inverted air, warm over cold, above the normal boundary air layer, cold over warm.

The inversion cap keeps the warm surface air from sinking and the air below the cap becomes more moist and it also allows the air above the cap to cool. This process creates a warm moist layer of air below a cooler air layer which becomes increasingly unstable. The warm air wants to rise and when the cap moves or weakens the system becomes explosive.

The features of a supercell include a wall cloud, Mammatus clouds, a precipitation free base, an area of precipitation, a flank line and an overshooting top. The wall cloud forms in the area between the precipitation free base and the precipitation area. The precipitation free base is usually on the southern side of the storm and is located below the main updraft. The precipitation area is where the most rain and hail occur. A flank line is a series of small cumulus clouds that form from rising warm air. The overshooting top is a dome that appears above the storm area and may or may not be visible.

Wall clouds form as cool air from the downdraft is pulled into the updraft. The clouds seem to descend from the rain free base. Wall clouds are fairly common and do not always indicated a supercell by themselves. When wall clouds appear to be moving violently, or last for more than ten minutes, it is an indication of a potential tornado. Mammatus clouds are pillow like in appearance and extend beneath the thunderstorm area. They are also not exclusive to supercells and are formed from cold air sinking into the warmer air beneath it.

Supercells will usually show up on Doppler radar as a hook shape or point that starts on the southwestern side and fans out to the northeast. The largest amount of rainfall normally occurs on the southwestern side. The precipitation is carried by the rear flank downdraft, counterclockwise to the north and northwest sides of the updraft. This produces a hook echo and indicates the presence of a mesocyclone.

References:

http://www.workingdog.eu/supercell_thunderstorm_en.html

http://www.absoluteastronomy.com/topics/Supercell