SUPERCELLS
Supercells are storms --- usually, but not necessarily, thunderstorms --- that contain updrafts
that rotate about a vertical axis. This rotation is derived from shear in the environmental
wind field (that is, a change in wind direction and / or speed with height) surrounding the storm
as it begins to grow. Supercells often produce damaging wind, large hail, and tornadoes, and
most strong to violent tornadoes are associated with supercells. Analogous to cancer cells in
a living organism, supercells differ from ordinary thunderstorms in that the rotation of their
updraft enables them to overcome the self-limiting mechanisms that bring demise to regular
storms. Once formed, a supercell may perpetuate itself for an appreciable length of time,
even upon encountering an environment that is hostile to the development of new storms.
The figure below shows the vertical wind field associated with a typical supercell thunderstorm. The background
photograph was taken from an airplane by T. Theodore Fujita, looking northeast over eastern Kansas on the
afternoon of April 21, 1961. The storm is rotating counter-clockwise (red arrows), which is typical for most
long-lived supercells in the Northern Hemisphere. The striations of the low-level clouds
reflect, in part, converging low-level winds (green and yellow arrows) curving into
the rotating updraft of the storm. This rotating updraft is known as a mesocyclone.
The anvil-shaped formation at the top of the storm marks the level at which the updrafts reach
the stratosphere, lose buoyancy (ie., stop rising) and begin to move downstream (blue arrow).
The schematic plan views below show how isolated supercell thunderstorms (panel a) appear on radar reflectivity displays in comparison to various
bow echo configurations (panels b-d). The arrows depict surface winds and downdrafts. Isolated supercells (a) often appear
as roughly circular or kidney-shaped blobs, with a point or hook-shaped appendage on the rear side of the echo, relative to its direction of motion.
("FFD" and "RFD" refer to the storm's front flank and rear flank downdrafts, respectively). Panels b, c, and d depict common bow echo configurations. Bow echoes
typically are larger in scale than individual supercells. However, supercells sometimes are embedded in bow echoes. Embedded supercell storms
may occur at or near the poleward (northern) end of a bow such as the one in (b), or at the intersection of the two legs of reflectivity in a
line echo wave pattern (LEWP) like that in (c). A classic serial derecho is depicted in (d); supercells in such
systems frequently occur near the crests of the LEWPs that connect adjacent bowing segments. In some cases, bow echoes and, ultimately, derecho-producing
convective systems, develop from individual supercell storms. Two examples of such evolution include the "Supercell Transition Derecho" of
July 28-29, 1986, and the "Texas Derecho" of May 4-5, 1989.
(Figure adapted from Johns and Doswell 1992).
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