SUPERCELLS

Supercells are storms (typically thunderstorms) that rotate around a vertical axis. Such storms often produce severe weather at ground level (damaging winds, large hail, and/or tornadoes). Further, most strong and violent tornadoes are produced by supercells.

The above figure shows the 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 orange lines in the above figure show how isolated supercell thunderstorms and bow echoes appear on weather radar displays (i.e., on displays of radar reflectivity). The left panel (a) depicts an isolated supercell. These often appear as roughly circular blobs, with a point or hook-shaped appendage on the rear side of the echo (in this case, on the lower left side). The other three panels (b, c, and d) depict common bow echo configurations. Bow echoes typically are larger in scale than individual supercells. However, sometimes supercells are embedded within bow echoes. For example, embedded supercell storms may occur near the "top" (typically, the northern-most) part of a bow such as the one in (b), or at the intersection of the two legs of reflectivity in the configuration shown in (c). A classic serial derecho is depicted in (d); supercells in such systems frequently occur near the crests of the "waves" 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.

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