Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 170

4.2 OBSERVED SUPERCELL BEHAVIOR AND EARLY THEORIES

In the late 1950s and early 1960s a few storms 1 were observed on conventional
(i.e., non-Doppler, reflectivity only) radar that persisted for a much longer period
of time than most other storms, whose individual cells lasted only on the order of
the advective time scale. Furthermore, ordinary cells move along approximately
with the pressure-weighted (i.e., mass-weighted) mean wind in the layer in which
they are embedded, while most long-lived cells instead propagate to the right of
the mean wind. In 1962, Keith Browning named these convective storms ''super-
cells'' 2 mainly owing to their longevity. It is fascinating to view time lapse radar
reflectivity imagery in which a field of convective cells propagate in one direction
and evolve quickly, while one, two, or several iconoclastic cells move more slowly
and off to the right of the others, and maintain their appearance longer. (It is
noted, however, that in multicell storms in which new-cell development occurs
along the right flank of the storm, the motion of the storm also deviates to the
right of the mean wind.)
Browning in 1977 proposed that the two basic building blocks of convective
storms are ordinary cells and supercells. While the definitions of these two convec-
tive building blocks depends on cell behavior (longevity, motion with respect to
the environmental wind profile, internal flow characteristics), it will be shown
that one could equally well define these two building blocks in terms of their
underlying, governing dynamics. Although the paradigm of two different types of
convective building blocks is useful for pedagogical purposes, in nature there is
more of a continuous spectrum of storm behavior; supercell behavior does not
suddenly begin and ordinary-cell behavior does not

suddenly end when a

threshold of environmental parameters is crossed.
Without Doppler radar it was dicult to determine precisely how the wind
field in supercells differed from that in ordinary cells. However, Ted Fujita and
collaborators inferred from analyses of time series of radar reflectivity and
analyses of wind data collected from aircraft located outside storms that the main
updraft in a supercell rotated and it was suggested that this characteristic was
responsible for their ''deviant'' motion and, at least in part indirectly, for their
longevity. Some supercells produced tornadoes and it was therefore thought that
there must be a connection between storm-scale rotation and the much smaller
scale tornado. Early analyses of supercell dynamics drew upon an analogy
between the interaction between spinning solid bodies and the airflow around
them, like baseballs curving through the air when spin is imparted to them
( Figure 4.4 ). Supercells, however, are not solid bodies embedded in the airflow:
They are part of the airflow itself and air circulates up, through them, and then
out from them. Furthermore, these early theories did not consider thermo-
dynamics or precipitation microphysics. However, it was recognized that the

1 For example, the Wokingham storm of July 9, 1959 in the U. K., and the Geary storm of
May 4, 1961 in Oklahoma.
2 The term ''supercell'' first appeared in the refereed literature in 1964, but had been used in
1963 at an American Meteorological Conference and in 1962 in the journal Meteorological
Magazine. Browning was initially criticized for using such a ''vulgar'' term.