Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 332

and consequently the most damaging, tornadoes are associated with the latter, a
mesocyclone produced in a parent supercell. Bob Davies-Jones at NSSL named
this type of tornado a ''Type I tornado'', also known as a ''supercell tornado''. By
''associated with'' we mean that a mesocyclone preceded the tornado and the
tornado is the mesocyclone contracted in size or is a separate vortex located
within the mesocyclone. A surface vortex on the scale intermediate between that
of the mesocyclone and that of the tornado has been referred to as a ''tornado
cyclone''. It got its name in 1949 from Ed Brooks, who found that tornadoes were
embedded in regions of relatively low pressure whose horizontal scale was greater
than that of the tornado. Perhaps there really is just one vortex, but it can be
resolved on several spatial scales. There is, however, some evidence from mobile
Doppler radars that a tornado cyclone or mesocyclone in some instances may be
broader than a tornado and coexist with a tornado, so that the tornado cyclone
does not necessarily always contract in scale radially down to that of a tornado
( Figure 6.23 ). Other tornadoes that are not associated with a mesocyclone pro-
duced by a supercell were named ''Type II tornadoes'' by Bob Davies-Jones, but
are therefore sometimes also referred to as ''non-supercell tornadoes (NSTs)''.
These tornadoes appear in a number of different situations:
(a) Some are found under growing cumuliform towers ( Figure 6.24 ), often even
before precipitation has formed or once it has formed but has not reached the
ground yet or just about to reach the ground. Since these tornadoes frequently
look like waterspouts that are pendant from lines of cumulus congestus, they are
colloquially known as ''landspouts'' (the origin of this term has been attributed
to the author who used the term in a conference publication, but the term may
have already been in use by other storm-chasers). 1 When the air is very dry,
landspouts appear as rotating whirls of dust near the ground, possibly without
any condensation funnel overhead. In some instances, rotating dust whirls pre-
cede the appearance of a funnel cloud aloft. They begin near the ground and
expand upwards toward cloud base. Although this type of tornado is found in
ordinary-cell convective storms, it is also found in supercells, but not in or near
the mesocyclone. When ''non-supercell'' tornadoes are found in supercells, they
are found along the rear-flank gust front. The term ''non-supercell tornado'' can
therefore be misleading. The term ''Type II tornado'' is therefore probably
preferable. From visual observations and radar observations we know that some
of the Type II landspout tornadoes must form in the absence of a downdraft,
especially when no radar echo has yet been detected and the parent cloud is still
growing, as the cloud top has not reached the tropopause.
It is thought that pre-existing vorticity along a surface boundary
characterized as a ''vortex sheet'' 2 may become barotropically unstable, breaking
up into small vortices, and that vorticity is stretched by an updraft in a convective
1 The term ''landspout'' appeared in the literature many years ago. Ironically, a tornado in the
early 20th century was once referred to as ''a violent landspout''.
2 A vortex sheet is the band of vorticity associated with a discontinuity across a line of the
component of the wind along the line.