Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 339

all types of convective storms, gustnadoes are found in both supercells and
ordinary-cell convective storms. Gustnadoes, like landspouts, are probably the
result of a barotropic shearing instability along a gust front boundary.
Gustnadoes do not require an updraft above and typically are very short lived.
In a sense, gustnadoes are non-supercell tornadoes in the absence of buoyant
updrafts aloft or with buoyant updrafts that get left behind as the convergence
boundary underneath the convective storm moves away from the convective
towers; landspouts, on the other hand, are non-supercell tornadoes with buoyant
updrafts aloft.
(c) Some are typically not readily visible, forming within regions of precipitation
along lines of convection, particularly along cold fronts and in bow echoes or
within MCSs ( Figure 6.28 ). Rit Carbone of NCAR has published some classic
studies of the former, while Greg Forbes and Roger Wakimoto have published a
classic study of tornadoes in bow echoes. It is thought that the vorticity in these
tornadoes comes from shear instability as in (a) or (b).
In the spirit of the so-called ''forecasting funnel'' in which one considers first
the influence of the large scale, and then the influences of progressively smaller
scales, we will first consider the formation of the parent vortex, and then the
smaller scale tornado vortex. Simply put, tornadogenesis associated with meso-
cyclones in supercells involves formation of a storm-scale vortex, a mesocyclone aloft
(i.e., above the boundary layer), and the interaction of the storm-scale vortex with
the surface in the boundary layer and its contraction in scale or the production of a
smaller scale vortex and its contraction in scale, and increase in intensity. Tornado-
genesis not associated with a mesocyclone aloft is associated with a boundary-layer
vortex that is intensified and advected upward.
6.5 TORNADO VORTEX FORMATION: TORNADOGENESIS
Rich Rotunno at NCAR has noted ''the tornado does not fit a simple model like
the spin-up that skaters experience when they pull in their arms.'' It may be
inferred from observations that the proximity of tornado formation to surface
boundaries separating warm, ambient air from evaporatively cooled outflow, and
of very strong updrafts near the ground, ''that complex boundary-layer interac-
tions are (also) important.'' To understand how tornadoes form, the source of
their vorticity must be identified and the mechanisms for its rapid increase must
be accounted for.
6.5.1 Tornado-like vortices in a vortex chamber
If a tornado is barotropic (as it is in a vortex chamber), then one may use vortex
line analysis to visualize tornado-vorticity dynamics. Rich Rotunno has pointed
out that vortex lines in a vortex chamber for steady-state flow with no swirling
motion (no azimuthal velocity) consist of a ring of clockwise-turning vortex lines