Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 336

Figure 6.26. Conceptual model of the evolution of non-supercell tornadoes along a weak
outflow boundary. In (I) a vortex sheet develops along the leading edge of the outflow when
there is a component of motion in the environment, ahead of the outflow boundary, along
(parallel to) the leading edge. The vortex sheet becomes unstable and small-scale vertical
vortices develop (II). Vortices interact and merge (III) through pairing of similar vortices
and the ''mining'' of weaker neighboring vortices by stronger vortices, leading to a wider
separation between neighboring vortices. Vorticity is advected upward out of the boundary
layer into growing convective cells and stretched. Frictionally induced radial inflow near the
ground (discussed in more detail in Section 6.6.1) stretches vorticity to tornadic strength (IV)
and environmental horizontal vorticity is tilted upward and also stretched. During the late
mature stage (V) cold pools produced by new convective storms increase low-level convergence
at the leading edge of the outflow boundary, increasing the stretching, and wrap relatively cold
air around the vortices. Downward-directed dynamic perturbation pressure gradient forces
develop above the vortices and force downdrafts, as does the negative buoyancy associated
with the strengthening cold pool (VI). The outflow boundary then accelerates and tilts the
vortex so that much of it is in descending air and the tornadoes dissipate. The tilting of
baroclinically generated horizontal vorticity along the outflow boundary does not appear to
be important in NST tornadogenesis. (from Lee and Wilhelmson, 1997).
cloud that is triggered by vertical motion associated with low-level convergence
along the boundary ( Figure 6.25 ). Bruce Lee at the University of Illinois Urbana-
Champaign and his collaborators have done seminal numerical studies of this
type of tornadoes. Baroclinicity along the surface boundary could also play a
role, especially as a source for horizontal vorticity that is tilted onto the vertical
along the edges of updrafts or downdrafts ( Figure 6.26 ), but it appears it is not of
primary importance.
(b) Some, which appear along the edge of gust fronts as rotating dust whirls ( Figure
6.27 ), are colloquially known as ''gustnadoes''. Since gust fronts are found in