Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 351

suggest that mesocyclone vertical propagation speed is nearly infinite (i.e.,
mesocyclones/TVSs instantly form in a column); earlier radar observations on
time scales of minutes simply could not resolve the true time scale of the vertical
propagation of mesocyclones or tornadoes. The DPE has therefore been called
into question, owing to a lack of observations of it actually occurring when
the time resolution of observations is such that observations are capable of
resolving it.
The DPE is a process by which vortices produced aloft propagate downward,
so that the origin of the tornado vortex is aloft. On the other hand, in vortex
chambers tornado-like vortices develop as a result of frictional convergence near
the surface driven by an updraft and vortex aloft. It is therefore thought that there
might be two different types of tornado formation: one in which the parent vortex
begins aloft and builds downward and intensifies via either the DPE (if it actually
exists) or builds downward through transport by a downdraft, and one in which
the tornado forms at low levels through frictional convergence and a strong
updraft aloft and is advected upward so that it appears to build upward from the
ground. The distinction between these two types of tornado formation may be
artificial: any vortex that gets near enough to the ground can be intensified as will
be discussed subsequently. Any vortex that forms aloft, away from the effects of
surface friction, may descend via the DPE process or be advected downward.
When it gets near enough to the surface, friction may take over. In a sense, the
DPE process and friction are similar, because they both act to draw air radially
inward below the vortex.
How can we explain how seemingly different processes through which a
mid-level mesocyclone and a low-level mesocyclone form can result in a tornado?
Tornadogenesis probably owes its existence more to the intensification of a low-
level mesocyclone rather than the intensification of a mid-level mesocyclone. The
role of a mid-level mesocyclone may be to promote the sustenance and intensification
of a low-level mesocyclone. For example, a mid-level mesocyclone promotes super-
cell updraft propagation normal to vertical shear, which may allow surface air
parcels to pass into the nearby downdraft so that streamwise vorticity is tilted
onto the vertical and then can be stretched underneath the main updraft. In a
sense, mid-level and low-level mesocyclones may be like the upper-level and
surface isentropic potential vorticity (IPV) (or potential temperature) anomalies
in the analysis of baroclinic instability of synoptic-scale flows. Should a mid-level
mesocyclone not be situated above a low-level mesocyclone, the low-level meso-
cyclone might not be able to intensify. This hypothesis probably requires further
examination.
6.5.5 Role of downdrafts in enhancing and transporting vorticity
Bob Davies-Jones at NSSL has considered the possibility that the hook echo in
supercells, which has been thought for decades to be formed passively as precipita-
tion is transported in a curved trajectory by a low-level mesocyclone, actually
plays an active role in tornadogenesis. Raindrops may be advected around just