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It is also possible that horizontal vorticity is tilted onto the vertical as air
parcels encounter an outflow boundary and turn upward as they pass over the
cold dome. Unless air parcels turn very sharply upward, tilting along the gradient
of an updraft alone is insucient to create a tornado because vertical vorticity is
rapidly advected upward, away from the ground. While gust fronts may present
opportunities for air parcels to be turned sharply upward ( Figure 4.5 ), in many
instances air parcels begin to turn upward ahead of the gust front not so sharply
( Figures 3.37 and 3.39a ).
When a downdraft forms adjacent to an updraft (underneath which there is
stretching), the vertical vorticity created through tilting remains closer to the
ground, as noted in Section 4.7, because material curves are brought to the
ground by the downdraft—not by the updraft. In many tornadoes, the tornado is
indeed located along a gradient in vertical motion, in between the rear-flank
downdraft (RFD) and the tip of the horseshoe-shaped updraft along the leading
edge of the RFD ( Figure 4.15 ). The role of baroclinically generated horizontal
vorticity in the FFD region of a supercell has been called into question recently,
owing to observations that temperature gradients at the surface near strong torna-
does seem to be much weaker than those in the absence of tornadoes. On the
other hand, upstream of the tornado, in some direction, air parcels may have
passed through regions of stronger temperature gradients or there is strong low-
level shear in the environment, such that environmental horizontal vorticity is the
main source for vorticity in the tornado. In recent field campaigns such as
VORTEX2 efforts have been made to map out the temperature at the surface
using probes and the complete results are forthcoming at the time of this writing.
Evidence from observational and numerical work so far suggests that the baro-
clinic generation of horizontal vorticity is more important than the import of
existing horizontal vorticity associated with vertical shear in the environment.
Since surface temperatures realized in numerical simulations of supercells are very
sensitive to the type of microphysical parameterization employed, results from
numerical simulations, however, are to be viewed with extreme caution.
While Rotunno and Klemp showed using circulation analysis how cyclonic
circulation is produced in a low-level mesocyclone (cf. Chapter 4) as a result of
part of the material curve being pushed downward by the downdraft in the FFD
and then the material curve in the convergent area shrinks under the main
updraft, Bob Davies-Jones and Harold Brooks have more recently offered, also
based upon numerical experiments, an explanation for the appearance of strong
low-level cyclonic vorticity in terms of both the main updraft and the rear-flank
downdraft using (local/individual) vortex line analysis rather than an area-
averaged (non-local/group of vortex lines) circulation analysis: consider a down-
draft in which (1) there is streamwise vorticity; (2) storm-relative flow through the
downdraft is parallel to isentropic surfaces, with the colder air lying to the right,
as it would in the RFD, west/northwest/north of a low-level mesocyclone (in the
Northern Hemisphere); and (3) that environmental flow has a component of
motion that decreases with height, as there would be if there were a density
current (gust front, surge of cool air) approaching from the north ( Figure 6.33 ).
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