Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 348

Figure 6.33. Schematic showing how cyclonic vorticity may be generated, as a combination of
tilting and baroclinic generation causes the vorticity of parcels to change from anticyclonic to
cyclonic while descending in a downdraft. (Top) Consider the vertical plane indicated by the
dashed line. The orange steamline represents streamwise vorticity in the forward flank—not as
it approaches the downdraft, but where it represents the trajectory. Otherwise, as in Figure
6.32. (Bottom) Suppose that (a) just inside the cold side of an RFD that is wrapping around a
mesocyclone, the temperature gradient vector points into the page, to the left of the flow, so
that baroclinic generation of vorticity is in the direction of the arrow at the right. As the parcel
sinks baroclinic vorticity continues to be generated, while the vorticity vector, which was tilted
downward by the downdraft (accompanied by anticyclonic vorticity) becomes tilted upward
(accompanied by cyclonic vorticity) as it is advected faster southward below than it is aloft, as
happens at the ''foot'' of a density current (where the flow of the density current is toward the
ambient air to the south, but much weaker above the density current) or in the presence of
strong low-level environmental vertical shear in the southerly direction (as in Figure 4.14g,
upper-left inset) and (c) finally enters the base of the updraft where it is stretched. Trajectories
in the vertical plane are denoted by solid curved lines with arrows; the three-dimensional
vorticity vector is indicated by the vectors. (The reader is cautioned that in Figure 6.32 vorticity
generated in the downdraft is on the opposite side of the RFD, so that the horizontal
temperature gradient is reversed and vorticity is anti-streamwise.) Figure 6.33 effectively shows
how an air parcel coming from the forward flank may wrap around the mesocyclone and then
enter the downdraft, but become tilted upward by vertical shear and then pass underneath the
updraft. Figure 6.32, on the other hand, shows how anti-streamwise vorticity can be tilted
downward just as it enters the downdraft and be advected to the ground where no more tilting is
possible, and then pass underneath the updraft.