Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 299

Figure 5.27. Idealized illustration of how small-scale counter-rotating vortices may be pro-
duced later in the life of a squall line MCS by the tilting of baroclinically generated horizontal
vorticity in the y-direction ( y points toward the north; the cold pool lies to the west) by (top)
an updraft and (bottom) a downdraft.
act against any updraft above the vortex. This suppression of updrafts near low-
level vortices acts to break up an existing continuous line into segments when
there are vortices produced at low levels ( Figure 5.28 ). Bookend vortices then
develop along the new line ends, and the process may be repeated.
The vortices in MCSs at low levels are unlike mesocyclones in supercells in
that they are not characterized by upward-directed, dynamically forced accelera-
tions and rising motion at mid-levels. In addition, they tend to move along with
the mean wind and do not propagate normal to vertical shear, so that the helicity
paradigm is not applicable to their dynamics. Furthermore, these ''mesovortices''
are produced from the tilting of crosswise vorticity—not streamwise vorticity.
Forecasters and nowcasters must be careful not to assume that mesocyclones/