Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 254

Figure 4.63. Idealized illustration of how a cyclonically rotating downdraft may form in a
convective storm when the hodograph curves in a clockwise manner with height. A cyclonic ( þ )
and anticyclonic ( ) couplet forms from tilting by the updraft; precipitation falls out at high
levels at B; the mid-level vorticity at B is stretched as the downdraft accelerates downward due
to evaporative cooling (from Parsons and Weisman, 1993).
height; Figure 4.63 ). Early on in the life of a convective storm, a cyclonic-
anticyclonic vortex couplet is produced at mid-levels as a result of the tilting of
environmental horizontal vorticity associated with vertical shear, as in a supercell,
with the mesocyclone to the right and the meso-anticyclone to the left of the
vertical shear vector at mid-levels where the updraft and shear are strongest.
Precipitation is produced in the updraft and then falls out in the downshear direc-
tion (with respect to the vertical shear aloft) into the cyclonic member of the
vorticity couplet. As the air is dragged downward by falling precipitation and
evaporative cooling makes the air negatively buoyant, it accelerates downward,
creating convergence that acts to intensify the mesocyclone and advect it down-
ward. When the shear vector backs with height, an anticyclonically rotating
downdraft is produced.
It is noted that the mechanism just described does not occur in supercells:
when vertical shear is relatively strong, a downdraft produces an anticyclonic-
cyclonic vortex couplet at mid-levels through tilting. When the vertical shear
vector veers with height, the linear dynamic perturbation pressure term is negative
upshear from the downdraft and positive downshear from the downdraft, so that
on the anticyclonic side there is a downward-directed perturbation pressure
gradient force and on the cyclonic side there is an upward-directed perturbation
pressure gradient force. Thus, anticyclonic downdrafts and cyclonic updrafts are
produced, which is opposite to what is observed when the vertical shear is weak.