Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 245

Figure 4.56. Conceptual model of couplets of vorticity produced as streamwise vorticity ( ! s )is

tilted upward and downward as it flows from right to left over a head at the edge of a cold pool
(vertical cross section shown) at the rear of a supercell and then exits the cold pool at the left in
a similar manner. With time (a ! f ) the cold pool expands and the vorticity couplets become
more separated from each other (from Kramar et al., 2005).

stronger than or displaced from the center of the mesocyclone aloft, so that a
downward-directed, dynamically driven, perturbation pressure gradient forms and
forces a downdraft, the occlusion downdraft ( Figure 4.58 ), which is not to be con-
fused with the RFD (which may be driven both thermodynamically and
dynamically), since it may be located nearby or within the broader RFD. The
RFD is thought to be involved in the increase in vorticity near the surface, so that
the occlusion downdraft may be a consequence of the RFD. The occlusion down-