Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 246

Figure 4.57. Conceptual model of the ''owl horn'' echo. Radar reflectivity factor color-coded
in dBZ at 250m AGL. Shaded region identifies cold air protrusions. Outer and inner unshaded
bounded areas enclosed by thin black contours are regions of cyclonic ( þ ) and anticyclonic ( )
vertical vorticity. Updrafts denoted by ''U''. Stippled streamlines indicate the general low-level
storm-relative flow. The line labeled ''A'' identifies the location of the vertical cross sections
seen in Figure 4.56 (from Kramar et al., 2005).
draft, however, is not co-located with the surface vorticity maximum where one
would expect the greatest downward force to be. If the surface vorticity maximum
is located within an updraft, then a downdraft may be discernible only along the
edge of the updraft. Another possible explanation for the lack of exact corre-
spondence between the location of the occlusion downdraft and the vorticity
maximum at the surface is that there are comparable contributions to the net ver-
tical force from other dynamic pressure effects or buoyancy. It is still also possible
that the vorticity maximum tilts with height.
Following the occlusion, a new mesocyclone may form along the rear-flank
gust front where there is a surge in the RFGF, initiating a new updraft separate
from the original, previous updraft, and increasing surface vorticity through con-
vergence at the leading edge of the RFGF; subsequently, the new mesocyclone
becomes occluded, and this periodic behavior may continue. In a series of influen-
tial papers by Ed Adlerman and Kelvin Droegemeier in the early part of the
second millennium, results from studies of cyclic mesocyclogenesis simulated
numerically have revealed how sensitive it is to environmental conditions (e.g., the
vertical shear profile). It is easily seen how the intensity of the cold pool produced
by the storm, if any, plays a role in whether or not there is cyclic mesocyclo-
genesis. Within the category of cyclic mesocyclogenesis, there are instances in
which the mesocyclone occludes (OCM, occluding cyclic mesocyclogenesis; Figure