Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 243

which is produced by the convective storm, that is at least partly frictional—not
baroclinic. Such vortex lines, however, must be elevated above the cold pool.

4.7.1 The ''owl horn'' echo

Matthew Kramar and collaborators have found while some supercells are
developing that the rear side of the radar reflectivity pattern resembles the profile
of the great horned owl. This owl horn echo ( Figure 4.55 ) is evident at low alti-
tude and lasts only for around 5-10min. The signature is created when horizontal
vorticity is tilted onto the vertical as cold/cool outflow at the rear expands through
a positive feedback mechanism with a rotational couplet produced inside the cold
pool ( Figure 4.56 ).
When the leading edge of the outflow has a head, a raised portion (cf. Figure
3.35 ), air that approaches the right front-flank side of the outflow head with
streamwise vorticity tilts upward and cyclonic vorticity is produced; as the air
sinks after passing the head anticyclonic vorticity is produced. The opposite
happens on the other side of the cold pool. The cyclonic-anticyclonic couplet pro-
duced inside the cold pool acts to increase the flow of cold air farther rearward
along the leading edges of the cold pool toward the storm ( Figure 4.57 ). A curved
environmental hodograph is necessary to produce the owl horn echo when there is
an outflow head.
When an outflow head is not present, as when the hodograph is straight, a
vortex couplet is not produced inside the cold pool. The same vortex structure is
observed, however, on the outer edge of the cold pool.

4.8 THE LIFE CYCLE OF THE MESOCYCLONE AND
CYCLIC MESOCYCLOGENESIS

The degree of steadiness of the main updraft in supercells is an aspect of their
behavior that has been analyzed and discussed for many years. It has been found,
from both numerical simulation studies and observational studies, that supercell
updrafts are not as steady as had first been postulated. Steadiness is not a neces-
sary attribute of supercells. While multicell convective storms exhibit pulses in
updrafts, sometimes at quasi-regular intervals, supercells do so also. At one end of
the spectrum of supercell behavior are supercells in which the updraft remains
intense, but undergoes some slight variations in intensity, along with relatively
long-lived mesocyclones, which eventually ''occlude'' (become detached from the
rear-flank gust front as downdraft air wraps completely around the mesocyclone,
in an attempted analogy to the life cycle of extratropical cyclones) and then decay.
At the other end of the spectrum are supercells in which discrete mesocyclones,
particularly at low levels, periodically form and undergo well-defined life cycles.
Such behavior is called ''cyclic mesocyclogenesis''. This process will be discussed
again in Chapter 6 regarding smaller scale cyclic tornadogenesis.
During surface mesocyclogenesis, the mesocyclone at low levels may be