Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 238

Figure 4.50. Illustration of how low-level horizontal vorticity may be enhanced by an anvil-
generated baroclinic zone due to a horizontal gradient in radiation (from Markowski et al.,
1998b).
the updraft. However, it has been demonstrated numerically that low-level, environ-
mental, horizontal vorticity in the form of low-level shear also affects low-level
mesocyclogenesis. In short, the mechanisms for low-level and mid-level
mesocyclogenesis are different. However, the two may interact, and this interaction
will be discussed in a subsequent section on tornadogenesis.
First, consider a vorticity analysis following air parcel trajectories. Air may
enter a low-level mesocyclone from the southeast (for simplicity we consider a
supercell in the Northern Hemisphere), which represents a flow of ambient,
relatively warm, and moist air. Air entering the low-level mesocyclone from the
north or northwest is relatively cold. During the daytime hours, there may also be
temperature differences created by anvil shading ( Figure 4.50 ); during the night-
time hours, the baroclinic effect due to anvil shading is reversed. Anvil shading is
not a fundamental process, but probably one that can enhance or diminish the
horizontal temperature gradient already existing due to differences in the amount
of evaporational/melting/sublimation cooling. When a storm and its surrounding
area is covered by anvils from other convective storms, there cannot be any
anvil-shading contribution to surface baroclinicity.
If there is a horizontal gradient in the coolness of the air underneath the
region of precipitation in the FFD area, then horizontal vorticity may be gener-
ated as air enters the low-level mesocyclone after having passed through this area.
Moreover, air entering the low-level mesocyclone from the east or northeast may
have passed over a baroclinic zone separating evaporatively cooled air underneath