Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 252

the low-level shear vector is oriented along the gust front such that cooler air lies
to the left of the shear vector, RKW theory is not applicable, but horizontal
vorticity is tilted onto the vertical as, relative to the movement of the rear-flank
gust front, air parcels are tilted upward at the leading edge of the gust front
( Figure 4.61, middle panel). When the low-level shear vector is oriented at an
angle of 45 from the gust front, then both RKW theory and the tilting of
horizontal vorticity onto the vertical must be considered ( Figure 4.61, bottom
panel).
4.9.2 Movement across outflow boundaries or fronts
Cells initiated along a surface boundary sometimes cross the boundary. Nolan
Atkins and colleagues considered what happens when a numerically simulated
supercell interacts with a pre-existing surface boundary in idealized numerical
experiments. They found that low-level mesocyclones form earlier, are stronger,
and are more long lived when a boundary is present. When a supercell crosses the
boundary from the warmer side to the cooler side, the low-level mesocyclone
weakens; when a supercell propagates along the boundary or has a component of
motion that is toward the warm side, the low-level mesocyclone strengthens.
In a homogeneous environment, the source of vorticity in low-level
mesocyclones in supercells is largely from baroclinally generated horizontal
vorticity that is associated with the forward-flank outflow boundary and whose
baroclinicity is generated by the convective storm itself. When a supercell interacts
with a baroclinic boundary such as an outflow boundary or a front, the forward-
flank area of the storm is less likely to affect the formation of the low-level
mesocyclone, while baroclinically generated horizontal vorticity associated with
the pre-existing boundary makes a significant contribution to the formation of the
mesocyclone at low levels.
It is thus concluded that, just as the orientation of a boundary along which a
broken line of storms is initiated is important in determining whether or not the
cells can evolve into long-lived supercells, the orientation of a boundary is also
important in determining whether or not a low-level mesocyclone in an isolated
supercell will intensify or decay ( Figure 4.62 ). When a supercell crosses a baro-
clinic boundary and the surface air becomes less potentially buoyant, the supercell
and the mid-level mesocyclone may persist, but the low-level mesocyclone
weakens. Thus, supercells that cross boundaries may maintain their intensity and
rotational characteristics aloft, but it is unlikely that sustained rotation will be
produced at low levels. On the other hand, there a number of observational cases
in which a tornado forms when a supercell crosses a surface boundary from the
warm side to the cooler side. In these cases, winds tend to be backed on the
cooler side, so that low-level vertical shear is stronger. Evidently, the effects of
increased vertical shear can sometimes be more important than the effects of
reduced CAPE.