Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 249

4.9 SUPERCELL STRUCTURE AND BEHAVIOR IN RELATION TO
INHOMOGENEITIES IN THE ENVIRONMENT, AND
INTERACTIONS WITH NEIGHBORING STORMS AND
SURFACE BOUNDARIES
In most numerical studies of supercell behavior, the simulated supercell is isolated
and embedded in a homogeneous environment. In nature, supercells may move
from one environment to another or the environment may change with time. For
example, storms may move across baroclinic zones, across zones of moisture gra-
dients, and across zones of gradients in vertical shear. Storm environmental shear
may be increased substantially as night-time approaches as the low-level jet forms.
Yvette Richardson and colleagues, in considering the influence of horizontal varia-
tions in vertical shear and low-level moisture on convective storm behavior, have
found that they can have significant effects. Variations in low-level moisture can
affect the location, timing, and intensity of new-cell development. When vertical
shear is strong, a convective storm may survive even if it moves into a region too
dry to support convective initiation. Forecasters must anticipate whether a super-
cell will decay or devolve into an ordinary-cell or multicell complex or whether an
ordinary-cell or multicell complex will evolve into a supercell as the environment
changes. There are many documented cases of storms changing character as they
move through different environments or as the environment changes with time.
There are also some distinctively different behavioral aspects of supercells that
are related to the supercell's interaction with neighboring storms and boundaries,
and its movement across surface boundaries.
4.9.1 Neighboring cell interaction
Even when the environment is favorable for the formation of a supercell (i.e.,
when vertical shear and CAPE are matched so that the bulk Richardson number
is within the range such that vertical perturbation gradients are significant and
vortices form), a supercell may not necessarily evolve from convective storms that
are initiated. In some instances, when convective storms are initiated along a
surface boundary, neighboring cells may interact with each other so that supercells
cannot evolve when cold pools block warm, moist air from entering updrafts.
When relatively widely spaced convective storms are initiated along a
boundary, the orientation of the boundary with respect to mean vertical shear in
the lower half of the troposphere matters ( Figure 4.60 ). If the boundary is oriented
normal to the mean shear vector, then convective storms split into right and left-
moving members and adjacent left and right-moving cells collide with each other.
Only the right-moving, cyclonically rotating member at the upshear end of the line
and the left-moving, anticyclonically rotating member on the downshear end of
the line do not interact with their neighbors and can behave like isolated super-
cells. If the boundary is oriented along the mean shear vector, then left-moving,
anticyclonically rotating supercells move across the boundary into cooler or drier