Geoscience Reference
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500m or if the environmental wind profile is such that U is not representative of
storm-relative inflow or if gust front propagation also makes U not representative
of storm-relative inflow. It is generally accepted that when
10
<
R
<
50
ð 4
:
10 Þ
supercell convection is possible. However, (4.10) expresses a necessary, but not a
sucient condition and ordinary cells/multicells may also be present. In practice,
severe storm forecasters are usually concerned more with the upper bound and not
with the lower bound.
It is seen in idealized numerical simulation experiments that the upward
acceleration due to the vertical perturbation pressure gradient force is sometimes
much greater than that due to buoyancy, especially at low levels when buoyancy is
not high and when vertical shear is very strong, as it sometimes is, for example, in
the environment of landfalling hurricanes ( Figure 4.2 ). Storms in landfalling
hurricanes can have dynamically driven updrafts that are as strong as updrafts in
supercells in the Great Plains of the U. S.
It is possible that R can be small enough to satisfy the supercell criterion
(4.10) when CAPE is very low and vertical shear is relatively high, but still not
very strong in an absolute sense. When CAPE is low, buoyancy is also low, so
that the vertical accelerations are weak in an absolute sense and it is not possible
to produce a supercell because updrafts are simply not very strong, even though
they are due, in significant part, to dynamic vertical pressure gradients.
On the other hand, when shear is extremely strong, even though CAPE is
relatively high, but R is small, it is dicult for a convective storm to develop
because initially shear makes the updraft lean over so much that the top of the
storm may become detached from the updraft. The use of R for forecasting must
be applied with caution and it is probably more useful to consider CAPE, shear,
and R all together.
We end this section with a gedunken (''thought'') experiment to show how the
dynamical vertical pressure gradient force can act to augment the updraft intensity
beyond that predicted from CAPE alone when R is small (i.e., when the effects of
vertical shear are significant). Consider what happens when the vertical dynamic
perturbation pressure gradient force ð 1
p 0 d =@
z Þ augments the net effect of
upward-directed buoyancy and downward-directed buoyancy perturbation press-
ure gradient force ( ½ð 1
=
@
p 0 b =@
z þ B ) (4.1). To do so, we derive a Bernoulli-like
equation for steady-state, frictionless flow.
The steady-state, frictionless form of the three-dimensional equation of motion
is as follows:
=
Þ@
Dt ¼ð v EJ Þ v ¼ 0 r p 0 þ B k
Dv =
ð 4
:
11 Þ
where
0 is the mean specific density. The advective part of the inertial term
1
( ð v
v, where
the first term is called the ''Bernoulli'' term and the second term is called the
''Lamb'' term. We define a differential position vector
dr ¼ dx i þ dy j þ dz k
EJ Þ v) may be expressed using a vector identity as
J ð
2 v
v Þþð JT
v Þ T
E
ð 4
:
12 Þ
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