Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 143

( Figure 3.39 ). Evidence of turbulent mixing underneath the arcus is often seen and
colloquially known by storm-chasers as the ''whale's mouth'', since it has the
texture of the inside of the mouth of the whale (actually simply the ''Terrible
Dogfish'') in Disney's animated version of the children's tale Pinocchio.
Because air moves up and over the cold, low-level pool of air associated with
the density current, the air acts as if it is being lifted over a mountain range. Of
course, in this case it is not really moving over a solid body and there is some
mixing at the interface between the ambient air and the colder air below. Under
the right conditions, lifting of the air triggers gravity waves, which behave like
waves in the lee of a mountain range. Such waves, depending on static stability,
the strength of the wind, and vertical shear, may be vertically propagating or
trapped. The reader is referred elsewhere for detailed analyses of mountain waves.
Above a density current, gravity waves may modulate the production of clouds
and also provide sources of horizontal vorticity parallel to the leading edge of the
density current.
Studies of density current behavior for conditions that depart from the highly
idealized ones we have assumed have been done using both analytical and numer-
ical models. A basic problem is to determine the transient behavior of the cold
pool under idealized conditions (two dimensions, inviscid, etc.). That is, we start
out with a cold pool and let it go such that it eventually may reach a quasi-steady
state, as happens in convective storms: A cold pool forms and then begins to
propagate and may reach a steady state. In nature, of course, the cold pool itself
takes time to be built up as well. Microphysics is obviously very important and so
is the nature of the airflow. What happens before the cold pool reaches a steady
state, if it ever does? This problem has been referred to as the dam-breaking
problem, as noted previously, owing to its analogy to a wall of water constrained
by a dam, which is suddenly opened and involves an analysis of propagating
gravity waves generated by the density current, which is beyond the scope of our
introductory discussion.
In our analysis, we have assumed that the density current is stagnant and that
there is no environmental flow ahead of it. The motion, relative to the movement
of the updraft(s) in a storm, of a density current generated in a convective storm is
important for the subsequent evolution of the storm: If the density current outruns
the updraft, then the updraft will no longer have potentially unstable low-level
environmental air available to feed it. If the density current moves along with the
updraft, at the same speed that the updraft is moving, then the updraft may be
long lived. Strong opposing flow in the environment slows down the motion of
the density current and may even stop it. Of equal or greater importance for the
initiation of convection is environmental vertical shear.
3.2.2 Gust fronts in the presence of vertical shear: RKW theory
Typically, in the absence of vertical wind shear, a cold pool will spread out (like a
pancake) at the surface and new convective cells will not be initiated along the
periphery of the cold pool in response to the lifting of ambient air, unless the