Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 134

sublimation is also a necessary condition for mamma formation: the importance of
sublimation and dry air underneath the anvil were emphasized. Neither liquid
water nor radiative effects were necessary for the process of forming mammatus
clouds.
Mammatus are little more than ''embroidery'' under the anvil and do not
appear to play an important role in the dynamics or behavior of severe convective
storms. They are included in this text only because they are sometimes prominent
visual features and may be of concern to aviation interests.
3.2.1.4 Cold pools and density currents
The depth of a pool of cold air near the ground is important dynamically, because
it determines the motion of the leading edge of the cold pool. Generally, the depth
of the cold pool behind a gust front ranges from several hundred meters to several
kilometers. The deeper and colder a cold pool is, the greater the hydrostatic press-
ure excess behind the cold pool. At the leading edge of the cold pool, a
hydrostatic pressure gradient force is directed from the cold side to the warm side.
The leading edge of the cold pool is then forced toward the warm air ( Figure
3.33 ). Much of the cold pool moves as a material surface like a ''density current'',
also known as a ''gravity current''. The density current is forced by the pressure
gradient force acting along it from the denser, colder side to the less dense,
warmer side, but is retarded by the drag of the flow along the ground and a
dynamic pressure gradient force just along the leading edge.
The reader is referred to the great Boston molasses flood of 1919, when a wall
of molasses (surrogate for cold, dense air) from a storage tank swept through a
part of the city: This event was probably the ultimate density current, albeit a
non-meteorological one. Perhaps an even more dramatic, but fictional, account is
that of a ''living'' density current, the alien life form in the 1958 science fiction/
horror movie The Blob. More recently, engineers have studied oil spills that
behave like density currents. Other examples include gust fronts in thunderstorms
in arid regions when almost all of the precipitation has evaporated and the winds
have kicked up a wall of blowing dust called a ''haboob'' ( Figure 3.34 ).
In the simplest model of a density current, the air behind the gust front is
assumed to be at rest (there is no density current-relative flow behind the density
Figure 3.33. Idealized vertical cross section across a cold pool of air near the surface. A
horizontal hydrostatic pressure gradient force (PGF) is directed from the cold side to the
ambient, warm side at low levels.