Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 162

4

Supercells

''Lear: Blow, winds, and crack your cheeks! rage! blow!
You cataracts and hurricanoes, spout
Till you have drench'd our steeples, drown'd the cocks!
You sulphurous and thought-executing fires,
Vaunt-couriers to oak-cleaving thunderbolts,
Singe my white head! And thou all-shaking thunder
Smite flat the thick rotundity o' the world!''
William Shakespeare—Act III, Scene II, King Lear

In the previous chapter we considered the behavior of ordinary cells and multicell
convective storms, which consist of a succession of ordinary cells for which
positive buoyancy is of primary importance. This positive buoyancy is realized by
air parcels accelerating upward in a conditionally unstable atmosphere, which is
counteracted by a downward-directed perturbation pressure gradient force that
depends on the aspect ratio of the spatial dimensions of the air parcel, and by
precipitation and cloud-particle loading, and by the entrainment of cooler or drier
environmental air.
In addition, density currents near the ground driven by evaporative cooling
and melting of precipitation or cloud material interact with the low-level
environmental shear to initiate new convective cells adjacent to earlier cells or to
cut off any new convective growth. Multicell storms may be either two dimen-
sional or three dimensional and the individual cells that make up the multicell
complex do not typically achieve any steady state (multicell storms that become
organized on the mesoscale, however, can achieve a steady state and will be
discussed in Chapter 5).
When the magnitude of

the (mainly deep-layer) vertical shear in the

environment exceeds some threshold, dynamic vertical perturbation pressure