Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 165

Figure 4.1. Cumulus congestus being sheared off as it develops during several spurts of growth
in central Oklahoma on (top) April 27, 1983 and (bottom) October 12, 2011 (photograph by
the author).
velocity), then for low values of R the dynamic vertical perturbation pressure
gradient force is at least as important as the buoyancy effects. We will soon see
what the dynamic consequences of this condition are in detail.
The bulk Richardson number can be interpreted physically in terms of energy
and integrated vertical shear. The scale of the horizontal component of the storm-
relative wind (U) is a measure of the vertical shear of the environmental wind
(multiplied by the difference between the altitude of the level under consideration
and the mean altitude of the subcloud boundary layer). The denominator of (4.9)
is therefore a measure of the square of the vertically integrated vertical shear, or
the kinetic energy associated with the horizontal component of the wind in an air
parcel that goes up in a storm's updraft. The numerator is a measure of the
kinetic energy associated with the vertical component of the wind of an air parcel
(converted from potential energy associated with buoyancy) that has risen to the
top of a storm. When R is low, the kinetic energy associated with the horizontal
wind component of the air going upward in the updraft is greater than the kinetic
energy associated with the vertical wind component in the updraft due to
buoyancy. If R is too low, however, the amount of kinetic energy going into the
updraft is so small that the updraft gets sheared over so much that the updraft
aloft gets separated from the updraft below ( Figure 4.1 ) and a storm will not
persist. This latter finding will be discussed later on in more detail in terms of
horizontal vorticity dynamics.
Severe storm forecasters use the bulk Richardson number criterion to
determine whether or not supercell convection is possible. CAPE is computed
from the LFC up to the equilibrium level (EL). It is noted that R is a qualita-
tive—not quantitative—parameter: it does not take into account moisture or
water loading in the calculation of CAPE or CIN, and may not include a good
measure of U if the depth of the boundary layer is substantially different from