Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 101

=

2 . For example, a CIN
of just 50 j kg 1 requires an initial upward velocity burst of at least 10m s 1 to
reach the LFC with a nonzero upward velocity. Very often a low-level moist
boundary layer is topped by an inversion, which acts as a cap to boundary layer-
based thermals and is responsible for CIN ( Figure 3.1 ). In nature, the mesoscale
lift is much weaker ( 1ms 1 or even 10 cm s 1 ).
If the environmental air that is entrained is unsaturated, some of the cloud
particles may evaporate and cool, so that buoyancy is even further reduced. When
the air outside the cloud is nearly saturated, the reduction in buoyancy is
negligible as a result of evaporation. In the tropics, the relative humidity of air in
many instances is high, so that the potential for evaporation and cooling is low; in
mid-latitudes, when the relative humidity in many instances is low, the potential
for evaporation is high. Thus, not only is the environmental lapse rate important,
but the humidity of the environment and its variation with height are too.
In an environment in which the water vapor mixing ratio decreases with
height (this is typically the case, since the source of water vapor is water at the
Earth's surface; however, the water vapor mixing ratio is often layered, with a
number of relative maxima), lift on the mesocale or synoptic scale, dynamic or
orographic, tends to increase humidity aloft in the ''environment'': convective
clouds that form in an environment of ascent have a better chance of growing
into deep convective clouds than those that form in an environment of descent, in
which humidity in the environment is decreased.
In addition, the environmental lapse rate is changed when there is vertical
motion (cf. the static stability tendency equation in Bluestein, 1992) owing to the
combined effects of vertical advection of temperature and of adiabatic cooling/
heating due to the expansion/contraction of air. At low levels, rising motion is
associated with horizontal convergence, which increases the lapse rate as potential
temperature isotherms (isentropes) are spread apart in the vertical ( Figure 3.6 ).
It has generally been observed that convectively enhanced environments are
those that sustain rising motion on the synoptic scale and convectively suppressed
environments are those that sustain sinking motion. Since synoptic-scale vertical
motion is two-three orders of magnitude (cm s 1 vs. m s 1 ) less than vertical
motion in a convective cloud and that needed to overcome CIN, synoptic-scale
motion is not directly implicated in initiating convection; instead, it makes the
environment more or less susceptible to convection by moistening the environment
and thereby reducing the deleterious effects of entrainment and reducing static
stability, all of which act to reduce CIN rather than overcome it.

1

if w ð z ¼ LFC Þ¼ 0. For w ð z ¼ LFC Þ > 0, w 0 > ð 2 CIN Þ

3.1.3 Observed life cycle and vertical velocity

After deep convection has been initiated, the cloud grows until the ''equilibrium
level'' (EL) is reached ( Figure 3.1 ), at which the rising air parcel's positive buoy-
ancy has been reduced to zero. Kinetic energy of the rising air parcel is nonzero,
so the cloud keeps growing upward until its kinetic energy has been used up. It