Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 95

air in the cloud (parcel) over its environment (and consequently its buoyancy)
increases with height ( Figure 3.1 ). We will consider for simplicity what happens
when the aspect ratio of the cloud parcel is large (i.e., the parcel is narrow), so
that we can neglect the counteracting downward perturbation pressure gradient
force due to buoyancy (cf. (2.70) and (2.75)) and, consequently, buoyancy is the
only vertical force.
When the layer of air in which the environmental lapse rate is greater than
moist adiabatic the atmosphere is unstable because a parcel's upward vertical
motion increases with time/height if it is given a nudge upward. The atmosphere
is said to be in a state of ''conditional instability'': the air continues to accelerate
upward in the absence of any more lifting if given an initial push upward. If the air
were not saturated, it would not be unstable, whence the adjective ''conditional'' is
used. If the air were unsaturated, it would cool when lifted at the more rapid dry
adiabatic lapse rate and the air would be stable. The same analysis and results
apply if the air parcel is pushed downward: if the lapse rate in the environment is
greater than moist adiabatic, downward motion increases with time/decreasing
height if it is given a nudge downward.
While it is a challenge to forecast exactly where and when the LCL or CCL
may be reached and convection initiated, some convective storms are triggered in
the exact same place with regularity for part of the year; some of these storms
have been given names. The onset of these storms is connected to topography/
orography and related solenoidal vertical circulations. The most famous is
''Hector'', which is a complex of storms that form over the Tiwi Islands off the
coast of northern Australia. Owing to a high tropopause and strong updrafts,
Hector is one of Earth's tallest convective storms, extending up to 20 km. In the
case of Hector, it is the sea breeze circulation from several coasts that is in part
responsible for its intensity and regularity.
The boundary layer is not the only source of moist air that may be lifted to its
LCL or LFC. Air that originates above the boundary layer may also be lifted to
its LCL and beyond to its LFC. Such convection that develops is referred to as
''elevated'' convection ( Figure 3.2 ). Elevated convection may develop as a result of
quasi-geostrophic, synoptic-scale ascent in a conditionally unstable environment,
or as a result of mesoscale ascent in a conditionally unstable environment.
Prominent cases of the latter include lift at and just behind a frontal zone that
tilts with height toward the cold air ( Figure 3.3 ), lift over an outflow boundary
( Figure 3.3 ), and orographic lift ( Figure 3.3 ) as air is forced upslope when there is
enough kinetic energy for the air to be lifted a sucient distance to reach the LFC
in the face of initial stability. Lift may also occur along the rising branch of
boundary layer rolls or the rising branch of coherent gravity waves.
3.1.2 Entrainment and convective initiation
In the real (non-idealized) world, as air rises environmental air is ''entrained''
through turbulent eddies into the cloud, from both the sides and/or the top and/
or bottom ( Figure 3.4 ): try to visualize the leading edge and the sides of a