Geography Reference
In-Depth Information
one-dimensional cloud models have been very popular in the past. Unfortunately,
observed cloud properties, such as maximum cloud depth and cloud water concen-
tration, cannot simultaneously be satisfactorily predicted by this type of model.
In reality, pressure perturbations generated by the convective cells are important
in the momentum budget, and entrainment does not occur instantaneously, but in
a sporadic manner that allows some cells to rise through most of the troposphere
with very little dilution. Although more sophisticated one-dimensional models can
partly overcome these deficiencies, some of the most important aspects of thun-
derstorm dynamics (e.g., the influence of vertical shear of the environmental wind)
can only be included in multidimensional models.
9.6
CONVECTIVE STORMS
Convective storms can take a large variety of forms. These range in scale from
isolated thunderstorms involving a single convective cloud (or
cell
) to mesoscale
convective complexes consisting of ensembles of multicelled thunderstorms. Here
we distinguish three primary types: the single cell, the multicell, and the supercell
storm. As shown in the previous section, convective available potential energy mea-
sures whether thermodynamical conditions are favorable for the development of
cumulus convection. CAPE, therefore, provides a guide to the strength of convec-
tion. It does not, however, provide any notion of the most likely type of mesoscale
organization. It turns out, as suggested above, that storm type also depends on the
vertical shear of the lower tropospheric environment.
When vertical shear is weak (< 10 m s
−
1
below 4 km), single cell storms occur.
These tend to be short lived (
30 min) and move with the mean wind in the
lowest 8 km. When there is moderate vertical shear (
∼
10-20 m s
−
1
below 4 km),
multicell storms arise in which individual cells have lifetimes of
∼
30 min, but
the storm lifetime may be many hours. In multicell storms the downdrafts induced
by the evaporation of precipitation form a dome of cold outflowing air at the
surface. New cells tend to develop along the
gust front
where the cold outflow
lifts conditionally unstable surface air. When vertical shear is large (> 20m s
−
1
below 4 km), the strong tilting of convective cells tends to delay storm development
even in a thermodynamically favorable environment so that an hour or more may
be required for the initial cell to completely develop. This development may be
followed by a split into two storms, which move to the left and right of the mean
wind. Usually the left-moving storm dies rapidly while the right-moving storm
slowly evolves into a rotating circulation with a single updraft core and trailing
downdrafts, as discussed in the next section. Such supercell storms often produce
heavy rain, hail, and damaging tornadoes. Ensembles of multicell or supercell
storms are often organized along lines referred to as
squall lines
, which may move
in a different direction than the individual thunderstorms.
∼
