Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 236

ordinary supercells, they can produce tornadoes (and waterspouts). They have
been documented in the Great Plains of the U. S. during the spring and summer,
along the Gulf Coast of the U. S. during the hurricane season, in Japan during
the typhoon season, and in the cool season in California and Australia, among
others.
Numerically simulated supercells have stronger surface outflow when the mid-
troposphere is relatively dry; when shear is strong or when dryness is concentrated
at higher altitudes, however, this effect is less. The potential for evaporatively
cooled downdrafts increases with increasing dryness, especially when vertical shear
is not too strong. Strong downdrafts are detrimental to storm longevity: when
surface outflow moves at the same speed as the updraft and mid-level meso-
cyclone, then storms can persist and intensify; when surface outflow moves faster
than the updraft and mid-level mesocylone, storms weaken.
The depth of the environmental, moist, boundary layer also significantly
affects numerically simulated supercell morphology and behavior. Numerical
experiments have been conducted to explain contrasting supercell behavior in the
relatively moist environments of the Central Plains of the U. S. from that in the
relatively dry environments of the High Plains. For example, in the former, the
LCL and LFC are at relatively low altitude, while in the latter the LCL and LFC
are relatively high. Therefore, the potential for evaporative cooling near the
surface is higher in the latter case, since precipitation has farther to fall through a
layer of unsaturated air; so, the dynamics of the cold pool then play a more
important role in storm behavior. An interesting finding is that, under some cir-
cumstances, storms in a low-CAPE environment may be more intense than storms
in a high-CAPE environment, owing to vertical perturbation pressure gradients
that act to enhance updraft intensity.
The effects of microphysical parameterization schemes on numerically
simulated supercell behavior have been investigated. When the amount of rainfall
is high relative to the amount of ice material, evaporation is higher and colder
surface cold pools may be generated, thus decreasing the likelihood of storm
longevity. When freezing processes occur, warming is increased due to latent heat
release, while cooling is increased due to melting.
From studies of the effect of dryness and microphysics, it is concluded that the
behavior of supercells is influenced not only by vertical shear and CAPE, but also
by the intensity of the surface cold pool and its effect on decoupling surface
features from those aloft that are not affected by the cold pool.
Studies that elucidate the behavior of convective storms based on vertical
shear and CAPE alone must be regarded with some caution because, for example,
the same CAPE at different temperature regimes may be associated with varying
amounts of hydrometeors of different types, as temperature and pressure vary.
Varying amounts of hydrometeor types can lead to different cold pool strengths.
A good place for a student to experiment by simply varying shear and CAPE and
exploring the effect on storm type is the COMET (Cooperative Program for
Operational Meteorology, Education, and Training) module for the ''Convective
Storm Matrix'' ( http://www.meted.ucar.edu ).