Supercells - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 203

nearby ( Figures 4.6, 4.25- 4.28 ). This observation is interpreted as meaning that
the region of heaviest precipitation is composed of widely scattered, large rain-
drops and hailstones, and the region of less intense precipitation is composed of
more densely packed smaller raindrops and hailstones.
Storm-chasers have noticed that the region behind the RFD is sometimes
optically translucent and contains little if any precipitation and the region where
there is typically the most intense precipitation is also optically translucent and
contains almost no rain, but some hail ( Figure 4.25 ). The only rain observed falls
out from the anvil, relatively far from the storm's main updraft. Such storms are
called ''low-precipitation (LP)'' supercells, the name originating with the author of
this text; Don Burgess and Bob Davies-Jones at NSSL first called these storms
''dryline storms'' because they tended to be found near the dryline: LP supercells
do not require a dryline for their existence and not all supercells near the dryline
are of the LP type.
On the other hand, the region behind the RFD is sometimes optically opaque
and contains an abundance of precipitation and the region where there is typically
the most intense precipitation is also optically opaque and contains rain and hail
( Figure 4.26 ). Such storms are called ''high-precipitation (HP)'' supercells, the
name being given originally by Al Moller and collaborators. LP and HP supercells
are the extreme ends of a spectrum of a variation of supercell types in which
precipitation coverage near the rear-flank gust front and wall cloud (i.e., near the
updraft) is variable. The idealized visual model depicts the classic supercell (i.e., a
supercell in which precipitation eciency is greater than that of an LP supercell,
but less than that of an HP supercell; Figure 4.27 ).
Interesting questions concerning differences in the thermodynamics of LP and
HP storms arise in the context of tornado formation and are addressed in a later
section. When there is little if any rain, the potential for the production of an
evaporatively cooled pool of air near the ground is very low; when there is a lot
of rain that falls out into relatively dry air, the potential for the production of an
evaporatively cooled pool of air is very high. Thus, the temperature gradient near
the surface across the rear-flank gust front and near the edge of the forward-flank
downdraft region should be greatest in HP supercells and lowest in LP supercells,
when air is unsaturated near the ground.
The reason(s) precipitation eciency varies so widely in supercells is (are) not
known very well because details of precipitation processes are not very well
understood. When vertical shear is relatively weak or nonexistent at high levels,
ice particles from the anvil can fall back into the storm's updraft and seed
growing convective towers, so that the precipitation process is enhanced; on the
other hand, when shear is relatively strong at high levels, ice particles from the
anvil are blown far downstream and do not seed the same storm in which the ice
particles were formed. Thus, the character of high-level vertical shear is thought to
be important in determining whether or not a supercell belongs to the LP, classic,
or HP archetype. It is possible, however, for ice particles produced in an anvil in
an environment of strong upper shear to fall out into the updraft of a nearby
supercell and seed it, thereby enhancing precipitation in the adjacent storm. Such