Geoscience Reference
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component, is an indicator of possible severe weather. In eastern Massachusetts,
however, an easterly wind transports relatively cool air off the Atlantic inland,
decreasing CAPE and increasing CIN, although vertical shear might be increased
if the winds aloft have a westerly component. The forecasting rule that upslope,
easterly flow in eastern Colorado is often associated with severe weather does
not work in eastern Massachusetts. On the other hand, the basic physics and
thermodynamics of severe convection are independent of location.
Forecasters of severe convection therefore often use what is called an
''ingredients-based'' methodology. The ''ingredients'' are the necessary, but not
sucient, physical conditions for the occurrence of severe convection. The ingredi-
ents are not to be confused with diagnostics, which are the measured quantities
that can be used to determine if the ingredients are present.
The main ingredients for supercells are adequate moisture for storm initiation,
CAPE, vertical shear, and mesoscale ascent or surface heating or both. Without
adequate moisture, there will be no condensation and thus no clouds at all. The
minimum moisture content needed depends upon the vertical profile of tempera-
ture; surface dewpoints of 50 F or higher are usually needed, though severe
convection above elevated terrain can occur with surface dewpoints as low as the
40-49 F range. CAPE is necessary for the release of conditional instability in deep
cumulus and cumulonimbus clouds. What constitutes the minimum CAPE (for air
parcels originating in the well-mixed boundary layer) needed is not clear, but
observational studies show that a minimum of 500-1,000 j kg 1 is usually required
( Figure 7.1 ); in some locations (such as Europe or the West Coast of the U. S.
during the winter) tornadoes can occur when CAPE is very low. Deep-layer shear
is probably the most important requirement for supercells ( Figure 7.2 ) when the
other two ingredients for cumulus convection in general are present. Vertical shear
in the lowest 6 km of at least 20m s 1 is usually necessary ( Figure 7.2 ) for typical
hodographs in the Great Plains of the U. S., though some variations occur
depending on the depth of the layer used and the shape of the hodograph.
Some forecasters have found the product of vertical shear and CAPE or other
combinations of vertical shear and CAPE to be useful. Mesoscale ascent or
surface heating or both are necessary to trigger cumulus convection. One cannot
set a threshold for the intensity of the ascent or the net amount of surface heating
needed because they depend on the nature of the vertical profile of temperature
and moisture.
It is not known precisely how to distinguish tornadic from non-tornadic
supercells yet, but strong low-level shear/storm-relative environmental helicity
( Figures 7.3, 7.4 ) , and especially low-level shear normal to the shear above, is
suspected as a possible ingredient. ''Significant'' tornadoes (those inflicting F2 or
greater damage) tend to occur when the surface-1 km shear exceeds a threshold
value and the mixed-layer LCL height is not too high ( Figure 7.4 ) .
The ingredients needed for non-supercell tornadoes—or large hail or strong
straight-line winds at the surface—depend on the nature of the parent storm.
Isolated convective cells and mesoscale convective systems can produce severe
weather via different physical processes. The reader is encouraged to reread
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