WMOD00132

• Contract/Permit #: #95-5
• Applicant: Western Kansas Groundwater
• Project Name: Kansas Weather Modification
• Date: 1/1/1995
• Weather Modification - Doc Type: Report

Air-mass storms can transition rapidly into a line containing
multiple cells exhibiting some characteristics similar to that of
a squall line. During the development of these multi-cellular
lines} cloud base updrafts frequently shift location although still
are found around several of the individual cells comprising it.
making proper cloud base treatment difficult. The location of the
important updrafts on these type of cloud systems usually develop
along a line in its frontal portion and can be many miles in
length. Other times, the best seeding area may be around only one
end of the line. MUlti-celled lines may also appear as a remnant
of a weakened squall line or as part of a line of storms associated
with fronts, surface troughs and storm outflows.

The cloud system known as the squall line is an organized line
of cumulonimbus clouds with important updraft areas found along its
leading edge. Updrafts pertinent to the precipitation process (and
hail) are seldom found along the trailing edges of squall lines
except at the end of a line or at significant breaks within it.
Squall lines c~n be extensive, crossing a few counties in a state,
or even several states; frequently they are associated with surface
troughing or frontal passages. Cloud base updrafts can exceed 2000
feet per minute and produce "scud" clouds visible nearly to the
ground. Inflow areas are usually smooth ahead of the line; however,
when storms collapse within the line strong outflows are pushed out
rapidly ahead of the line, usually causing severe turbulence.

Convective Scale Interaction is a term describing a process in
which a collapsing storm produces both precipitation and associated
downdrafts, downdrafts which are strong enough to create a gust
front (or outflow boundary) which initiates new storm growth some
distance away from its "parent storm", even up to a hundred miles
or more. As storms collapse, a considerable volume of air is thrust
below cloud base which often undercuts warm, moist air as it fans
out below the cloud base lifting the warm, moist air ahead of it
into conditionally unstable air in which new storms can grow
rapidly, mature, collapse and produce its own gust front, thereby
repeating the same process. Satellite views of clouds seen forming
along these moving gust fronts which align themselves in a semi-
circular, fan-shaped orientation are called "arc-clouds" and can
develop into large, severe convective storm systems. Single storms,
multiple storms and supercells all have been identified as forming
along these gust fronts. One study in the southeastern U.S.A.
estimated 60% - 75% of the storms existing in late afternoon on a
typical storm day were caused by scale interaction. Two, or more,
colliding gust fronts frequently create extremely severe storms.
However, despite the severe turbulence found between the storm and
the leading edge of the gust front, the air in front of and just
above the gust fronts is usually very smooth for cloud base seeding
aircraft. Scale interaction as described here occurs frequently on
the WKWM Program and when identified on radar or seen by pilots,
its occurrence and direction of movement is monitored carefully for
being a preferred area for new severe storms to begin developing.

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