WMOD00057

• Contract/Permit #: #90-3
• Applicant: Western Kansas Groundwater
• Project Name: Kansas Weather Modification
• Date: 9/13/1990
• Weather Modification - Doc Type: Report

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Air-mass storms often transition into a line of storms
containing a multiple number of cells that eventually exhibit
characteristics approaching t~e squall line variety. Storms growing
into a multicellular line frequently have multiple, shifting
updraft areas that sometimes can make proper cloud b~se treatment
difficult. The location of the important updrafts on these types of
cloud systems are usually, but not always, found along a line in
the frontal portion and frow a few miles to many miles in length.
Other times, the best seeding area is around only one end of the
line of cells. Multiple-celled lines may also appear as remnants of
a weakened squall line or commonly as part of a line of storms
associated with fronts or surface ~roughs.

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The cloud system known as the squall line is an organized line
of cumulonimbus clouds with its important updraft area found along
the leading edge. Squall lines may cross a few counties within a
state or cross more than one state and frequently associated with
surface traugh1ng or frontal passages. Vertical updraft speeds can
easily exceed 2000 feet per minute and often produce updraft "scud"
clouds almost visible to the ground. Inflow areas are often smooth
ahead of the line, however, collapsing storms within the line can
cause severe turbulence as strong outflows push out rapidly ahead
of the line.

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Significant updrafts are seldom found along the trailing edges
of squall lines except occasionally at the end of the line or at
significant breaks within the line.

More recently, interest has increased on severe storm
development resulting from "convective scale interactions". These
interactions are capable of producing roost storm types just
mentioned. In the southeastern U.S.A. one study estimated that 60%
- 75% of the storms existing later in an afternoon on a typical
storm day are the result of being previously triggered by
"convective scale lnteractionH.

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Convective Scale Interaction 1s a term that describes a
process in which a storm collapses, producing both precipitation
and attendant downdrafts strong enough to create a gust front near
the surface of the ground. These gust fronts eventually initiate
new storm development at 50Itl€. eristance from the "parent storm". As
these gust fronts spread radially from the precipitation area of
the parent storm) they act like cold fronts inasmuch as cooler air
1s thrust outward ahead af the dissipating storm. Such gust fronts
can undercut warml moist air and lift it into an unstable
atmosphere to create mare new 5eve~e thunderstorms. Clouds that
form along these moving gust frontg often align themselves in a
semi-circular fashion 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 <also called "outflow boundaries").

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