WMOD00097

• Applicant: Western Kansas Groundwater Management
• Project Name: Cloud Seeding
• Date: 5/1/1989
• Weather Modification - Doc Type: Report

~

More recently, interest has increased on severe storm development
resulting from "convective scale interactions". The interactions are
capable of producing most storm types mentioned above. In the south-
eastern 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 interaction",

Convective Scale Interaction is 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 can eventually initiate new storm
develop_nt.at so_ distance from the "parent storm". As these gust
fronts spread radially from the precipitation area of the parent
storm, they act like cold fronts as cooler air is thrust outward ahead
of the dissipating storm. Such gust fronts can undercut warm, moist
air and lift it into an unstable atmosphere to create more new severe
thunderstorms. Clouds that form along these moving gust fronts often
align themselves in a semi-circular fashion into "arc-clouds" that 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"),

Often, two or more collapsing storms produce outflow boundaries
that can eventually collide. When that happens, very severe storms
may be brought to life along these intersections and in turn repeat
the process. Although severe turbulence is found along these outflow
boundaries, air in front and above them is usually smooth.

J

There is one other form of cloud system that appears to haVe
important seeding potential to produce precipitation in Kansas: the
multiple celled convective system. This starts as a cluster of small,
weak, air-mass clouds, or storms, that develops over a relatvely small
area---typically about 10 - 30 miles in diameter. If one, or more, of
the clouds grow sufficiently, it can merge with others nearby and
continue growing larger. The added growth allows the merged storm to
continue capturing_additional smaller sto.rms, further increasing its
size and strength. Such storms are eventuallly capable of producing
precipitation over large areas and will persist longer than "average"
thunderstorms. Updrafts initially found within the cluster of cells
are often embedded or difficult to locate, however, once the storm
grows to sufficient size, updrafts generally organize better. This
type of storm system has often responded positively to cloud seeding
and can produce abundant rainfall over 200 - 300 square miles, ar
more, at a time,

Althaugh almast all research an the dynamics af the multiple cell
system has been dane nearby in Texas, earlier radar studies of Western
Kansas clouds by KSU Professar L. Dean Bark from 1972 - 1974 indicate
Kansas is a fertile area for the accurrence af these smaller clusters
af cells---perhaps even better than Texas. As a result, aver the past
four years these targets of oppartunity have been eagerly anticipated.
Time was, nat too. many years ago., that little attentian wauld have
been paid to these weak-appearing cloud clusters. However, over the
past few years we have faund that when we persist in our seeding
efforts we are aften able to produce spectacular results.

i.

9