Weather Mod Critical Issues Report

• Project Type: General OWC
• Project Name: Weather Modification
• Title: Critical Issues in Weather Modification Research
• Date: 1/1/2004
• Water Conservation - Doc Type: Final Report

-

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

3. Supplemental Information to the NRC Report

3.1 Hail suppression

Extensive research has been accomplished regarding hailstorms and hailstone
growth since the 1970's, The National Hail Research Experiment (NHRE), conducted
from 1972 through 1976, produced two volumes devoted to the topic (Knight and
Squires, Eds., 1982). Volume I concentrated on the general aspects of hailstorms of the
central High Plains and Volume 11 on several case studies of hailstorms observed during
NHRE. Many field projects and scientific studies were conducted in western Canada
during the Alberta Hail Project (Renick, 1975) in the 70's and 80's. In Switzerland the
Grossversuch hail experiment was run for five years during this period and produced
many research papers (Federer et aI., 1986). Numerous studies of convective storms
continued through the 80's and 90's with several hailstorms among the sampled storms in
the Cooperative Convective Precipitation Experiment (CCOPE), the North Dakota
Thunderstorm Project (NDTP), and the North Dakota Tracer Experiment (NOTE)
programs. Studies of these storms and the growth of hailstones within the storms have
led to the refining of several of the hail suppression concepts that guide most current
operations. A recent review of hailstorms by Knight and Knight (2001) concentrates on
the growth of hailstones, A worthwhile review panel response follows that review, and
elaborates on several of these hail suppression concepts. The Knights point out that there
are nearly 1500 literature citations keyed to hailstorms and hailstones in the period from
1976 to 1996.

3.1.1 Hail suppression concepts

The NRC review panel failed to discuss the rationale and any conceptual model
for hail suppression. We provide such a discussion here, basing it largely on a World
Meteorological Organization (WMO) report (WMO, 1996), and the Board on
Atmospheric Sciences and Climate (BASC, 2001) report, which in turn depended on the
many research studies and field experiments reported in the literature in the past 30 years.

Three ingredients are necessary to produce hail: (I) the raw material from which
the stones develop (supercooled liquid water, or SL W), (2) nascent hail embryos
(commonly graupel and/or frozen raindrops), and (3) updrafts of sufficient magnitude to
support the growing hailstones, If any of the three are absent, hail does not develop.
When all three are present, the hail growth is limited by the available SLW, and/or the
updraft strength, It logically follows that ample SL Wand updraft, coupled with limited
numbers of hail embryos, will result in the largest hailstones the updrafts can support.
When the hailstones grow to the maximum mass supportable by the updraft, they begin to
descend, If the stones are not too large and the subcloud layer warm, significant melting
occurs during descent, and those hailstones reaching the ground are likely to be small.

Thus, the most often cited hail suppression concept is intended to increase the
numbers of nascent hail embryos, and thus, through competition, reduce the amount of
supercooled liquid available to grow hail. Instead of growing hailstones large enough to

9