WMOD00438

• Title: HIPLEX: A Cooperative Program on Rain Augmentation in the High Plains
• Date: 6/1/1984
• Weather Modification - Doc Type: Report

1. HIPLEX RESULTS AND
IMPLICATIONS
FOR THE FUTURE OF
WEATHER MODIFICATION
TECHNOLOGY

HIPLEX (High Plains Cooperative Program) was a
complex program involving studies of both the tech-
nology of summer cumulus cloud seeding to
increase growing season rainfall and the economic
and environmental impact of potential rainfall
increases through cloud seeding.

The HIPLEX approach to the scientific advancement
of cumulus cloud seeding technology was to empha-
size initially the smaller clouds that had some poten-
tial for producing rain and were well enough
understood to permit the statement and testing of
a detailed physical hypothesis of their step-by-step
response to seeding. Such a test was made in
HIPLEX-l. which was conducted at Miles City. Mon-
tana. in 1979-80. Understanding of larger. more
complex cloud systems was and still is insufficient to
permit formal testing of such a hypothesis for those
systems. However. while HIPLEX-l was being car-
ried out. and again during the 1981 CCOPE (Coop-
erative Convective Precipitation Experiment)
program. many larger clouds and cloud complexes
were observed in detail by several aircraft. radar sets.
ground weather stations. and satellite. It was
intended that analyses of these data would lead to
formal physical-statistical experiments with larger
clouds after HIPLEX-l was completed.

The HIPLEX-l physical hypothesis envisaged the for-
mation of ice crystals by dry ice seeding. the growth
of the ice crystals by deposition of water vapor and
then by riming. and the fallout of the resultant
graupel particles. perhaps already melted into rain-
drops. from cloud base. The desigr, of HIPLEX-1 set
forth detailed criteria for the selection and classifica-
tion of test cases and detailed instructions for
seeding and for the collection of observations to be
used in calculating response variables (see sec. 6). It
was expected that the seeding effects would be
easiest to detect in clouds with tops in the -6 to
-12 0 C range that probably would not rain naturally
(type A). but the experimental design permitted
selection of rain clouds with tops in the -1 2 to
-20 0 C interval (type B) as test cases when no suita-
ble type A clouds were available.

Because HIPLEX was ended sooner than expected.
HIPLEX-1 itself was truncated with only 20 official
test cases recorded instead of the 50 to 150
considered necessary at the start to permit strong
statistical conclusions to be drawn. Nevertheless.
certain conclusions could be drawn because of the
step-by-step physical and statistical evaluation

procedures employed (Braham. 1981). The
HIPLEX-1 results were statistically significant only
for the response variables observed by the cloud
physics aircraft in the 5-minute period after seeding.
The physical hypothesis was confirmed up to the
start of the riming process.

Detailed physical studies showed that the LWC (liq-
uid water concentration) generally decl'eased rapidly
following test case selection. The decrease is attri-
buted to mixing with the dry air around the cloud.
Some clouds dissipated entirely; others persisted but
with peak values of LWC too low to make riming effi-
cient. There was evidence that the artifically pro-
duced ice crystals aggregated into snowflakes in
some of those clouds to yield very light precipi-
tation. Rimed particles (graupel) were observed in
only a few of the seeded clouds. and those few were
type B clouds.

In assessing the implications of HIPLEX. it is reasona-
ble to ask what should be done if and when funding
again becomes available for research on convective
cloud modification to increase summer rainfall.
Should HIPLEX-l be completed by similar
experimentation on type A clouds (with tops warmer
than -12 0 C). or should experimentation imme-
diately progress to larger clouds? The Bureau of
Reclamation recommends the latter course.

The primary reason for that recommendation is that
the physical studies have shown that the HIPLEX-1
physical hypothesis cannot be operative in many of
the small clouds treated in that experiment. Most
type A clouds dissipate about 8 to 12 minutes after
treatment time. This does not allow sufficient time
for the precipitation embryos produced by seeding
to reach the riming threshold. grow into graupel. and
fall out as precipitation before the dissipation of the
clouds. This HI PLEX-1 result agrees with those of
recent extensive but nonrandomized experiments
carried out near Yellowknife. North West Territories.
and near Thunder Bay. Ontario. by the Atmospheric
Environment Service of Canada. In those Canadian
experiments. seeding cumulus clouds almost always
produced additional cloud ice and precipitation
embryos. but only a fraction of the Yellowknife
clouds and hardly any of the Thunde,r Bay clouds
ever rained (Schemenauer et a/.. 1982). The
HIPLEX-l results also agree with Braham's findings.
which suggest that only about half of the type A
clouds would last long enough ,for in-cloud precipi-
tation to develop. and. of these. only a fraction would
last long enough for seeding to be effective (Braham.
1960). Therefore. it is not reasonable to expect a
change in the HIPLEX-l results as a result of running
more type A test cases. However. the results on type
B clouds are intriguing; they suggest that in the right
clouds. the physical hypothesis could be followed to