WMOD00439

• Title: HIPLEX-1: Experimental Design and Response Variables
• Date: 4/4/1984
• Weather Modification - Doc Type: Report

APRIL 1984

SMITH ET AL.

505

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FiG. 3. Illustration of the first post-treatment pass ("2-min pass")
by the cloud physics aircraft. After the treatment pass by the seeding
aircraft, the cloud physics aircraft penetrated the cloud at about the
-goc level in an orthogonal direction that would intersect the curtain
of ice crystals produced by the dry ice pellets. Meanwhile the seeding
aircraft continued to monitor the development of the cloud top:

for the various measurements, because several of the
response variables are expressed in terms of times since
treatment.
Close coordination of the multiple aircraft and radar
operations was needed in order to collect compatible
data on the same cloud for the case duration of ap-
proximately 45 min. The navigation systems on the
aircraft made it possible to repeatedly sample the de-
sired portion of the selected cloud, even as it drifted
with the environmental winds. Thus it was possible in
most cases to follow the development of the seeded
plume within a cloud over times exceeding 15 min.

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b. Execution of the procedures

The experimental procedure worked fairly well in
practice. Table 4 presents the values of the "selection
variables" determined for each HIPLEX-I test case
cloud. More than half of the clouds had average liquid
- water concentrations (A WCO) under I g m-3, indicating
that the overall sample comprised relatively dry clouds.
Within Class A-I, all three of the nonseeded cases had
AWCO> I g m-3, while two of the four seeded cases
had A WCO < I g m-3. On the other hand, the A-I'
seeded cases generally had stronger updrafts and greater
buoyancy than the nonseeded cases. The distributions
of values among the larger sample of Class B clouds
are better balanced in these respects.
The measured initial ice particle concentrations
(AICO) were all 0.1 L -lor less, except for Case 7. That
case had negative buoyancy, and the ice-particle con-
centrations decreased on subsequent penetrations

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around the-80C level (cf. Table 6); The vertical air
motions (VVLO) ranged from slight downdrafts to an
updraft of just over 10 m S-I, with the median being
a 4 m S-I updraft. The cloud widths (LPNO) ranged
from just over 2 km to a maximum of 7.9 km, with
a median of about 3.2 km.. The median A-I cloud
width was 2.8 km, compared to a Class B median of
3.4 km; such a difference is consistent with the overall
selection criteria for the two classes. The Class B seeded
clouds were generally broader than the nonseeded
clouds, and had somewhat colder cloud bases. The
overall cloud base temperature (CBTO) ranged from
+ 1.5 to + II oC, with a median of +60C.
An error was made in the selection of Test Case
No. 16 (16 June 1980), and it should not have entered
the HIPLEX-I experiment. The error arose from the
measurement of vertical air motion. The system nor-
mally used for real-time determination of the vertical
wind was based on a vertically-stabilized accelerometer;
during the 16 June flight, the circuit breaker controlling
power to that accelerometer was open. This caused
the calculated vertical wind to have a large positive
value, and the cloud passed the real-time selection
criteria for Class A-I. However, when the data were
reprocessed using a technique that does not depend
on the accelerometer, the resulting updraft at the target
point was found to be -4.2 m S-I, a value low enough
to have disqualified the cloud.
Throughout HIPLEX-I, the real-time measurements
of vertical wind were based on the technique described
by Cooper (1978a), while subsequent data processing
was based on the improved scheme described by Rodi

FIG. 4. The cloud physics aircraft made passes through the cloud
at the -80C level at about 2 and 5 min after treatment, along re-
ciprocal tracks. It then descended to the - 5 oC level for a series of
passes at about 8, II, 14, and 17 min after treatment. All these
passes were made along tracks orthogonal to the expected ice crystal
curtain. The seeding aircraft remained at cloud top until about 10
min after treatment.