WMOD00439

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

500

TABLE 1. Cloud selection criteria for HIPLEX-1.

JOURNAL OF CLIMATE AND APPLIED METEOROLOGY

VOLUME 2:3

A Class A-I cloud criteria:

Average cloud liquid water concentration greater than 0.5 g
m-3 over approximately a I-km-Iong cloud region
determined by 10 s of flight at approximately 100 m S-I
(A wco)a
2 Average ice-crystal concentrationb less than 1.0 L -I in the 1-
km-Iong (10 s of flight) cloud region of maximum average
liquid water concentration (AI CO)
3 Maximum ice-crystaI concentrationb less than 5.0 L-t for any
I-km-Iong (10 s of flight) cloud region (defined by FSSP
liquid water concentration greater than 0.01 g m-3) during
the test pass (MICO)
4 Vertical air velocity greater than -1.0 m S-I in the region
defined by item I ,but if the vertical velocity is greater than
10.0 m S-I and the buoyancy is greater than 10C, reject the
candidate (VLLO and BOYO) ,
5 Length of the test penetration more than 2 and less than 8 km
as defined by an FSSP liquid water concentration greater
than om g m",3 (LPNO)
6 No radar echo detectable on the aircraft weather radar (RFLO)
7 Cloud-top temperature lower than -60C but higher than
-120C (CTTO)
8 Cloud-base temperature higher than OOC (CBTO)
9 Minimum separation between the current test cloud and
previous test clouds greater than 15 km to insure the
meteorological independence of the clouds (DBCO)

B Class A-2 cloud criteria:

I Items AI to A9 above
2 An average wind direction between the surface and 800 mb
from 250 to 0400 true (A WOO)
3 A 30-mb-thick stable layer present with its base between 0 and
-lOoC and its top temperature at least 1.50C higher than
the temperature extrapolated from the base of the layer to
the top using pseudoadiabatic ascent (DTCO)
4 A 100C dewpoint depression present somewhere within the 30-
mb layer of B3 above (DPoo)

C Class B cloud criteria:

I Items AI-A3, A5, A6, A8, and A9 abovec
2 Cloud-top temperature lower than -60C but higher than
-200C (CITO)
3 Vertical air velocity greater than -1.0 m S-I in the region
defined by item A I, but no other vertical velocity or
buoyancy restrictions (VVLO and BOYO)

a Symbols in parentheses indicate the names assigned to the vari-
ables in the computer programs.
b Measured by the depolarization signal from the PMS 2D-C probe;
not the regUlar 2D-C image data. Although the depolarization signal
is believed to indicate only about 25% of the actual ice-crystal con-
centration, it was used because the image data include spurious
images that could not be rejected in real time.
C Clouds that failed to meet criteria A4 and A 7 for Class A were
then evaluated as candidates for Class B.

table and elsewhere, the following notation is used to
indicate microphysical probes on the aircraft:

FSSP: A Particle Measuring Systems (PMS) forward
scattering spectrometer probe.
2D-C: A PMS OAP-2D-C two-dimensional optical
array probe with image resolution of 25 ILm.
2D-P: A PMS OAP-2D-P two-dimensional optical
array probe with image resolution of 0.2 mm.

The in-cloud measurements were made by a cloud
physics aircraft at the -80C level or about 300 IlIl
below cloud top, whichever was lower. The physical
separation between the clouds (DBCO) was specified
to eliminate any possibility of microphysical contam~
ination from one test case cloud to another. The severa~
variables were evaluated in real-time computers O]tl
board the aircraft to determine whether each cloud
met the selection criteria for one class or another.

4. Treatment and randomization procedure

The seeding for HIPLEX -I was conducted by drop-
ping a line of dry-ice pellets from a jet aircraft. The
treatment pass (Fig. 2) was made on a parallel track
with, and about 300 meters above the altitude of, the
pre-treatment pass made by the cloud physics aircrafl:,
around the -IOoC level, and had to begin within 2
min of the time when the cloud had passed the selection
tests. The basis for selecting dry ice for the seeding
agent is discussed in Holroyd et al. (1978a), the as-
sociated correspondence (Hobbs et al., 1978; Holroyd
et al., I978b), and Appendix D of the HIPLEX-I design
document (Bureau of Reclamation, 1979). The pellets
were about 1.5 cm in diameter and 0.5-2.5 cm long.
The intended seeding rate was about 0.1 kg per ki-
lometer of flight path.
Among the significant advantages of dry-ice seeding
is the fact that the activity of the seeding material
(_1012_1013 ice crystals per gram of dry ice) is nearly
instantaneous and not strongly dependent on tem~
perature (Hom et a/., 1982). The warmest regions of
the supercooled cloud, where there is a minimum of
competition from the natural ice particles, can bt~
seeded effectively. The seeding can be directed at a
specific region of the cloud, and it is effective only in

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FIG. 2. Illustration of flight procedures for experimental unit se-
lection and seeding. The cloud physics aircraft first penetrated the
, candidate cloud at about the -80C level while the seeding aircraft
determined the cloud top temperature. If the cloud met the selection
criteria (Table I), the seeding aircraft then made one treatment pass
at about the -lOoe level along a line parallel to the cloud physics
aircraft pass, to drop dry ice pellets (in the event of a seed decision).