WMOD00510

• Title: The Practicability of Dry Ice for On-Top Seeding of Convective Clouds
• Date: 1/1/1978
• Weather Modification - Doc Type: Report

52

JOURNAL OF APPLIED METEOROLOGY

VOLUME 17,

TABLE 3. Approximate normalization of ice concentration
measurements (CSI's IPC= 100 i-I).

Counter code

Date

IPC
CSI UW

PMS2
CSI UW

16 June 76
8 July 76

100
100

23

42
33

50 i-I
i-I

[ice particle counters (IPC) and Particle Measuring
Systems (PMS) probes] readily produce data (Turner
and Radke, 1973; Knollenberg, 1970), but spurious
measurements can occur. Scattered sunlight and large
waterdrops can increase the counts of the IPC. devices,
and an adjustable detector threshold prevents detection
of miniature crystals. Theoretically, crystals having
their c axis parallel to the laser beam should not be
detected at all. Heavily rimed crystals may block the
beam rather than rotate it and thus escape detection.
The PMS one dimensional counters cannot distinguish
among water drops, ice and artifacts. Crystals whose
smallest dimension is small compared to a channel
width may not be counted. The 50% light extinction
threshold may allow some flat transparent crystals to
escape detection. Intercomparisons of all the instru-
ments in the same cloud are therefore needed.
For a preliminary intercomparison, data from two
abundantly glaciated clouds were examined. One was
a natural cloud top penetrated by both the Convergence
Systems, Inc. (CSI) Aero Commander and the Univer-
sity of Washington (UW) B-23 on 16 June 1976. The
cloud had crystal concentrations well in excess of
100 t-1. The other was a dry ice seeded cloud penetrated
by CSI on 8 July 1976 and having typical concentra-
tions of about 45 t-1. CSI's ice particle counter almost
always yielded the highest readings. Table 3 presents
the approximate measurements of three other instru-
ments when CSI's IPC is normalized to 100 crystals
t-1. These normalization factors have a variance of at
least 25%, due especially to crystal size and type, and
should be used as general guides only. The instruments
have been normalized to CSI's IPC for convenience and
because all optical counters are likely to miss some
crystals; unless a counter gives multiple counts for
some crystals or has an incorrectly determined sample
volume, the highest counting instrument should be
most nearly correct. The time constant on the CSI-IPC
lets ice particles up to 1 cm diameter be counted only
once.
On the 16 June 1976 intercomparison flight, the
two aircraft flew a similar but not necessarily identical
track through the same cloud with about a 3 km
separation between aircraft. Intercomparison data
between the other instruments in Table 2 have not
yet been examined. In discussions with CSI and UW
personnel it was agreed that the UW-IPC instrument
was less sensitive than the CSI-IPC; its threshold was

adjusted to ensure no counting of large water drops but _
at the expense of not counting all small crystals. .,
Cold room calibrations (CSI, personal communica-
tion) of the CSI-IPC (modeled after the UW version)
showed near zero counting efficiency for 50 J.lm diameter
crystals and about 20% counting efficiency for 150 J.lm
diameters for presumably planar crystals grown
between - 16 and - 20oC. A personal communication
from PMS personnel confirms a counting efficiency
of about 20% for depolarization instruments, with
variations due to crystal type and orientation. The
efficiency-size trend was confirmed on the 8 July 1976
experiment. While the crystals were still very small
immediately after seeding, the CSI-IPC counted fewer
crystals than the PMS2 instrument; after the crystals
had grown, it counted more than the PMS2 instrument.
These efficiency corrections have not been applied to
the data discussed herein; to apply them would, of
course, increase the crystal concentration values given.
The CSIRO foil impactor (Bethwaite et at., 1966;
Hobbs, 1977) was cautiously used because it did not
have heaters to prevent rime from building up and
immobilizing the shutter or tearing the foil. It was
rarely operated on initial passes to avoid the rime
problems. Its use was delayed until the cloud began
to visibly glaciate or could be assumed to have a
significant consumption 'of supercooled water by the
growing crystals. The natural background of ice
crystals at the time of seeding is therefore not available a
for most Australian clouds. The CSIRO impactor was .,
operated at about 6 s intervals with an exposure time
of about 0.2 s to avoid overexposure of the foil. These
values were varied slightly from year to year but were
precisely determined with an oscilloscope.
No absolute calibration of the collection efficiency
of the foil impactor was made. The 25 J.lm thick
aluminum foil was examined under an 8 to 40 X
microscope. The illumination was adjusted so that the
clean foil appeared bright and any dents dark. Unex-
posed portions were also examined for comparison.
On 8 December 1972 an altocumulus cloud was
seeded along a line with dry ice. The - 70C columns
produced were sampled 12-36 min later. Linear
impressions of random orientation could be observed
among the dents in the exposed portions of the foil.
It is assumed that these are crystal length impressions.
A spectrum of these lengths (sample size 829 impres-
sions) is shown in Fig. 2. Ryan et at. (1974) have
indicated that -70C columns can reach these lengths '
in about 5 min. The lower sizes represent either the
smallest sizes sampled or the threshold of detecta.bility
on foil, or a combination of the two.
In Fig. 2b the cumulative distribution appears to be
lognormal (straight line). The departures from the line
are possibly indicative of irregularities in the selection
of crystals to be measured. Departures to the left of
the line at small sizes, had they occurred, could have
been indicative of possible truncation due to insensitiv- _