WMOD00510

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

JANUARY 1978

HOLROYD, SUPER AND SILVERMAN

51

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limation rate of pellets in a wind tunnel. Their pellet
density was 1.4 g cm-s. They found that the sublimation
time t (s) was

fTs-25oe)
t = 153Mo.42(
" Ts-Ta

for initial mass M (g). Slower sublimation rates
occurred at colder environmental temperatures Ta.
They appear to have used Ts=-79oe as the dry ice
surface temperature. Their equation may be inverted
and differentiated (ignoring lapse rates) to obtain

dm (Ts-Ta)
--=1.556XlO-zMo.58 -
dt T8-25

(-79-Ta)
=1.556XlO-zMo.58 -- ,
-104

where Ta is the air temperature (Oe). No corrections
for pressure were included in their work.
Both sublimation rates were used in a numerical
simulation to determine the amount of dry ice that
sublimed within the supercooled region of the clouds
investigated. The typical dry ice pellet spectrum,
expressing cylindrical sizes, was converted to a spectrum
of spherical pellets having the same volume or mass.
The terminal velocity of Fukuta et al. (1971),

.

( Td)!
u=1.15X102 p [cm S-IJ,

was used in an atmosphere having a lapse rate of loe
(152 m)-I. For the experiments conducted, dry ice
was typically dropped at -woe and 5800 m (all

(2)

elevations above mean sea level unless otherwise noted),
for which the Mee and Eadie sublimation rate indicates
about 32% of the mass was consumed in the supercooled
regions, assuming no vertical velocity. The Fukuta
et al. sublimation rate is slower as a result of a colder
pellet surface temperature and gives 13% of the mass
consumed in the subzero regions. In. both cases the
remains of the larger pellets arrived at a ground level
surface defined at 800 m. This suggests that a smaller
size spectrum would have resulted in a more efficient
use of the dry ice for the conditions stated. Pellets
equivalent to a 7 mm diameter sphere (the smallest
size considered in the numerical experiment) fell to the
20e level in the simulation using the Mee and Eadie
sublimation rate; they reached the 150e level using
the Fukuta et al. rate.
The dry ice pellets would have a longer residence time
in the supercooled region of clouds with strong updrafts,
and might even completely sublime above the ooe
level in very vigorous turrets. However, the clouds
sampled in these experiments typically had both
updrafts and downdrafts of only light to moderate
( < 5 m S-I) magnitude.

(3)

(4)

c. Ice crystal counters

The clouds investigated were sampled with several
types of airborne instruments. They are listed in
Table 2 with abbreviations used later in this text.
Some of the instruments could discriminate between
water and ice while others yielded particle sizes only.
Each of the instruments has distinct capabilities and
limitations. The physical samplers (foil, Formvar,
oil-coated slides) are reasonably unambiguous in their
records, but they are laborious to examine and some-
times require interpretation. The electronic samplers

TABLE 2. Instruments used for detecting or inferr:ing ice crystal concentration.

Instrument*

CSI

Aircraft operator
MRI UW CSIRO

IPC
CSI-IPC
UW -IPC
PMS2

Ice particle counter (polarized light)
a. CSI version, vertical beam
b. UW version, diagonal beam
PMS I-D probe (20-300 }Lm) (optical array)
a. vertical beam
b. horizontal beam
PMS I-D probe (300-4500 pm) (optical array)
a. vertical beam
b. horizontal beam
Foil impactor (various brands)
Formvar replicator
Decelerator, oil-coated slide impactor
(University of Wyoming system)

PMS3

Foil

Slide

x

x

x

x

x

x

x

x
S
S

s

* Continuous operation, x; selected operation, s.
CSI: Convergence Systems, Inc.
wIRI: Meteorology Research, Inc.
UW; University of Washington.
CSIRO: Commonwealth Scientific and Industrial Research Organization.
PMS: Particle Measuring Systems.

.

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