WMOD00510

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

.

I

JANUARY 1978

H 0 L ROY D, SUP ERA N D S I LV E R.M A N

57

.

poorly sampled. On two other days with CO2 seeding
some natural clouds, usually dissimilar to the seeded
clouds, were also sampled. Those days, however, are
also days excludable for poor sampling of the seeded
clouds or for suspected ice multiplication. Cloud 1
on 8 December 1972 was a noteworthy layer cloud in
that the only snow observed to fall anywhere in the
region of this altocumulus cloud was from the seeded
line. In summary, it is possible that some of the ice
crystals sampled in the seeded cloud in the Australian
experiments resulted from causes other than seeding.
In Table S the air temperature at the seeding level
can be considered to approximate the cloud-top
'temperature at the time of seeding; the true value may
be up to SOC colder, as in the 8 July 1976 case. None of
the adequately sampled clouds appreciably increased
their cloud-top altitude after seeding; so for them the
seeding level temperatures also approximate the coldest
temperatures achieved. Cloud-top temperatures may
also be approximated by multiplying the cloud thickness
above OOC in Table 4 by a moist adiabatic lapse rate
appropriate to the region of SOO mb and - lOoe. The
Australian experiments lacked temperature soundings
above the flight levels, so that the true cloud top
temperatures are unknown for those cases. Cloud-base
temperatures for all experiments were at or warmer

,
I
e
I
... 0
\I)
ct
1&I en
CI)
E ....
oX
..10
UJ
0
Z
~
(f)
5
w
> l\I
j::
<t
..J
W
Q:
0
... ;-
\I)
f I&J
~N
2340

KEV:

LIQUID WATER CONTENT
I .-
0..02 ()5 2 4gm-3

than O"C except for 8 December 1972 cloud 1. Base
temperatures could not be monitored during the
single-aircraft Australian experiments. '
The four reliable American experiments show an
average nucleation effectiveness of about 2 X 1011
crystals per gram, using the Mee and Eadie (1963)
sublimation rate. Using the Fukuta et at. (1971)
sublimation rate raises this average to SXlOll. These
estimates are supported by the reliable Australian
values, which average a factor of 6 lower but which
must still be normalized upward to correct for the
expected lower crystal counting efficiency of the foil
impactor. Corrections for the absolute ice particle
sampling efficiencies for all instruments and for the
numbers of crystals within aggregates have not been
applied and should raise the effectiveness values further.

5. Discussion of the 8 July 1976 experiment

A pa.tch of tall altocumulus clouds was selected for
dry-ice seeding on 8 July 1976. There were no towering
cumulus clouds in the area capable of raining to the
ground. There were a few cumulonimbus clouds visible
over 100 km away to the north and northwest.
Four aircraft were involved in the study. The seeder
and a doud physics aircraft were near cloud top and

ICE PARTICLE CONCENTRATION

I I I
o 40 80 160 (I

.....
r
. .
. '.
0, '"
..... '" '.,,'
i ~i i
13 /5 /7
55 0000 05 10 15 20 25 30 35 0040
TIME., GMT

FIG. 4. A length-time diagram of the seeded cloud of 8 July 1976. The Cloud was penetrated through the center of the cloud at the
indicated times in a generally east-west orientation. The heavy line represents the boundaries of the liquid cloud; the shaded regions
give contours of liquid water content. The thin line represents the boundaries of the ice cloud; the segmented lines and the interior thin
solid line give the contours of ice crystal concentration. Mid-level convection continued to stir the crystals and regenerate the liquid
cloud for nearly 1 h. .

l~.,l":ii "'~"" . ..:.'f,',': ,i..' ,.;, ,,;"'';;,i.i... ".' ....
,..., :, '"'~'"", '=,"".lL~V,">'A'%'.d ","..; ,:,.j,.(l;~.:, c. ......€i,;iiliIl"i:"""."..,;"""....,'......,. :....:. .,)
-..~ "","",.;.~_~,' ..~."" :"-"'1s-.~::-~~t.:::.;.."