WMOD00442

• Title: A Model of Hygroscopic Seeding in Cumulus Clouds
• Date: 12/12/1978
• Weather Modification - Doc Type: Report

1764

JOURNAL OF APPLIED METEOROLOGY

VOLUME 17

10
9
8
7.
E
.x
6
:r:
l-
e..
w 5
0
0
::J 4
0
-'
u
3
2

~ ~~~ ~~ ~
<:> V' ,0 V' nO V' 0.0 V' -00 V' 00 V' 00 V'
.. ~ 'lJ 0.
15 1510 5

I 2 3 4 5 6 7. 8 9 10

15
UPDRAFT (m s-I)

-150 C

50C

8m= 23, 20C
BASE=3km
M.R. = 11.3 g kg-I

20

25

30

FIG. 5. As in Fig. 4 except for cooler, drier cloud.

30

51""INITIAL SIZE
em~23.2.C
BASE ~ I km
MR~16.2 9 kg-I

- -- - 51""INITIAL S'ZE
em ~ 16.C
BASE~2km
M.R.~7.6g kg-"

E
.5.

20

25

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w
~ 20
f'!
I-
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a:
w 15
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w
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X 10
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/ \
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10 15
UPDRAFT (m ,-II

FIG. 6. Variation of the maximum drop size attained as a
function of updraft speed for initial hygroscopic drop diameter
of 5 ILm for moist and relatively dry cloud conditions (model
computation).

25

5. Comparative analyses

a. Comparison with natural embryos and AgI nuclei

Prior to seeding, a determination has to be made
of the suitability of a particular day for hygroscopic
seeding. Even if a given day is considered suitable,
it may well be that only clouds that have specific
physical characteristics are deemed seedable. Such
parameters as cloud-base height and temperature,
cloud thickness and diameter, updraft and cloud-water
profiles, and cloud droplet spectra all play a very
important role in determining' the efficiency with
which hygroscopic seeds will act to stimulate addi-
tional precipitation from a cloud.
It is a very difficult task to determine just how
much help a particular seeding effort may have con-
tributed to the total precipitation from a cloud system.
There are still many uncertainties to be resolved
before this problem is solved. For instance the sto-
chastic theory for the production of large droplets
(> 100 ~m) does not always adequately explain the
formation of these droplets in the times observed.
In-cloud measurements have shown particles of this
general size to already exist at cloud base in concen-
trations of 0.5 to 1.0 e-I in Texas, Ariwna and South
Dakota (Takeuchi, 1972). Others have also found
evidence of the existence of large cloud droplets
(40-100 ~m) near the bases of cumulus clouds (e.g.,
Ludlam, 1959; MacCready and Takeuchi, 1968;
Kopcewicz, 1965; Rosinski and Kerrigan, 1969). Ac-
cordingly, it might be more appropriate in the present