WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

M
';:E
U

W 100
..J
U
:::J
Z

> 0.8
u
z:
w
~
g 06
a:
LL

~. 04
~
-'
:::J
?i 02
u

- 52 -

1000

"

(c~

10
~ I ro
SUPERSATURATION (PERCENT)

~.: (a) Experimentally measured supersatu-
ration spectra from aircraft measurements in
different geographic regions. Notg the system-
atic difference between maritime ~nd continent-
al air. The numbers on these curves indicate
the region. as follows: 1 = continental. Austra-
lia; 2 = continental. U.S.; 3 = CQntinental.
Africa; 4 = oceani~. South Pacifid; 5 = ocean-
ic. North Atlantic; 6 = oceanic. South Atlantic:
7 = oceanic. North Pacific. (b) Distribution
of the slope parameter k among the data leading
to the previous figure (From Twom~y. 1977).

',0 (b)

o (}2 04 06 08 10 12
AVERAGE SLOPE do OF NUCLEUS SPECTRA

"
i

3. Ice nuclei (IN)

Contrary to the fact that only small sup.ersaturations are possible in the atmosphere due
to the omnipresence of CCN. large supercoolings of the liquid droplets can occu~ due to
the scarcity of a rather special type of foreign particle. called ice nuclei. The fact that
natural clouds have a resistance to freezing is contrary to common experience. which indi-
cates that wat.er freezes when the temperature drops below 00 C. Our' experience ~s based
on observations of bulk water. in which a single nucleation event anywhere suffices to
cause the entire mass to freeze. A cloud. however, is an unusual system in whicr the
water mass is distributed over a large number of very small droplets. each one pf which
must experience a nucleation event somewhere within its boundffiies before the cloud can
become entirely frozen. A supercooled droplet is in an unstable state. In order: for freez-
ing to occur. enough water molecules must collect together within the droplet to form an
embryo of ice large enough to survive and grow. Since the formation of the ice ~tructure
is aided by the ice nucleus. heterogeneous nucleation can occur at much higher temperatures
than homogeneous nucleation without the aid of an ice nucleus (Thom ~ -400 C). ~ce nuclei
can initiate the new phase in 3 different modes:
1) The nucleus is contained within the droplet: freezing nucleus.
2) The nucleus in the air comes into contact with the droplet: contact nucleus.' There is
some evidence that a particle can cause a drop to freeze by contact nucleation at a
temperature several degrees higher than it would if it were embedded in the ~rop.
3) The nucleus serves as center upon which ice crystals form directly from the ~apour phase:
deposition (inverse sublimation) nucleus.

~