THE ROLE OF INSOLUBLE PARTICULATES IN THE ICE CRYSTAL FORMATION IN A SUPERCOOLED WATER CLOUD

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THE ROLE OF INSOLUBLE PARTICULATES IN THE ICE CRYSTAL FORMATION IN A

SUPERCOOLED WATER CLOUD

H. Andriambeloma, R. Montmory, J. Podzimek

To cite this version:

H. Andriambeloma, R. Montmory, J. Podzimek. THE ROLE OF INSOLUBLE PARTICULATES IN THE ICE CRYSTAL FORMATION IN A SUPERCOOLED WATER CLOUD. Journal de Physique Colloques, 1987, 48 (C1), pp.C1-369-C1-374. �10.1051/jphyscol:1987151�. �jpa-00226297�

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JOURNAL D E PHYSIQUE

Colloque C1, suppl6ment au n o 3, Tome 48, mars 1987

THE ROLE OF INSOLUBLE PARTICULATES IN THE ICE CRYSTAL FORMATION IN A SUPERCOOLED WATER CLOUD

H. ANDRIAMBELOMA, R. MONTMORY" and J. PODZIMEK

Graduate Center for Cloud Physics Research, University of Missouri, Rolla, MO 65401, U.S.A.

" ~ d b o r a t o i r e de Physique des Pr6cipitations U.S.T.M.G., Grenoble, Domaine Universitaire, F-38402 St-Martin-d'H&res Cedex, France

R6sume - Dans c e t t e e t u d e , nous d i s c u t o n s brievement du mecanisme de l a n u c l e a t i o n de l a g l a c e s u r AgI e t PbI2, e t d e s l i m i t e s d ' a p p l i c a t i o n d e s t h e o r i e s e x i s t a n t e s pour e x p l i q u e r l e taux de g l a c i a t i o n dans un nuage surfondu. Une a t t e n t i o n p a r t i c u - l i 6 r e e s t a u s s i p o r t e e s u r l ' e f f e t d e s p a r t i c u l e s i n s o l u b l e s i n e r t e s q u i f a v o r i s e n t l a c o n g e l a t i o n d e s g o u t t e l e t t e s p a r simgle c o n t a c t . Le mecanisme de d e p o s i t i o n de c e s p a r t i c u l e s s u r l e s g o u t t e l e t t e s d ' u n nuage surfondu s ' e f f e c t u e p r i n c i g a l e m e n t p a r un mouvement Brownien e t p a r d e s f o r c e s p h o r e t i q u e s .

A b s t r a c t - B r i e f l y , we d i s c u s s e d t h e main mechanism o f i c e n u c l e a t i o n on AgI and PbI2, and t h e a p p l i c a b i l i t y of t h e e x i s t i n g t h e o r i e s f o r e x p l a i n i n g t h e r a t e o f g l a c i a t i o n i n a supercooled w a t e r cloud. A t t e n t i o n i s a l s o p a i d t o n a t u r a l inadver- t e n t cloud s e e d i n g by i n a c t i v e i n s o l u b l e p a r t i c u l a t e s which through s i m ~ l e c o n t a c t and g e o m e t r i c a l c o n f i g u r a t i o n can speed up t h e d r o p l e t f r e e z i n g . The mechanism of p a r t i c l e d e p o s i t i o n and a c t i o n i n a supercooled w a t e r cloud i s most e f f e c t i v e l y through Brownian motion and p h o r e t i c f o r c e s .

1. I n t r o d u c t i o n

S e v e r a l works p o i n t e d o u t t h a t t h e p r i n c i ~ a l mode of i c e n u c l e i forming i s t h e f r e e z i n g mode, o r i n o t h e r words, t h e i c e n u c l e i i n i t i a t e e s s e n t i a l l y t h e i c e phase from i n s i d e a supercooled w a t e r drop o r a t c o n t a c t w i t h t h e supercooled drop s u r f a c e .

Thus, t h e presence of i c e n u c l e a t i n g and i n a c t i v e i n s o l u b l e p a r t i c u l a t e s i n a supercooled w a t e r cloud can under f a v o r a b l e c o n d i t i o n s speed up t h e g l a c i a t i o n pro- c e s s . On t h e o t h e r hand it can modify t h e t r a n s p o r t of w a t e r vapor towards t h e s u r - f a c e of t h e embryo. The aim o f t h e p r e s e n t paper i s t o t r y t o e x p l a i n t h e r o l e of s u i t a b l e p a r t i c u l a t e s i n t h e i c e c r y s t a l formation i n a supercooled w a t e r cloud.

F i r s t , w e w i l l d i s c u s s some r e s u l t s of i c e n u c l e a t i o n s t u d i e s on AgI, PbI2 and o t h e r i c e n u c l e a t i n g s u b s t a n c e s , and t h e l i m i t e d a p p l i c a b i l i t y o f t h e e x i s t i n g the- o r i e s f o r e x p l a i n i n g t h e r a t e of g l a c i a t i o n i n a supercooled w a t e r cloud. Second, we w i l l examine t h e f a v o r a b l e c o n d i t i o n s t o speed up t h e g l a c i a t i o n by t h e presence of s u i t a b l e p a r t i c u l a t e s a t o p t i m a l s i z e and c o n c e n t r a t i o n i n a c l o u d , and by t h e e x i s t e n c e o f an e f f e c t i v e mechanism o f p a r t i c l e d e p o s i t i o n on t h e s u r f a c e o f supercooled d r o p l e t s .

2. The mechanism of i c e n u c l e a t i o n on AgI, PbI2 and o t h e r i c e n u c l e a t i o n s u b s t a n c e s Drost-Hansen ( 1 ) p o i n t e d o u t t h e e x i s t e n c e of o r d e r e d elements s i m i l a r t o w a t e r n e a r c e r t a i n solid/aqueous i n t e r f a c e s . The o r d e r i n g i s eptended i n t o t h e b u l k l i q u i d and d e c r e a s e s a s a f u n c t i o n o f d i s t a n c e from t h e i n t e r f a c e . Re showed t h a t thermal anomalies f r e q u e n t l y o c c u r i n t h e p r o p e r t i e s o f w a t e r n e a r i n t e r f a c e s and claimed t h a t t h i s may be taken a s evidence f o r t h e e x i s t e n c e of o r g a n i z e d u n i t s of w a t e r

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987151

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Cl-370 JOURNAL DE PHYSIQUE

near i n t e r f a c e s . According t o Drost Hansen, t h e o r d e r e d u n i t s n e a r some t y p e s of i n t e r f a c e s may b e c a g e - l i k e c l a t h r a t e s , o r h i g h p r e s s u r e i c e t y p e s .

The n u c l e a t i n g e f f i c i e n c y of c e r t a i n m a t e r i a l s (such a s AgI) may b e due t o f a c i - l i t a t i n g t h e n u c l e a t i o n i n t h e d i s o r d e r e d w a t e r between t h e v i c i n a l w a t e r and t h e b u l k s t r u c t u r e . Because of t h e c l o s e resemblance of i t s molecular s t r u c t u r e t o t h a t of i c e , s i l v e r i o d i d e (AgI) h a s been used e x t e n s i v e l y f o r t h e i n i t i a t i o n o f t h e i c e phase i n supercooled w a t e r clouds i n t h e atmosphere s i n c e t h e d i s c o v e r y of i t s i c e n u c l e a t i n g a b i l i t y by Vonnegut i n 1947. The n u c l e a t i n g a b i l i t y of a p a r t i c l e may be s p e c i f i e d a s t h e temperature below O°C a t which i c e n u c l e a t i o n t a k e s p l a c e .

Experimental i n v e s t i g a t i o n o f i c e n u c l e a t i o n on s u b s t r a t e s made by Isono and I s h i z a k a ( 2 ) p o i n t e d o u t t h a t t h e v a l u e s o f s u p e r s a t u r a t i o n r e l a t i v e t o i c e a t which i c e c r y s t a l s f i r s t appear on t h e s i l v e r i o d i d e s u b s t r a t e s depend on t h e i r c r y s t a l i n e s t r u c t u r e s and f a c e s . According t o F l e t c h e r ( 3 ) t h e (0001) ,plane of AgI i s n o t s u i - t a b l e f o r i c e n u c l e a t i o n ; t h i s i s due t o t h e c o n f i g u r a t i o n a l e n t r o p y d e c r e a s e a t t h e embryo o f a n i c e c r y s t a l on t h i s k i n d o f plane. Experimental v e r i f i c a t i o n s of this p r o c e s s were c a r r i e d o u t by Montmory and Andriambeloma ( 4 ) on f l a t s u r f a c e s of PbIz d e p o s i t e d by e p i t a x y on mica. The same r e s u l t h a s been o b t a i n e d by Andriambeloma ( 5 ) who showed t h a t i c e c r y s t a l n u c l e a t i o n on (0001) p l a n e of AgI s u b s t r a t e r e q u i r e d a g r e a t energy, and i c e c r y s t a l s appear p r e f e r e n t i a l l y a t cleavage and qrowth s t e p s , a t c r a c k s and i n c a v i t i e s of the s u b s t r a t e s s u r f a c e . The main r e s u l t s of t h e s e ice n u c l e a t i o n experiments a r e p l o t t e d i n F i g s . 1 and 2. Our measurements concern v e r y high p u r i t y PbI2 and AgI with a l a r g e s p e c i f i c a r e a (10 m2g-l) and p r e f e r e n t i a l

(0001) f a c e s . Between -5OC and -30°C, a t o m i c a l l y c l e a n e p i t a x i a l f i l m s f r o m P b I 2 and AgI, p r e p a r e d i n u l t r a - h i g h vacuum, do n o t show any i c e forming p r o p e r t i e s i n sub- s a t u r a t i o n w i t h r e f e r e n c e t o supercooled w a t e r . A t s u p e r s a t u r a t i o n o n l y , an impor- t a n t group o f m i c r o d r o p l e t s appear.The i c e n o c l e a t i o n i s r e l a t e d t o ifhe formation of +

macroscopic s t e p s (10i0) o r (1071) e t c . . . , o r t o p o i n t d e f e c t s (Ag s i t e s ) ( F i g . 1 ) .

i56.

C o n d e n s a t i o n * *. ⁹

*Crystallization on s.te'&

120 -

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100 l ^L ^L

- ⁵ - ¹⁰ ^-15 ^-20 ^-25

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1

- 5 -10 -15 -20 -25

Temperature (OC)

F i g . 1 Nucleation o f i c e c r y s t a l s F i g . 2 Nucleation o f i c e c r y s t a l s on "pure" AgI s u b s t r a t e s . on Pb12 s u b s t r a t e s .

The t h e o r e t i c a l t r e a t m e n t o f t h e n u c l e a t i n g e f f i c i e n c y o f s m a l l p a r t i c l e s , once c o l L e c t e d by t h e f r e e z i n g d r o p l e t f o l l o w s t h e work of F l e t c h e r ( 6 ) . He modified t h e c l a s s i c a l thermodynamical approach t o account f o r t h e p o s s i b i l i t y o f " a c t i v e s i t e s "

on the s u r f a c e of t h e n u c l e a t i n g s u b s t r a t e . A f o r e i g n p a r t i c l e ( i n e r t ) can a l s o promote i c e n u c l e a t i o n by p r o v i d i n g a s t a b l e s u r f a c e on which an embryo can grow and

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by reducing t h e f r e e energy involved i n t h e f o r m a t i o n of a c r i t i c a l embryo. To b e an e f f i c i e n t n u c l e u s , t h e p a r t i c l e must b e a t l e a s t comparable i n s i z e w i t h t h e c r i t i - c a l embryo, and t h e f r e e energy a s s o c i a t e d with t h e p a r t i c l e i c e i n t e r f a c e must be a s small a s p o s s i b l e . Zettlemoyer e t a l . ( 7 ) performed experiments with hydrophobic m a t e r i a l which suggested t h a t i m p u r i t i e s may p l a y an i m p o r t a n t r o l e i n some c a s e s , though r a t h e r enhancing t h a n i n h i b i t i n g i c e n u c l e a t i o n a c t i v i t y . Adsorption s t u d i e s by Gravenhorst and C o r r i n ( 8 ) have e s t a b l i s h e d t h a t p a r t i c l e s from AgI samples con- t a i n i n g i m p u r i t y i o n s have a c o n s i d e r a b l y h i g h e r i c e n u c l e a t i o n e f f i c i e n c y t h a n

"pure " AgI .

Our r e s u l t s concerning s e v e r a l s e r i e s of i c e n u c l e a t i o n experiments from s t a n - d a r d commercial AgI s u b s t r a t e s a r e p l o t t e d i n F i g . 3 . This f i p u r e shows t h a t t h e r e i s no c r i t i c a l t h r e s h o l d f o r n u c l e a t i o n by " d e ~ o s i t i o n " a s p r e d i c t e d by S c h a l l e r and Fukuta ( 9 ) o r o t h e r a u t h o r s . The AgI s u b s t r a t e s used i n t h e s e experiments were a l s o prepared i n u l t r a - h i g h vacuum, l i k e i n t h e former c a s e , w i t h t h e same e p i t a x i a l o r i e n t a t i o n ; t h e i c e n u c l e a t i o n procedure i s a l s o t h e s a m e ( 5 ) . However, t h i s " s t a n - d a r d commercial AgI" i s c h a r a c t e r i z e d by t h e p r e s e n c e of i m p u r i t i e s which probably s t i m u l a t e t h e n u c l e a t i o n of i c e c r y s t a l s .

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Nucleation of ice crystals on "standard commercial AgI"

substrates (with impurities).

3 . The mechanism of p a r t i c l e d e p o s i t i o n and a c t i o n i n a s u p e r c o o l e d w a t e r c l o u d S e v e r a l q u e s t i o n s might b e asked concerning t h e p a r t i c l e d e p o s i t i o n on t h e S U - percooled d r o p l e t o r i c e c r y s t a l s u r f a c e . The o l d scheme of t h e i n t e r a c t i o n through d i f f u s i o n , i n e r t i a l and t u r b u l e n t motion f o r c e s a f f e c t i n g t h e a e r o s o l p a r t i c l e depo- s i t i o n on cloud elements--established by G r e e n f i e l d i n 1957--is s t i l l i n p r i n c i p l e v a l i d . More r e c e n t i n v e s t i g a t i o n s by C a r s t e n s and Martin (10) c o r r o b o r a t e d t h e f i n - d i n g by Podzimek (11) t h a t t h e thermo- and d i f f u s i o p h o r e t i c a l f o r c e s might p l a y an

important r o l e i n p a r t i c l e d e p o s i t i o n i n t h e s o c a l l e d "Greenfield gap"

0 . 1 v m < d < 1.0 pm where Brownian d i f f u s i o n , i n e r t i a l f o r c e s and microturbulence do n o t play an i m p o r t a n t r o l e . The model by C a r s t e n s and Martin (10) s t r e s s e s more t h e importance of t h e r m o p h o r e t i c a l f o r c e s - - i n comparison with d i f f u s i o p h o r e s i s - - i n ae2 r o s o l d e p o s i t i o n on c l o u d and p r e c i p i t a t i o n elements. This was a l r e a d y shown by Young ( 1 2 ) . Another i m p o r t a n t r e s u l t i s t h e comparison of t h e e f f e c t of Brownian d i f f u s i o n i n comparison w i t h d i f f u s i o - and thermophoresis. I f one assumes t h e whole p r o c e s s o f d r o p l e t ( o f mean r a d i u s r = 10 pm) e v a p o r a t i o n i n a c l o u d w i t h i t s impact on p a r t i c l e d e p o s i t i o n due t o t h e a c t i o n of d i f f u s i o n a l and p h o r e t i c f o r c e s , one o b t a i n s a r e l a t i o n s h i p

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JOURNAL DE PHYSIQUE

n T L - n ) m D N = N exp {

P PO Mv [exp (C/Dp - ^{I ) ]}³

with m e q u a l t o l i q u i d water c o n t e n t o f a c l o u d and M a s molecular weight of w a t e r vapor qT, q a r e p r o p o r t i o n a l i t y f a c t o r s from t h e t r a n s p o r t e q u a t i o n of h e a t and v w a t e r vapor: c h a r a c t e r i z e d by t h e w a t e r vapor d i f f u s i o n c o e f f i c i e n t , w a t e r vapor p r e s s u r e a t t h e s u r f a c e o f t h e d r o p l e t and f a r from i t ; L i s t h e l a t e n t h e a t r e l e a - s e d a t w a t e r phase t r a n s i t i o n . I n F i g . 4 a r e p r e s e n t e d c u r v e s f o r t h e f a c t o r C/D which f e a t u r e t h e p a r t i c l e d e p o s i t i o n r a t e N /N on a c l o u d d r o p l e t i n a g l a c i a B i o n s t a g e ( C D ) and i n c l o u d downdraft ( C /D 7 J t h o u t and w i t h (dashed l i n e ) v e n t i - l a t i o n e@egt. The f i n a l e f f e c t on part?21ePdeposition depends on atmospheric condi- t i o n s and on whether t h e d i f f u s i o n and p h o r e t i c f l u x e s a r e a d d i t i v e . The a n a l y s i s shows t h a t t h e simple a d d i t i o n of f l u x e s might c a u s e an e r r o r up t o 30% i n p a r t i c l e d e p o s i t i o n r a t e . I n g e n e r a l , one can show t h a t i n t h e p a r t i c l e s i z e range

0.01 < r < 0.05 urn t h e p h o r e t i c f o r c e s a r e l e s s e f f e c t i v e i n t h e p a r t i c l e d e p o s i t i o n under atmospheric c o n d i t i o n s t h a n t h e p a r t i c l e d i f f u s i o n .

100.

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The r e a l mechanism o f d r o p l e t f r e e z i n g i s n o t y e t well-known and t h e hypotheses p r e s e n t e d till now a r e n o t v o i d of c o n t r a d i c t i o n s o r n o t s a t i s f a c t o r y e x p l a n a t i o n s . Young (12) s t a t e s c o r r e c t l y t h a t t h e Brownian d i f f u s i o n and p h o r e t i c f o r c e s p l a y an i m p o r t a n t r o l e i n c o n t a c t f r e e z i n g which n u c l e a t e s t h e i c e ~ h a S e a t temperatures si- g n i f i c a n t l y h i g h e r t h a n o t h e r n u c l e a t i o n mechanism. I n 1964 Podzimek (13) c a l l e d a t t e n t i o n t o t h e o r i g i n o f columner type c r y s t a l s due t o t h e c o a g u l a t i o n of two h i g h l y supercooled d r o p l e t s of 2 0 p.m i n diameter (Fig. 5 ) . He s t r e s s e d t h e impor- t a n c e of s a t u r a t e d a i r "pockets" a f t e r t h e c e n t r a l impaction o r touching o f two d r o p l e t s . T h i s f i n a l l y l e a d s t o a banded i c e column, which had o f t e n a i r bubbles i n i t s c e n t r a l p a r t . Also s e v e r a l i d e a s and o b s e r v a t i o n s r e l a t e d t o i c e s i n t e r i n g could s u p p o r t this p i c t u r e o f t h e o r i g i n of columnar type i c e c r y s t a l s i n v e r y c o l d envi- ronment ( 1 4 ) . Another i n t e r e s t i n g c r y s t a l e v o l u t i o n i s t h e formation of two p a r a l l e l p l a n a r c r y s t a l s s e p a r a t e d by a f r o z e n d r o p l e t ( 1 5 ) . They were observed i n n a t u r e and a l s o i n t h e l a b o r a t o r y e s p e c i a l l y a f t e r s e e d i n g t h e cloud o f supercooled d r o p l e t s by a r t i f i c i a l i c e n u c l e i . (e.g. [ I 6 1 i n Fig. 6 was used AgI). Heymsfield (17) p r e s e n t e d l a t e r microphotographs s u p p o r t i n g h i s h y p o t h e s i s t h a t c i r r u s columnar type c r y s t a l s might o r i g i n a t e through t h e e v o l u t i o n of a double l e v e l c r y s t a l on t h e f r o z e n dro- p l e t i n t o a banded column. The main q u e s t i o n o f what i s t h e r o l e of n a t u r a l o r a r t i - f i c i a l n u c l e i i n t h e s e i c e c r y s t a l e v o l u t i o n remains l a r g e l y unanswered. I n t u i t i v e l y one can assume t h a t any i n s o l u b l e i n a c t i v e p a r t i c l e d e p o s i t e d on t h e p a i r of super-

0.1

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ventilation (f) .

0.05 0.1 0.5 1.0

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cooled d r o p l e t s w i l l b e d r i v e n by s u r f a c e d i f f u s i o n i n t o t h e neck o f t h e d r o p l e t s and t h e r e enhance t h e w a t e r phase t r a n s i t i o n a s d e s c r i b e d i n t h e f i r s t p a r t o f t h i s c o n t r i b u t i o n . Also t h e n u c l e i enclosed i n t h e a i r "pocket" between two c o l l i d i n g drops have b e t t e r chance t o cause t h e d r o p l e t f r e e z i n q eue t o t h e s a t u r a t e d a i r i n t h e e n c l o s u r e .

Fig. 5 Two coagulated waterdrops Fig. 6 "Two level" ice crystals start to form a hexagonal grow around supercooled column at -16.2OC. water drops at -11.5OC

(artificial fog was seeded by AgI aerosol).

4. Conclusion

Our e x p e r i m e n t a l r e s u l t s of i c e n u c l e a t i o n on AgI and PbIz s u b s t r a t e s show c l e - a r l y t h a t s t e p s , c o r n e r s and i m p u r i t i e s a r e p r e f e r r e d n u c l e a t i o n s i t e s i n which t h e f r e e energy b a r r i e r t o n u c l e a t i o n i s g r e a t l y reduced while t h e n u c l e a t i o n on p e r f e c t s u r f a c e s r e q u i r e s v e r y l a r g e s u p e r s a t u r a t i o n s o r s u p e r c o o l i n g s . These r e s u l t s account f o r t h e r o l e o f s u i t a b l e n a r t i c u l a t e s ( i n a c t i v e and i n s o l u b l e ) t o i n i t i a t e i c e phase i n a supercooled w a t e r cloud.

The p a r t i c l e d e p o s i t i o n towards t h e s u r f a c e of supercooled d r o p l e t s proceeds most e f f e c t i v e l y through Brownian motion and a l s o by p h o r e t i c f o r c e s (thermo- and d i f f u s i o p h o r e t i c ) . Two mechanisms of i c e c r y s t a l formation from supercooled d r o p l e t s a r e i m p o r t a n t a t low temperatures and i n presence o f atmospheric p a r t i c u l a t e s : Formation of columnar c r y s t a l s from two supercooled water drops and t h e development of double p l a t e ( d e n d r i t e , o r columnar c r y s t a l ) from one f r o z e n drop.

REFERENCES

( 1 ) Drost-Hansen, W., Ind. Eng. Chem. 61, (1969), 10-47.

( 2 ) Isono, K. and I s h i z a k a , Y . , J. Rech. Atmos., 5, ( 1 9 7 2 ) , 283-300.

( 3 ) F l e t c h e r , N.H., J. Chem. Phys., 29, (1959), 572-576.

( 4 ) Montmory, R. and Andriambeloma, H . , C.R. Acad. S c i . P a r i s , 285 (19771, 61-64 ( 5 ) Andriambeloma, H., These D o c t o r a t d ' E t a t - Univ. GliENOBLE FRANCE, ( 1 9 8 4 ) ,

133-167 .

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JOURNAL DE PHYSIQUE

( 6 ) F l e t c h e r , N . H . , J . Atmos. S c i . , 2, (19691, 1266-1271.

(7) Zettlemoyer, A.C., Tcheurekdjian, IJ., H o s l e r , C.L., 2. Angew Math P h y s i c s , 14,

(1963) , 496-502.

( 8 ) Gravenhorst, G . , and C o r r i n , M.L., J. Rech. Atm., 6, ( 1 9 7 2 ) , 205-212.

( 9 ) S c h a l l e r , R.C., and Fukuta, N . , Denver Research I n s t i t u t e , June Vol. 1 , (1977) 116-138.

(10) C a r s t e n s , J . C . , and Martin, J.J., J. Atmos. S c i . , 2, (19821, 1124-1129.

(11) Podzimek, J . , G e o f i s . Pura s Applic., 50, (1961), 161-168.

(12) Young, K . C . , J. Atmos. S c i . , 2, ( 1 9 7 4 K 768-776.

(13) Podzimek, J . , Travaux I n s t f t u t Gbophys. ATS, Prague, 213, (1965) 551-583.

(14) Hobbs, P . V . , J. Atmos. S c i . , , ( 1 9 6 5 ) , 296-300.

(15) Weickmann, H.K., Katz, U. and S t e e l e , R.L., P r e p r i n t 2nd Nat. Conf. on Weather M o d i f i c a t i o n , S a n t a Barbara, AMS, Boston, ( 1 9 7 0 ) , 332-336.

(16) Podzimek, J., J . Xech. Atmos., 6, ( 1 9 7 2 ) , 419-438.

(17) Heymsfield, A . J . , Ph. D. T h e s i s , Univ. of Chicago, (19731, 269, a l s o i n J.

Atmos. S c i . 32, (1975) , 799-830.

COMMENTS N. FUKUTA

F l e t c h e r ' s e n t r o p y e f f e c t on i c e n u c l e a t i o n on b a s a l p l a n e o f AgI is o b s o l e t e . We have d i s p r o v e d t h e e f f e c t t h e o r e t i c a l l y some 15 y e a r s ago. A s a d s o r b e d w a t e r m o l e c u l e s a c c u m u l a t e on t h e b a s a l p l a n e o f AgI, t h e i n i t i a l l y o r i e n t e d w a t e r m o l e c u l e s begin t o m u t u a l l y i n t e r a c t t o a v o i d t h e e f f e c t . We have a c t u a l l y observed i c e n u c l e a t i o n on t h e b a s a l p l a n e o f AgI.

Answer :

As you mentionned i n your p u b l i c a t i o n 15 y e a r s ago, F l e t c h e r ' s e n t r o p y e f f e c t c o n c e r n s o n l y t h e first l a y e r s on b a s a l p l a n e o f AgI, and I t h i n k t h a t you a r e r i g h t .

But d o n ' t you t h i n k t h a t t h i s i s s u f f i c i e n t t o p r e v e n t i c e n u c l e a t i o n , i n t h e i d e a l c a s e ( v e r y f l a t and c l e a n s u r f a c e s ) .

You c a n o b s e r v e n u c l e a t i o n o f i c e c r y s t a l s i f t h e r e a r e i m p u r i t i e s o r c r a c k s and s t e p s . That i s t h e c a s e f o r t h e most p a r t i c l e s i n t h e atmosphere. The e x i s t e n c e o f a l i g h t s o u r c e o r p o i n t d e f e c t s c r e a t e s a l s o a c t i v e s i t e s on which w a t e r m o l e c u l e s a r e adsorbed, and i n f r e e z i n g g i v e i c e c r y s t a l s ( a s you o b s e r v e d ) .