WMOD00303

• Title: Observational and Numerical Studies of Cloud and Precipitation Development with a View to Rainfall Enhancement
• Date: 4/1/1992
• Weather Modification - Doc Type: Report

r"o

The mass of each newly nucleated particle is assumed to be 10-11 gm (sphere about .
3 p.m diameter). Thus the nucleation rate C". of type A ice particles is

~ -",'
, .

i..

6NIA X 10-11
C". = _
P

(13)

~. .

b. Nucleation of type B ice by collision of type A ice with rain drops (C".).

The formation of type B ice occurs when raindrops freeze due to a collision between
a type A ice crystal and a raindrop at temperatures below ooc. This results in the loss
or one type A ice particle and one raindrop for each type B ice p~ticle (graupel particle)
that is formed. The average mass of the type B particle equals the sum of the masses of
the two colliding particles. The rate of formation of type B particles as derived by K-M is

,. :~"
;

Li
l,;

36tN,1/4 (pqR)t I I
6NIB = AO A VTR - VTA NIAEU.
'&AOIPI.
where PI. is the density of water, and E" the collision efficiency, which is assumed to
be unity. AOI is a constant while No represents the intercept of the Marshall-Palmer
distribution curve at zero radius (see K-M). Consequently, the nucleation rate of type B
particles is

(14)

.

r-

'"
i

6NIB
C". = --=- (ms + m.).
P

(15)

c. DifFusional and riming growth of type A &I1d type B ice (C i'" ) i-.,5;i-10,.5'

Ice particles can grow by either vapor diftUsion or riming depending on the mean mass
of the particles in the individual categories and on the availability of liquid water. Based
on detailed c:alculations performed by Koenig (1972), the growth rates of the particles were
divided into four difFerent regimes. The growth rates in the difFerent growth regimes are
illustrated in Fig. 3 (After Koenig, 1972).

.

\....:..

33