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

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pa.rlicles mentioned in the previous section. Each ice pa.rlicle category is represented by two
variables, namely, the mixing ratio (<<Ii) and the number density (or number of pa.rlicles per
unit volume, Nj). The number density is related to the mixing ratio through the average
particle mass mj as

Njmj = qjp.

(10)

It is assumed that that particles of mass m; represent the entire spectrum. of particles of
category j.

The ice growth processes considered in the model are nucleation, difFusional growth
and sublimation, accretional growth, and melting of both types of ice particles. The
physical equations (in cgs units) describing each of these processes are as follows:

a. Nucleation of type A ice by heterogeneous sorption nucleation. of ice particles (C"a).

Type A ice are nucleated by heterogeneous ice nucleation processes. Initially type A ice
will be smaller than type B ice because type B ice are formed by the freezing of raindrops.
The nucleation of new pa.rlicles closely follows the K-M parameterization where nucleation
can occur whenever the temperature is below OOC and the air is saturated with respect to
water or when the temperature is below a certain threshold (-120C) and the air saturated
with respect to ice. The number of ice particles that can be formed by this process as a
function of temperature is

NIP = Aoe exp[ln.f~) max((273.15 - T) ,20)] (11)

with a mR~mum. value at -2000 and a constant number at temperatures colder than -200C.
Aoe and AOT are constants and may be varied depending on the nucleation rate desired for
each experiment (see K-M, section 3). The actual number of ice crystals generated at a
certain time will also depend on the number already present

6NIA = max[(NIP - NIA) ,0]

(12)

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