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

. -.

..

.

.

; -

.

eft'orts is provided by Orville (1988) and Orville et al.(1991). In these modelling studies an
initial region of buoyant air is usually employed to initiate the convection. This framework.
is useful to study the interactions between the internal dynamics and the microphysics
within convective clouds but is questionable as far as studying the dynamics of cloud
populations and how they interact with the larger scale flow (Clark et al., 1986; Clark,
1991). A more re8.Iistic approach is to consider the cloud as part of a field solution where
the concept of an isolated cloud with a remote environment is rather obscure (Balaji and
Clark, 1988; Crook et al., 1991, Rasmussen et al., 1988 and 1989). Instead the cloud is
allowed to interact with the environment. To date no simulations have been conducted to
investigate seeding effects using the second approach.

Modelling studies investigating seeding effects on convective and stratiform clouds
have primarily concentrated on the microphysical processes within clouds, and the
interaction with the internal dynamics (Orville et al., 1991). Most of the modelling
studies indicated precipitation increases in moderate size cold-base (<+100C) convective
clouds (20-100%) but in some larger storm systems decreases in iainfall were noted. The
increases were due to earlier development of preapitation size particles at lower elevations
in the cloud as a result of cloud seeding. In larger clouds the interaction between the
precipitation and the internal circulation led to less rain in some instances. In warm.
base convective clouds the results seem to be rather inconclusive at this stage due to
more complex precipitation formation processes. Usually an active coalescence process
operates in combination with an ice process. In addition ice multiplication processes are
usually active in such clouds. The method in which the microphysical processes are treated
and siIt1Jllated have been upgraded substantially as understanding of these processes has
increased over the past twenty years, with a resultant effect on predicted effects of seeding.
While in a study by Hsie et al.(1980), on the eft'ects of seeding on a warm. base cloud,
a significant increase in rAin"'l\l1 was predicted, Orville et al., (1989) in their simulations
of similar clouds found a decrease in rAin......u in the larger clouds and a small increase in
smaller clouds. The apparent difference is ascribed to the inclusion of a snow mixing ratio
field in the latter study (Orville et al., 1991). This example emphasizes the importance of

20