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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In physical experiments like those in the Cascades in Washington (Hobbs, 1975a and
b; Hobbs and Radke, 1975), to a lesser extent the Israeli experiments (Gagin and Ne11Tn..nn,
1981), and recently the High Plains Experiment, HIPLEX-1 (Bureau of Reclamation, 1979;
Smith et al., 1984; Cooper and Lawson, 1984) ~d the Sierra Cooperative Pilot Project,
SCPP (Bureau of Reclamation, 1983; Reynolds and Dennis, 1986) the aim was to test
the fundamental cause and effect relations in the physical chain of events of cloud and
precipitation development. However, most past field and modelling studies concentrated
on the links in the physical chain of events from the nucleation source to precipitation
at cloud base and subsequently at the surface, while to a large extent the forces (large
scale and cloud dynamics), that brought about the cloud were not taken into account.
The existence and development of clouds largely depend on factors such as, synoptic scale
disturbances which determine the stability and moisture content of the air in which the
clouds form, and mesoscale effects like orography and cilif'erential heating at the surface
which are primary forcing mechanisms in cloud development. Mesoscale convergence and
divergence areas, and smaller scale effects such as interaction with other clouds are also
important. Consequently, future work should concentrate on developing & new, more
complex physical hypothesis that considers both the interactions between the environment
and the cloud and between the microphysics and the internal dynamics of the cloud and
their response to seeding (Dennis, 1980; Braham, 1986; Silverman, 1986; Cotton, 1986;
Orville 1986; Orville, 1988; Orville et al., 1991; Reinlrlng and Meitin, 1989; Super, 1990;
Schaefer,1990).

Weather modification research requires the involvement of a large range of expertise
due to both the multifaceted nature of the problem and the large range of scales that
are addressed. The large scale dynamics determining the characteristics of the cloud
systems down to the small scale microphysics deterrniniJlg the nucleation and growth
characteristics of water droplets and ice particles all form part of the chain of events
leading to precipitation at the surface. Between these two' extremes there still exist major
gaps in our scientmc knowledge about certain physical processes. An attempt will be made
in the following section to identify and highlight some of the areas where in view of the

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