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<br />knowledge' and understanding of the natural microphysical processes in order to be able <br />to realisticallY' simulate these processes and to detect seeding dects. . <br /> <br />In both the warm and cold base cloud modelling studies dynamic effects were evident <br />through the release of latent heat of fusion which invigorated cloud growth. Changes in <br />precipitation loading due to seeding when the liquid water is frozen into ice particles also <br />affected cloud growth. However, this effect was usuallY' small. Precipitation development <br />further lead to regions of new cloud development due to enhanced downdra.:l'ts and outflow <br />regions. This process also modified internal cloud circulations. It must be stressed however, <br />that most of these modelling studies were periormed as an unforced initial value problem <br />where the cloud was modelled in isolation of the remote environment as discussed earlier. <br /> <br />Simulations of seeding effects in stratiform and orographic clouds are rather scarce to <br />date (Orville et al., 1984,1987; Young 1974a and bj Nickerson et al., 1976; Rasmussen <br />et al., 1988). Orville et ale (1984, 1987) simulated the seeding of stratiform clouds <br />using a two-dimensional time dependant model ior a case in Spain which iormed part <br />oi the Precipitation Enhancement Project (PEP) organized bY' the World Meteorological <br />Organization (WMO). The natural clouds were characterized bY' low amounts of . <br />supercooled water and low ice crystal concentrations. The silver iodide seeding simulations <br />did produce significant drnamic responses increasing cloud vertical motions from 10-20 cm <br />S-1 in the unseeded case to several m s-1 in the seeded case. According to their modelling <br />results the effects appear to be due to the heat released as the supercooled water freezes <br />and the cloud vapor mixing ratio changes from water to ice saturation. In terms of the <br />overall energetics of the cloud the heat release is much more significant than found in <br />simulations for convective situations. These results, however, did not consider that natural <br />processes are extremelY' time dependant. Although the results are signHicant theY' should <br />first be vermed with an independent model which is capable of simulating some of the time <br />dependent t'a.ctors mentioned earlier and testing the underlying assumptions of this study. <br />A studY' such &I this should preferablY' be done with a time dependant three-dimensional <br />model capable of realisticallY' simulating both the relevant drnamic and microphY'sical <br /> <br />processes. <br /> <br />. <br /> <br />21 <br />