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... , 5.1.7 A Description Of The Numerical Seeding Guidance Model The model used ~o simulate seeding effects in various parts of the state was developed at NAWC by Robert Elliott for ~he Bureau of Reclamation-sponsored Sierra Cooperative Pilot Project. The model has been described in detail in Elliott et aI, (198l). In si~ple terms, the model simulates airflow over a barrier and con~ains rrodules to compute artificial and natural nucleation of cloud water and the subsequent ice crystal fallout. Both ground-based and aerial seeding can be simulated. Convection is handled by the use of a pre-determined mixing depth and a ~ean convective updraft within the mixing layer. Da~a required by the model include a terrain profile (generally values averaged over a width of 20 km normal to the wind and spaced in 10 k~ downwind s~eps), a profile repre- senting a streamline i3 middle levels of the atmosphere, an upwind rawinsonde sounding, and cloud wa-:er profiles vertically above each terrain step. The model provides a variety of displays of computed resul~s. In this study, the principal display was a downwind distance VS height above ground plot that shows how far downwind and how high above ground a seeding plume drifts and where the artificially-nucleated ice crystals fall to the ground as snow or rain. This display also shows the seeding-related increment in precipitation at each fallout step downwind. An example of this display is shown in Figure 5.4. The seeding increment (called Footprint Precip in the figure) is the increase over what would have occurred without seeding. Typical wintertime 5-11