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<br />e <br /> <br />IV. CONCEPTUAL MODEL <br /> <br />A. <br /> <br />Characteristics of Typical SCPP-l Clouds <br /> <br />Weakly organized initially nonprecipitating convective clouds are treated <br />early in their life cycle. In a typical situation, which occurs on the back <br />side of storms, the airmass below about 4 km is convectively unstable and has <br />a barrier-normal wind component of <10 m/s. Since the slope of the Sierra <br />barrier is about 3 percent, the forced ascent rate is <0.3 m/s. The forced <br />ascent of convectively unstable air results in weakly organized convective <br />clouds consisting of towering cumulus in which new clouds continually develop <br />wi th i n the ensemb 1 e unt i 1 all the convect i ve i nstab il i ty is consumed. A <br />stable layer exists above 4 km (-10 to -20 OC), and occasionally there is <br />substantial wind shear across the stable layer. The air above is quite <br />dry. <br /> <br />e <br /> <br />The droplet spectra from these clouds are more continental in character tha~ <br />in other synoptic situations with concentrations of several hundred per cm <br />and mean di arneters about 15 Ilm. Many of these clouds subsequently develop <br />ice, radar echoes, and precipitation naturally, but when they do, the echoes <br />are weak (about 30 dBZ) and precipitation rates are light (<1 mm/hr). <br />Natural ice can develop with cloud top temperatures as warm as -10 oc, but <br />more often appears when the temperature is <-15 oc. The radar echo and <br />precipitation result from an ice-phase process which is predominantly dif- <br />fusional growth followed by a relatively slow accretional growth process. <br />There is some evidence that ice multiplication may occur by rime splintering <br />or by graupel-graupel collisions (Hobbs, 1983). <br /> <br />The principal difference between these clouds and High Plains cumulus con- <br />gestus treated in the HIPLEX (High Plains Experiment) is longer lifetime in <br />the Sierra. Entrainment in these clouds is effective in reducing liquid <br />water contents as in clouds on the High Plains, but the influence on precipi- <br />tation development is less severe because the environment is less dry and the <br />continued forced releases of convective instability result in new convective <br />towers. Nonetheless, liquid water contents are far below the adiabatic <br />levels. When very dry air or substantial wind shear is present at cloud top, <br />entrainment effectively suppresses natural and/or treated precipitation <br />development. <br /> <br />The turbulence in these clouds is moderate and results in a longitudinal <br />diffusion rate which will cause the seeding curtain to expand at a rate of <br />2 to 3 m/s (Lawson et al., 1980; Karacostas, 1981; Rodi, 1981). Diffusion is <br />enhanced by the growth and transport of crystal sin a sheared environment <br />resulting in diffusion rates which can exceed that by turbulence alone <br />(Stewart and Marwitz, 1982). <br /> <br />B. Treatment Justification <br /> <br />. <br /> <br />The conceptual model to justify the experimental seeding of such clouds to <br />augment their precipitation depends on increasing the clouds precipitation <br />efficiency microphysically. By artificially introducing the ice phase at an <br />earlier stage in the clouds life cycle, rapid diffusional growth, aggrega- <br />tion, and increased fallout of precipitation will ensue at an earlier time <br /> <br />11 <br />