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DECEMBER 1978 GERARD E. KLAZURA AND CLEMENT J. TODD 1767 at cloud base when they reached the -50C level. In the present study the nucleation of AgI particlb was assumed, which would lead to a longer growth period to precipitation sizes. The reason why Rokicki and Young found hygroscopic seeding to be more effective at the lower cloud-base temperatures (O-lOoe) may be due to their stochastic growth assumption which allows for the interaction with other large drops, whereas in the AgI seeded situation many of the interactions would be between crystals. Rokicki and Young also point out that the exclusion of sedimenta- tion in a parcel model results in overestimates of the response to large drop seeding. It was found that for faster updrafts the larger hygroscopic seeds grow into precipitation and stand a better chance of breaking up, while the small seeds are carried to cirrus level and are lost before they reach precipitation size. The requirement for larger seeds becomes more critical as cloud bases become higher and colder. Takeuchi found that the effective- ness of hygroscopic treatments can be improved with higher concentrations of larger hygroscopic particles for stronger updrafts. Rokicki and Young found that increasing the seed drop size hastens precipitation formation significantly. Hygroscopic seeding produces the greatest water yield from clouds with warmest base temperatures (and consequently highest liquid water concentra- tions). Related to this is the fact that the vertical depth of the Langmuir drop breakup region increases as the cloud-base temperature increases. Takeuchi found that initiation occurs earlier in clouds with a given top temperature and increasing base tempera- ture. He also concluded that in warmer base cloud cases with updrafts of 10-15 m S-1 the chain reaction of drop breakup occurs which results in complete conversion of available moisture. Rokicki and Young also found that hygroscopic seeding produced its largest effect in warm based clouds. Updraft speeds of 5-15 m S-1 are the optimum for the Langmuir chain reaction to occur which then should lead to the most rapid conversion of cloud water to precipitation. Takeuchi found the optimum updraft speed range for hygroscopic seeding to be 10-15 m S-1. For clouds with strong updrafts (2: 15 m S-1) only large hygroscopic seeds will have a chance to con- vert to precipitation, and in this situation hail is produced. Takeuchi's model computed appreciable fallout due to hygroscopic seeding for deeper and warmer Clouds for updrafts 15-20 m S-1. In these cases the particles falling were hail. He concluded that hygroscopic seeding would ,work well in deeper clouds with moderate liquid water concentrations and strong updrafts but that hail would result. 7. Remarks The results of this study and those of Takeuchi (1975) and Rokicki and Young (1978) suggest that the potential for hygroscopic seeding as an effective technique of enhancing precipitation from convective clouds is very high. Hygroscopic seeds can be very effective in producing precipitation by an all-water process, but they can also be effective through the ice mechanism as well since larg~ water drops freeze at warmer temperatures than small ones. Rokicki and Young (1978), in fact, come out strongly in favor of hygroscopic over AgI seeding. They conclude that its effect is generally greater than that of AgI, the physical processes involved are fairly well understood, and more reliable systems for dispensing hygroscopic material are now becoming available. There are many processes that have not been simulated in this model but will have to be considered before a complete understanding of precipitation physics is possible. Such factors as the interaction between large drops and the dynamic effects due to large drop accumulation zones have to be studied and understood before an accurate model will evolve. In the meantime, simple models such as those dis- cussed here can serve as one of the tools with which to reduce some of the major uncertainties connected with precipitation formation. One should, however, be cautious in the use of such models and not extract information that they were not designed to provide. Acknowledgments. The authors are grateful to Mr. Dennis :Musil, South Dakota School of Mines and Technology, and Dr. C. W. Chien, Meteorology Re- search, Inc., for providing us with their accretion and condensation computer programs, respectively. REFERENCES Ackerman, B., 1974: Illinois precipitation enhancement program phase L Interim Report, Bureau of Reclamation Contract 14-06,D-7197, Illinois State Water Survey, Urbana, 14 pp. Biswas, K. R., and A. S. Dennis, 1971: Formation of a rain shower by salt seeding. J. Appt. Meteor., 10, 780-784. -, and -, 1972: Calculations related to formation of a rain shower by salt seeding. J. Appt. Meteor., 11, 755-760. Chien, C. W., and D. P. Mack, 1966: Computation of droplet growth from giant salt particles in ascending air below cloud base. J. Atmos. Sci., 23, 810-812. Dennis, A. S., and A. Koscielski, 1972: Height and temperature of first echoes in un seeded and seeded convective clouds in South Dakota. J. Appl. Meteor., 6, 994-1000. Draginis, lVI., 1958: Liquid water content within the convective cloud~,. J. Meteor., 15,481-485. Heymsfieid, A. J., P. N. Johnson and J. E. Dye, 1978: Observa- tions of moist adiabatic ascent in northeast Colorado cu- mulus congestus clouds. J. Atmos. Sci., 35, 1689-1703. Hirsch, J. H., 1971: Computer modeling of cumulus clou?s during project cloud catcher. Rep. 71-7, Inst. Atmos. SC1., South Dakota School of Mines and Technology, 61 pp. -, and C. L. Schock, 1968: Cumulus cloud characteristics over western South Dakota. J. Appl. Meteor., 7, 882-885. Kopcewicz, T., 1965: Selected problems in hail physics and hail suppression. Przegl. Geofiz., Warsaw Univ., 10, 84 pp.