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<br />[Reprinted from BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, Vol. 67, No.5, May 1986] <br />Printed in U. S. A. <br /> <br />A Review of the Sierra <br />Cooperative Pi lot Project <br /> <br />David W. Reynoldsl <br />and Arnett S. Dennis2 <br /> <br />Abstract <br /> <br />The Sierra Cooperative Pilot Project (SCPP) is an investigation of <br />cloud seeding as a means of increasing winter precipitation on the <br />Sierra Nevada. It is a concerted effort in the development of a physi- <br />cally sound cloud-seeding technology. It involves the use ofremote- <br />sensing devices, in situ observations, and the application ofa numer- <br />ical targeting model in randomized seeding experiments. The results <br />have led SCPP scientists to believe that shallow but widespread oro- <br />graphic clouds provide the best opportunity for glaciogenic seeding <br />in the central Sierra Nevada. <br /> <br />1. Introduction <br /> <br />There are indications from statistical analyses of operational <br />and randomized cloud-seeding experiments that, under cer- <br />tain conditions, it is possible to increase precipitation from <br />winter storms over mountainous regions of the western United <br />States by glaciogenic cloud seeding (e.g., Advisory Commit- <br />tee on Weather Control, 1957; National Academy of Science, <br /> <br />observations, documented the growth and fallout of snow- <br />flakes following the glaciogenic seeding of winter clouds over <br />the Cascade Mountains. New types of in situ and remote sen- <br />sors, coupled with numerical models of hydro meteor growth <br />in air flowing past mountain barriers, show promise that seed- <br />ing effects can be documented with increasing precision as <br />technology progresses. Table 1 lists several recent or ongoing <br />research programs on winter orographic clouds that are fol- <br />lowing a scientific approach. This paper describes one of <br />those projects, the Sierra Cooperative Pilot Project (SCPP), <br />to exemplify current trends in research on winter orographic <br />clouds. <br /> <br />2. Historical background <br /> <br />SCPP is an investigation of cloud seeding as a means of mod- <br />ifying precipitation over the Sierra Nevada during the winter. <br />SCPP is conducted by the Department of the Interior, Bu- <br />reau of Reclamation, in cooperation with the states of Cali- <br /> <br />TABLE I. Recent or ongoing winter orographic research programs with a strong in situ and remote-sensing observational base. <br /> <br /> Period of Primary <br />Project Agency/Funding operation study area <br />COSE (Colorado Colorado State Winter 1979 Park Range Colorado <br />Orographic Seeding) Univ./NSF Fall 1979 <br /> Winter 1981/82 <br /> Winter 1984/85 <br />CRADP (Colorado River Bureau of Winter 1982/83 Grand Mesa, Colorado <br />Augmentation Reclamation Winter 1983/84 <br />Demonstration Program) Winter 1984/85 <br /> Ongoing <br />Nevada Desert Research Winter 1983/84 Central Sierra Nevada <br /> Institute/NOAA Winter 1984/85 Walker-Carson Range <br /> Ongoing <br />SCPP (Sierra Bureau of Winters 1976/77 Central Sierra Nevada <br />Cooperative Pilot Reclamation through 1984/85 <br />Project) Ongoing <br />Utah Multiple Winter 1980/81 Tushar Mountains, Utah <br /> Agencies/NOAA Winter 1982/83 <br /> Winter 1984/85 <br /> <br />'. <br /> <br />1973). However, statistical evidence alone is insufficient to <br />convince many atmospheric scientists of the efficacy of cloud <br />seeding. They require, in addition, firm knowledge of the <br />physical mechanisms responsible for any apparent increase <br />in precipitation. <br />Evidence of the latter type has been reported by Hobbs <br />(1975) who, using aircraft, radar, and ground microphysical <br /> <br />1 U.S. Bureau of Reclamation, Auburn, CA 95603. <br />2 U.S. Bureau of Reclamation, Denver, CO 80225. <br /> <br />@ 1986 American Meteorological Society <br /> <br />Bulletin American Meteorological Society <br /> <br />_.J <br /> <br />fornia and Nevada, local government agencies, and private <br />firms conducting operational cloud-seeding projects in the <br />same general area. <br />Operational cloud-seeding projects to increase precipita- <br />tion and runoff from the Sierra Nevada have existed since <br />1948. Nearly all of these projects have been based on the hy- <br />pothesis that the precipitation rate from orographic clouds <br />can be increased by increasing the concentration of ice crys- <br />tals at temperatures above -20oC. This hypothesis is based <br />on thl~ observation that some orographic clouds contain <br />supercooled liquid water (SL W), which is inefficiently util- <br /> <br />513 <br />