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<br />- 11 - <br /> <br />Review of PEP Concept and Approval of <br />Operations Plan <br /> <br />Start of randomized experiment <br /> <br />Completion of randomized experiment: <br /> <br />Completion of major programm,~ (PEP) <br /> <br />June 1981 or 1982 <br /> <br />January 1982 or 1983 <br />May 1986 or 1987 <br />May 1987 or 1988 <br /> <br />3.4 The Evaluation Period <br /> <br />The farther we look into' the future, the less defined our tasks are. This is <br />particularly true for the evaluation phase because it depends primarily on the actual <br />operations plan for the seeding experiment, and this plan cannot be established until <br />after the conclusion of SSP-3 (which should offer a seeding strategy). <br /> <br />* <br />4. THE OPERATIONS PLAN FOR THE SITE SELECTION - PHASE 3 <br /> <br />4.1 Cloud Seedability <br /> <br />The purpose of SSP-3 has been giveIl in Section 3.2 together with some key char- <br />acteristics of the Spanish site. What can be said about seedability? <br /> <br />At the present time, seedable clouds c,an be characterized as follows: <br /> <br />(a) clouds where the coalescence process does not dominate the natural precipitation <br />mechanism; <br /> <br />(b) clouds with a large body of supercooled water; <br /> <br />(c) clouds with a lack of natural ice crystals; <br /> <br />(d) clouds where there is an absence of large drops <::'25 ~m) in the active part of the <br />cloud indicating an absence of secondary natural ice production; <br /> <br />(e) clouds that are deep and have a cloud-base sufficiently close to the surface so that <br />precipitation does not significantly evaporate before reaching the Earth's surface. <br /> <br />However, a more detailed understanding of the physical, synoptic and climatic <br />situation is needed to refine "seedability" further and to assess properly the probability <br />of occurrence of weather situations suitable for seeding. Such studies include: <br /> <br />(a) the natural precipitation mechanism; <br /> <br />(b) the efficiency of the natural processes in terms of producing rainfall; <br /> <br />(c) the synoptic and mesoscale characteristics of the rain-producing systems; <br /> <br />(d) the variability and frequency of rain-producing disturbances; <br /> <br />(e) the predictability of rain-producing disturbances; <br /> <br />(0 the frequency of destructive storms (floods). <br /> <br />4.2 The SSP-3 Measurement Programme <br /> <br />In order to assess which clouds conform to the above characterization, a micro- <br />physical measurement programme conducted by aircraft flying through clouds is required. At <br />least one meteorological radar is also considered essential to keep a continuous record of <br />cellular and mesoscale precipitation patterns and vertical cloud/precipitation structure. <br />The aircraft measurements should be made over the entire site for a significant part of the <br /> <br />*As quoted to quite some extent from the "Operations Plan for Site-Selection Phase 3", PEP <br />Report, No. 11. <br />