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<br />requirements applied only to the initial qualification pass). In some cases, seeding or <br />simulated seeding was done 1,000 feet or less above the top of an especially vigorous hard <br />tower when previous cloud passes on a particular day had established the suitability of the <br />subject clouds. In the simulated seeding passes, no flares were actually ejected when the <br />button was pressed, but the event was still recorded in the aircraft data system by activating <br />an event switch. The treatment decision for each experimental unit was revealed after the <br />qualification pass. <br /> <br />2.4.3 Results to date <br /> <br />One experimental unit was qualified in 1991, and 14 were qualified in 1993, giving a total <br />of 8 seed and 7 no seed units. One of the seed units cannot be analyzed because of improper <br />radar operation. Appendix B contains summary information for these cases. <br /> <br />A total of 151 convective cells (87 Sand 64 NS) have been identified within the 14 <br />experimental units, and their properties have been computed through analysis of thrE!e- <br />dimensional, volume-scan, S-band radar data using cell tracking software. The results <br />indicate that AgI seeding may have increased the maximum cell areas by 25 percent, <br />durations by 14 percent, and rain volumes by 69 percent. Little effect is indicated on <br />maximum cell heights, which may be real or caused by underestimation ofthe glaciated tops <br />of AgI-treated clouds by the S-band radar. None of the results have strong statistical <br />support. Additionally, within the height range of 7 to 11 kilometers, seeded cells of a given <br />maximum echo top produced more rain volume than unseeded cells of the same height. <br /> <br />Partitioning by cloud base temperature increased the apparent effect of seeding to 71 percent, <br />33 percent, and 125 percent for cell areas, durations, and rain volumes, respectively, within <br />the warm base (temperature> 16 OC) partition. The apparent effect on maximum cell height <br />is on the order of 6 percent. <br /> <br />The outcome for the 14 experimental units (7 Sand 7 NS) for which analysis is possible is <br />consistent with a positive effect of seeding. The natural variability is great, and the results <br />are highly sensitive to removal of the wettest unit from each of the S and NS samples. <br /> <br />The results of cold cloud seeding to date are consistent in most respects with the results that <br />have been published previously for Florida and Texas (Gagin et al., 1986; Rosenfeld and <br />Woodley, 1989; 1993). The Thai sample is still very small. Interested parties must be <br />cautious, therefore, in interpreting the results of the seeding until a larger sample has beEm <br />obtained. <br /> <br />The Thai cold cloud experiments appear to be on the right track, and the obvious <br />recommendation is that they continue in the context of a Phase 2 of Thailand's AARRP cold <br />cloud seeding effort. The design and operations plan for this effort is addressed later in this <br />report. <br /> <br />2.5 Implications of the Results of the Exploratory Experimentation on the Design <br />of the Cold Cloud Demonstration Project <br /> <br />The design ofthe cold cloud demonstration project will build on the results of the exploratory <br />studies and experiments. Thai cold clouds are clearly suitable for seeding intervention for <br />the enhancement of rainfall. The design of the exploratory experiments worked extremely <br /> <br />8 <br />