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<br />Evaluation <br /> <br />It is impossible to make a valid overall evaluation of the effect of seeding <br />by hygroscopic material or silver iodide in ODROP. Natural convective <br />precipitation is so varied that statistical evaluation on the basis of rainfall <br />over a large area is impossible in a program lasting less' than a decade. <br />Likewise, physical evaluation based on models and storm by storm <br />observations is beyond the state of the art and likely to remain so for <br />. some years. One does have more chance when looking at possible <br />seeding effects on individual cells or cell clusters. However, an <br />increase in rain from individual cells can have secondary effects on <br />adjacent and subsequent clouds which may increase or. decrease overall <br />rainfall. Intuitively, one would expect that increases from seeded cells <br />give a net increase when viewed areawide, but the negative alternative is <br />not precluded. <br /> <br />The FTR program. was not set up or funded for evaluation, and so none was <br />done. There were a few observations which could give hints of effects, but <br />the main faith one has in the usefulness of the seeding come s from research <br />projects elsewhere (such as Biswas and Dennis, 1971) and from the <br />simplicity of the physical model of the seeding. Notes and slides were taken <br />on the C-97 and these may be useful if one re-examines the operations later. <br /> <br />During the project it was possible to look at some of the NSSL radar records. <br />In all the main seeding events there was natural rain in the area of greatest <br />seeding potential, and it does not seem likely that one can isolate rain-from- <br />seeding from natural rain. Very careful examination of aircraft position and <br />precipitation timing and position may make it possible to find significant <br />effects, but that is beyond the scope of the FTR project. It was possible. how- <br />ever, to spend some time on the August 24 case, and a qualitative discussion <br />is presented below. <br /> <br />Ihe_A~[,u~t_2i ~~s~ <br /> <br />Weak shower activity was decaying, and 0-1 (the C-97) (with 0-2, the Navajo <br />accompanying) had to go far NNE to find an area of cumulus which was <br />maintaining some strength. 0-1 operated in 3 general areas. <br /> <br />In the first area between Webb, Taloga, and Putnam, 0-1 seeded 1605-1628 <br />near the 8000' bases. Upcurrents were erratic, but occasionally reached 300- <br />500 fpm. A bit of light rain was encountered. but no general rain developed <br />and no NSSL radar echo ever appeared. It is believed that occasionally one of <br />these cells reached a bit above the freezing level, but that all were short-lived. <br /> <br />29 <br />