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<br />4. SEEDING OPERATIONS <br /> <br />\Vinter storm cloud seeding operations for the 3-}T LOREP were conducted in 6-h randomized <br />blocks, based on a simple set of criteria: <br /> <br />1. Indications of rime icing at any of the mountaintop icing stations for at least 1 h. <br /> <br />2. Precipitation falling as snow at or below 5,000 ft m.s.I. <br /> <br />3. The numerical targeting model indicated the target area was being treated. <br /> <br />Each 6-h block meeting these criteria qualified as a "case" to be used in the statistical <br />analysis. If appropriate conditions continued past 6 h, seeding terminat~d for 1 h and then <br />resumed for 6 h. If a 6-h period was a no-seed, a new 6-h block could begin immediately. <br />This pattern was repeated until cloud conditions were no longer suitable. <br /> <br />Randomization was done on a 3-to-2 basis. On average, for every 5 6-h periods qualifying for <br />seeding, 3 were seeded and 2 were left unseeded for comparison. This ratio was chosen to <br />maximize snow production while still maintaining a similar ratio of seeded and unseeded <br />storms for statistical comparison. An envelope containing the seed or no-seed decision was <br />opened by pac (Project Operations Center) personnel after it was determined that a suitable <br />6-h period qualified for treatment. On-off times of seeding dispensers were logged by <br />personnel in the pac. <br /> <br />A control gauge was installed at LaPorte (see fig. 2.1), which was up"vind of the target area. <br />A 2-)T correlation using 6-h precipitation totals between LaPort~ and the target network <br />showed a linear correlation of 0.85. It was anticipat~d that this control site plus the use of <br />randomization would allow statistical evaluation of the seeding effects (see section 7). <br /> <br />Seeding equipment reliability was documented by recording hourly status information <br />transmitted by each dispenser back to the POC throughout the field season. Tank liquid <br />level measurements were found to be accurate only after liquid levels dropped by several <br />hundred gallons. <br /> <br />The seeding guidance model (GUIDE) was used to determine if seeding would target the <br />gauge network. The model was initialized using the Johns"ille sounding (fig. 2.1). The <br />model is a simple two-dimensional Gaussian plume model ",..ith parameterized microphysics. <br />The model outputs both the horizontal and vertical dispersion of the seeding plume. In <br />addition, the fallout point of a snow crystal initiated at the dispenser was predicted. Crystal <br />growth was assumed to be in a continuous cloud environment having a liquid water c{)nt~nt <br />of 0.1 g mOs. A graphical example of GUIDE model output is shown on figure 4.1. Stars at <br />the origin indicate that nucleation occurred at a nuclei source. Downwind stars represent <br />fallout points for fastest growing crystals. <br /> <br />Cloud seeding suspension criteria Oisted in appendix Bi were developed so that seeding <br />operations would be suspended prior to the development of hazardous situations, e.g., serious <br />flooding and ayalanche warnings. No seeding suspensions occurred during the 1991-92 and <br />1993-94 winter seasons, which were both relatively dry. However, for the 1992-93 winter <br />season, seeding operations were suspended on December 30, 1992, because of excessive <br />snowpack. The suspension remained in effect for the remainder of the field season. <br /> <br />23 <br />