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OCTOBER 1988 HOLROYD, McPARTLAND AND SUPER 1127 ~- G;, ........ '0 ~ '9'10: ~'S'! ~ ~ '9'..... ~ '''9. Go- o . I 2.0 :::. .0 2~i 3R~~J ~~S~ 5" E"' 3-': ... ~ "0 6- 39CN- 4" FIG, I. A map of the terrain near the Grand Mesa, Colorado, showing the locations of ground release sites for AgI and weather stations, The seeding sites are identified by numbers here and in Table 1. The weather stations are indicated by letters here and in the text, The contour interval is 500 m, with the terrain below 3000 m shaded as shown in the key in the upper left, reliably transported up and over the barrier to sufficient height and in sufficient concentrations to produce measurable cloud microphysical effects during typical storms. 2. The appropriate spacing between ground gener- ators, based on the lateral spread of ground-released plumes as they cross the target area. 3. The lateral diffusion rates of airborne-released lines of AgI and the resulting seeding repetition rate that would be required for plume mergers to occur prior to target cloud entry. The Grand Mesa is a large, relatively flat-topped mountain that rises steeply about 1500 m from the surrounding valley floors. The maximum mesa ele- vation in the region of the experiments is about 3200 m msl. The orientation of the barrier provides oro- graphic lift for storms from all principal directions (clockwise from South to North). Because of the lack of pronounced vertical relief on the Mesa top, Federal Aviation Administration (FAA) permitted aircraft op- 1- erations to within about 300 m above the highest point in the research area during IFR conditions. This is 300 m closer to the surface than is usually allowed in mountainous terrain. Figure 1 shows the area and il- lustrates the locations of ground sensors (indicated by letters) and seeding sites (indicated by numbers) in the Grand Mesa area. Table 1 further identifies the seeding sites. 3. Equipment a, Silver iodide generators The ground seeding unit was a modified Skyfire-type solution burner developed at Montana State Univer- sity. It utilizes a propane flame into which is injected an acetone solution containing dissolved AgI. A 3% by weight AgI solution complexed with ammonium iodide (NH4I) was used at a consumption rate of about 30 g AgI h -1 . Calibration tests conducted in the isothermal cloud chamber at Colorado State University (CSU)