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OcrOBER 1988 HOLROYD, McPARTLAND AND SUPER 1135 r up to the cloud base but only in insignificant concen- trations. Had the base been lower, an ice particle plume would have been expected. The northern limit of the AgI plume at flight level was near the northern rim, suggesting that the usual lee subsidence was sending the IN rapidly downhill. In the experiment in Fig. 2j a meandering ice particle plume was detected in the expected position downwind from seeding site number 6, which was high up on the side of the mesa. Later that day, as shown in Fig. 2k, the crest winds came from a similar direction as the aircraft again flew back and forth over the high ground of the mesa within the region shown. But the winds at the lowest seeding site used were nearly parallel to the axis of the mesa. The plume presumably traveled farther to the east, beyond the turn-around point of the aircraft, before flowing over the crest. The experi- ment shows that the winds at both the seeding and target sites should be monitored for more accurate tar- geting of the materials. The site itself was probably not the problem, because in Fig. 21 materials from the same seeding site were easily detected over the crest of the mesa. In fact, the plume from this lowest seeding site on that day, under broken altostratus and altocumulus conditions, rose over the crest to the highest altitude of detection in any experiment. The elevation of this site is 2515 m or about 700 m below the mesa top. The experiment illustrated last, in Fig. 2m, shows a plume traveling down the axis of the mesa. The direc- tion of flow was parallel to the upper winds measured by the acoustic sounder. The lowest passes close to the seeding site detected the plume just north of the site. Presumably either the winds at the seeding site were temporarily moving in a southerly direction or the di- rections were highly sheared. This low-level plume is shown between the short dotted lines north of the seed- ing site. This case also demonstrates that an AgI plume can be detected as much as 40 km downwind of the seeding site. Two experiments with AgI released from the north slope were not presented in Fig. 2, nor are they listed in Table 1. Although AgI or ice particle plumes were detected, the number of plume interceptions was in- sufficient to add meaningfully to this paper. But even including these experiments, the seeding plume was always detected by the aircraft. (The one exception is that illustrated in Fig. 2k, in which the aircraft appar- ently did not fly in the COlTect location.) This detection success rate is similar to that obtained by Super and Heimbach (1988) for AgI plumes over the Bridger Range of Montana. These examples show that the transport of a seeding plume can be estimated from local wind data. The plumes spread downwind through angles that are not necessarily related to the standard deviation or variance of the wind directions. The deflection of the wind di- rections by local telTain and by angular wind shear in the vertical, however, will always present a challenge 1/ \...., r to those wishing to properly target seeding materials released from the ground. b, Vertical extents The vertical extent of a plume released from the ground can be affected by several mechanisms. The buoyancy from the heat of combustion of the AgI gen- erator provides an initial boost, but it can be considered insignificant when it is dispersed in a plume with a cross section of several hundred meters. Turbulent ed- dies, either already present or freshly induced by the mechanics of air flowing over rough terrain, will nor- mally be the main contributors to the initial dispersion of the plume. The greatest vertical rises of the plume will occur with the aid of natural convective processes, especially if they are energized by local solar heating of the surface. Plume rise is typically inhibited by stable layers warm enough to make a penetrating plume neg- atively buoyant. It was found that the plume can rise over the mesa to higher than the base of a stable layer as measured away from it, because the horizontal mo- mentum of the air approaching the mesa can be con- verted into vertical momentum as it flows over it. In such cases, the base of the stable layer is lifted, per- mitting the plume to rise higher. When the aircraft was flying near such a boundary, the IN were usually found in the cooler air at any given level. The potential tem- perature of the seeded air at flight level was nearly the same as that known or estimated for the seeding site. Sometimes the tethersonde data showed bubbles of warm air, presumably rising up a dry adiabat. The mixing of the AgI to higher levels in these experiments was therefore accomplished by a variety of mecha- nisms, but no attempt has been made to assess their relative contributions. The stability situation shown in Fig. 3 is an extreme example of the vertical profile of potential (left) and equivalent potential temperature (right). The shaded areas show the range of temperatures encountered by the aircraft for all portions of the flight. The solid lines show the rawinsonde profiles recorded at GJT. Seeding material was released at an altitude just below 3.0 km in air with a potential temperature of 299 to 301 K. Hundreds ofIN per pass were measured up to 3.6 km, where the left side of the potential temperature profile becomes stable. A dwindling number ofIN reached as high as about 3.9 km. For all cases, the average potential temperature of the air containing the AgI plume and the span of the temperature variation are listed in Ta- ble 1. The GJT potential temperature profile in Fig. 3 sug- gests that the air at the seeding site should not have risen significantly, because the atmosphere was every- where stable above that level. The aircraft temperature measurements, however, show that air with a potential temperature of about 301 K could have reached to just beyond the 3.6 km level. At that level, cold air from