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.. . point is difficult for aircraft sampling. Adjusting for cloud chamber mixing, the maximum aircraft- measured concentration of IN was less than 20 IN L.1 effective at -20 oe. The temperature at this level was -8 oC, implying the maximum concentration of effective IN was less than 1.5 L-1. Zones of SL W over the Plateau were targeted by IN, though indications are that this was in a shallow layer. Later in the experiment, IN concentrations on the Plateau were reduced to near-background levels. For the present case study, modeling suggests that the valley-released AgI had an initial vertical impetus by the gravity wave mechanism which provided transport above the surface inversion. This was followed by orographic forcing in a more organized westerly flow. The model confirmed the confinement of the plume to a shallow layer and subsequent lifting again due to a gravity wave further eastward in the vicinity of the TAR site. It may be possible to exploit the influence of gravity waves in. surface-based seeding strategies. This was explored by Heimbach and Hall (1994) who applied the model to several source configurations. Super (1994) suggested that targeting might be enhanced if releases were from a barrier upwind of the Plateau, in this case the San Pitch Mountains. Targeting from high altitude releases is generally successful provided there is an organized cross-barrier wind component. On the other hand, valley releases have uncertainties in terms of reaching the target and doing so in sufficient concentrations (ibid). Therefore, the gravity wave mechanism would be best exploited with valley releases which can "load" an entire valley floor with high IN concentrations. The wave structures can be wide ranging depending on speed and stability, making a broad source area or pool of aerosol advantageous. Seeding from the valley sites could not take advantage of fast-acting forced-condensation nuclei (Finnegan and Pitter 1988) because the temperatures are frequently above -6 oC (Li and Pitter 1997). Therefore, a contact nucleant would be appropriate provided the gravity wave mechanism contributed to a deep enough lifting to reach sufficiently colder temperatures. . The criteria for judging the possibility of gravity wave excitation are uncomplicated: (1) a cross-barrier wind component exists, and (2) the atmosphere is stable. For the latter, this implies potential temperature increases with height which includes moist adiabatic profiles commonly observed in project soundings. A more objective criteria can be devised using the Proude number, Fr. This is a nondimensional ratio of vertical perturbation to horizontal wind speed, Fr= vh/ U, (2) where U= cross-barrier wind speed, v ="the Brunt-Vaisalla frequency (Eq. 1), and h = the height of the barrier. Gravity waves are stimulated when Fr <1 (Bruintjeset al. 1995), a criteria easily tested with a sounding. Por Froude numbers between 1 and 2, high amplitude waves can be formed over the lee slope of a barrier (Smolarkeiwicz and Rotunno 1989). This suggests that for low wind speed and high stability conditions, the lee of the San Pitch Mountains could be a reasonable release zone. However, dilution, the subsequent downward portion ofthe wave and horizontal targeting issues complicate this application. For the current case, Fig. 6 suggests that the upwind side of this barrier would be appropriate. Super. (1995a) suggests the Utah operational netWork in the Sanpete Valley, though' sometimes successful at targeting the Plateau, often provided low concentrations effective at the ambient temperatures. Effecti"etargeting would require a strong source configuration with many more generators and/or higher output generators. Figure 6 also indicates that the west side of the Sanpete Valley could provide some initial upward impetus; however, the subsidence portion of the gravity wave could have a detrimental influence on targeting. Releases further upwind of the current valley sites have the disadvantage of . .. 78