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3. Do conditions exist such that seeding will reduce the snowfall on the Mogollon Rim? There is conflicting evidence on this point and more physical evidence is needed to test limited statistical suggestions that such decreases can be significant. The reality and magnitude of any seeding-caused decreases should be known prior to implementation of operational seeding. 4. How frequently can ground-released AgI result in significant concentrations of ice crystals well upwind of the barrier to allow for crystal growth and fallout? 5. Does nature enhance the IPC near the ground? Super et al. (1989) discuss large apparent increases in IPC between lowest aircraft sampling levels and the ground. Because of differences in sampling times between the aircraft and ground systems, and artificial increases due to snowflake breakup in the ground probe, it could not be determined how much of the apparent increase was real. However, low-level ice crystal development has been observed at other mountain barriers, possibly because transient supersaturations in the ascending air allow natural ice nuclei to become effective. The amount of natural ice particle formation in the lowest several hundred meters above the Mogollon Rim needs to be documented, as any such formation would be competing with seeding caused crystals for the available SL W. These uncertainties all suggest caution should be exercised in using numerical model predictions. Model outputs should certainly be used for general guidance in conducting seeding experiments. Models will no doubt improve as better observational evidence becomes available and leads to more through theoretical understanding. However, at this stage an empirical approach continues to be required, with the effect of seeding under particular conditions demonstrated by comprehensive physical observations. ~ 1.5.2 General Overview. - A number of sophisticated observational systems are required for detection of key physical processes in comprehensive physical seeding experiments, whether ground- based or airborne seeding is used. Additional measurement systems will provide supporting data to assist in conduct of experiments and post hoc analys.is of the resulting data base. A specially instrumented aircraft, hereafter called the cloud physics aircraft, will be the primary airborne sensing platform, Ground observing systems will include a scanning Doppler radar; a microwave radiometer; a Doppler acoustic sounder; an aspirated ice particle imaging probe; a laser ceilometer; tower-mounted wind, temperature, humidity, and icing sensors; and a network of high-resolution precipitation gauges, In addition, manual observations will include frequent sampling of surface snowfall for precipitation rate and later silver content analysis, and microphotographs of ice crystals to reveal details of structure and evidence of riming growth, These systems will now be described, and brief discussion given of their respective roles in the experiments. The instrumentation required on the cloud physics and seeding aircraft will be described first, followed by the ground-based measurement systems, Sections 1.6 and 1.7 will provide more detail on how the various observing systems will be integrated in conduct of the ground-based and aircraft seeding experiments, respectively. 1.5.3 Cloud Physics Aircraft. - The most valuable cloud physics aircraft measurements will be SL W, IPC, and AgI concentration, all referenced to position and time. King and J - W hot wire probes and a PMS-FSSP sensor (see the appendix for listing of instrument sources) will be used to monitor SLW, which must be present for seeding to create significant concentrations of ice particles, The FSSP also yields cloud droplet concentrations and sizes, of special interest concerning possible ice multiplication (Hallet and Mossop, 1974). A PMS 2D-C particle imaging probe will provide IPC and particle sizes and an estimation of habits, If SL W is present, the seeded cloud volume should 13