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1146 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 Montana. They involved airborne sampling of the mi- crophysical characteristics of orographic clouds effected by seeding with silver iodide (AgI), which was pro- duced by a single ground-based generator located on the upwind barrier slope. The second set of experiments was conducted over the Grand Mesa of west-central Colorado during March 1986. The Grand Mesa experiments utilized both ground-based and airborne seeding with Agl. Evalua- tion of seeding effects was accomplished by airborne sampling of microphysical properties and by surface measurements of precipitation rates, ice crystal habits and concentrations. a. Physical hypothesis The following is a general physical hypothesis ap- plicable to both series of experiments: 1) Large numbers of AgI particles will be produced by the combustion of a solution of silver iodide, am- monium iodide, and acetone. These particles will be functional as ice nuclei in supercooled liquid water (SLW) clouds colder than about -90C. 2) These ice nuclei, released either aloft or from the ground, will be transported into orographic clouds that may contain SLW. 3) The AgI will be present in quantities sufficient for in-cloud detection of the seeding plume by an acoustical ice nucleus counter. 4) In sufficiently cold SL W cloud, the AgI will create ice crystals in concentrations significantly greater than natural background levels. (This zone of enhanced ice particle concentration is often referred to as the seeding signature or the seeding plume.) 5) The artificially nucleated ice crystals will grow at the expense of the SLW, and with habits character- istic of the prevailing moisture regime and temperature. Their size will rapidly exceed 0.1 mm, the detection limit employed for the aircraft-borne optical imaging probe. 6) The resulting enhanced ice particle concentration (IPC) will remain significantly greater than the natural background concentration as the plume crosses the target area. 7) Because of the enhanced IPC, the ice water con- tent within the seeded volume will significantly exceed that in adjacent nonseeded volumes above the target area. 8) Melted-equivalent precipitation rates recorded at the surface in the target area, coincident with the pres- ence of the plume aloft, will significantly exceed rates recorded immediately prior to or subsequent to the plume passage (tested only over the Grand Mesa). b. Experimental design The experiments had several objectives, all related to verification of the physical hypothesis. They in- cluded: , -I) Demonstrating that properly conducted seeding operations result in significant populations of AgI ice nuclei reaching the targeted clouds. 2) Documenting the resulting microphysical changes within the seeded SL W clouds, specifically the ice particle concentrations, habits and sizes. 3) Comparing the microphysical composition of the seeded clouds with that of nearby nonseeded clouds. 4) Demonstrating that the growth of ice crystals initially generated by seeding with AgI eventually leads to enhanced precipitation at the surface. If the first objective can be met, the second and third objectives should be obtainable by positioning the ap- propriate airborne instrumentation in the proper places at the desired times. The final objective, perhaps best described as "ground truth," can be successfully at- tained only when precipitation data are available at the ground in the target area, as in the airborne seeding experiments conducted over the Grand Mesa. In keeping with these goals, key time- and position- referenced variables in the experiments were ice particle habit, size, and concentration; ice nucleus concentra- tion; and cloud liquid water content. Supporting mea- surements included horizontal winds, air temperature, and stability parameters. In addition, cloud droplet spectra were recorded during the Bridger Range ex- periments. 1 3. Instrumentation a. Aircraft The National Center for Atmospheric Research (NCAR) Beechcraft King Air turboprop aircraft was used in the Bridger Range experiments. A Rockwell Turbo Commander 690A turboprop aircraft was op- erated by Aero Systems over the Grand Mesa. Both aircraft were capable of sustained flight in known icing conditions and were usually flown in the 85-100 m S-1 true air speed range during sampling passes. The King Air's inertial navigation system (INS) po- sition data were used in analysis, while Loran-C posi- tions were the most accurate available from the Turbo Commander. Absolute differences from a known lo- cation measured prior to takeoff and after landing were usually less than 1.5 km with the INS and less than 1.0 km with the Loran-C. Relative differences from pass to pass should be very small with the INS (typical drift 0.4 km h -1) and are believed to be usually less than 0.5 km with the Loran-C. b. Optical array spectrometer probes The primary aircraft probe for detection of seeding effects was the same on both aircraft. Particle Measur- ing Systems (PMS) 2D-C probes with a 25-800 Ilm range and a 25 Ilm resolution were used to examine IPC, crystal sizes and habits. The sampling volume of