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OCTOBER 1988 ARLIN B. SUPER AND BRUCE A. BOE 1167 GJT ~ker Field ,...",-.-. + North I ::::::::::::: ::GrIII... J I'll hur .. GMO Eo.t .. * .GMO .. GMO West sC8lelkm) A .5 KEY "Snow Lab . PROBE station *DoPPler acoustic sounder .. PROBE station with Icing rate meter 0,8 Sillier iodide _ding sites .28.7 em orifice precipitation gage .45.4 cm orifice precipitation gage GMO=Grand Mesa Observatory FIG. 1. Map of the Grand Mesa experimental ~rrea in western Colorado. south slopes. Twice daily upper-air soundings are ob- tained from rawinsondes released by the National Weather Service near Grand Junction, 45 km WNW of the Snow Lab. Because of the absence of higher ter- rain nearby, a special waiver was obtained from the Federal Aviation Administration (FAA) permitting Instrument Flight Rules (IFR) flight to within 300 m of the highest terrain on the mesa, half the minimum clearance normally required. The Snow Lab surface target for the seeding exper- iments was located in a small forest clearing just south of and about 100 m below the southern edge of the mesa caprock, where the wind was typically very light. Instrumentation at the Snow Lab included an ice crys- tal photography system; other instrumentation loca- tions on the mesa are shown in Fig. 1. The PROBE stations and doppler acoustic sounder provided near- surface and 30-570 m (agl) layer winds, respectively. (A more complete description of the instrumentation and related characteristics can be found in Part I.) Attempts to detect seeding effects directly had certain operational constraints, for example, the minimum aircraft altitude restriction of 300 m above the highest terrain. This placed the aircraft 500-600 m agl in the area of operations. Thus, a significant layer without ~ microphysical measurements existed between the low- est aircraft sampling level and the surface. Ice crystals having terminal velocities of 50 cm S-I would require about 1000 s to traverse this "layer of uncertainty." Significant additional growth could occur within this layer if supercooled liquid water (SLW) was present. Particle trajectories might also change markedly within this layer, depending on growth rates and wind shear. The presence of AgI and SL W within this layer was not recorded, although the latter was sometimes de- tected by tower-mounted icing rate meters at two mesa- top locations (Fig. 1). Sampling by the single research aircraft was done primarily at the 3.8 km level, the altitude of the AgI release. Consequently, the AgI and cloud microphysics were only occasionally monitored at higher levels. 3. Ground-based seeding experiments Experiments were conducted on 19 and 20 Mar to observe by aircraft the ground-released AgI and the associated microphysical effects in the orographic clouds over the Grand Mesa. Flow over western Col- orado was northerly, with Grand Junction (GJT) 70 kPa rawinsonde winds ranging from 7 to 12 m s -1 and 330 to 0100 true.