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3. RESEARCH AIRCRAFT OPERATIONS Through a Cooperatiye Agreement between the Reclamation and the X ational Oceanic and Atmospheric Administration, a "i;ell-instrumented King-Air C-90 research aircraft was proyided to the LOREP for a 2-month field program (January 20 to :\Iarch 19. 1993). The aircraft was based at Reno-Cannon International Airport in Reno, Xeyada (see fig 3.1l. The aircraft canied a full complement of research instruments, including sensors to measure air pressure, ambient temperature and dew point, cloud liquid water content, cloud droplets (fon,,-ard scattering spectrometer probe [FSSP]) in the size range of 1 to 45 p.m, cloud ice particles t2D-C Probe) in the size range from 25 to 800 p.m, and aircraft position lLoran-C and GPS). Horizontal wind speed and direction could be derived from aircraft obsen-ations of heading and true airspeed. In addition, a ScienTech Inc., precision tracer gas analyzer was operated on the aircraft to provide real-time measurements of SF" tracer gas concentrations (Benner and Lamb, 19851. This analyzer was the same type that was used at the Jackson Creek Obseryatory. The same calibration procedures were followed for aircraft operation as were used for the ground analyzer. The analyzer is capable of detecting SF" concentrations dnwn to 5 pit. In-flight testing (injection of calibration gas into the sampling tube) determined that a 10- to 12-s lag time occuned between ingestion of SF" and registration by the analyzer 0 This lag needs to be accounted for when comparing the aircraft SF 5 data to other aircraft data. An on-board computer-based data acquisition system recorded all obseITations at a rate of lis. A waiyer obtained from the Federal Ayiation Administration permitted flight to within 300 vertical m (1000 ft) ofthe highest terrain within 2.5 km (5 statute mil of the flight path during IFR (instrument flight rules) weather conditions. This allowance usually meant the aircraft was within 600 m (2000 ft) of most underlying terrain. All research flights were flown between dawn and dusk. A research flight was terminated at the discretion of the chief pilot when horizontal winds exceeded 20 rnIs or \vhen airframe icing became excessiye. During YFR lvisual flight rules-) weather conditions, flight altitudes could be lowered to as close to the terrain as the pilot felt safe to fly. The flight pattern used during research missions is shown on figure 3.2. The flight leg flown between PI and P2 was the valley track flown at 2500 m (8300 ft) on a northwest heading. The flight leg between P3 and P4 was the ridge track flown at 2680 m (8800 ft) on a southeast heading. During VFR flights, these same tracks were flown, but at flight levels down to 2100 m (7000 ft). The two SF" release locations are annotated with the numbers d7 and d9, meaning site d7 and site d9 dispenser locations on figure 3.2. A typical flight operation consisted of takeoff from Reno and climb-out to the west oyer the Siena Nevada. The aircraft would then descend from 3960 to 2500 m (13,000 to 8300 ft), entering the project area at PI. A typical flight lasted 3 h, completing about eigh: valley passes and eight lidge passes. During the early 1993 study period, the aircraft logged 35 research flight hours on 10 separate flights on 10 separate days. LOREP cloud seeding operations were suspended during this period because of excess snnwpack. Howeyer, the operation of one propane dispenser for a I-h period was allowed for each research aircraft flight. 20