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482 JOURNAL OF APPLIED METEOROLOGY VOLUME 29 Based on research aircraft measurements during the experiment, 1700-2000, the orographic cloud consisted of ice, 30-100 L -Ion the 2D-C, 6-20 L -( on the 2D- P, and almost no liquid water in the FSSP size range. FSSP concentrations, when they existed, were <40 cm -3, with droplets predominantly between 7 and 19 ~m diameter. Images from the 2D-C and 2D-P indi- cated that the ice was primarily small graupel and nee- dles. Analysis of the 2D-C, and comparison of the CSIRO and JW liquid water meters, indicated the pos- sibility of many regions of large drops, 100-200 ~m (Rauber and Heggli 1988). At times water drops dom- inated the particle concentrations sensed by the 2D-C and 2D-P, still only low concentrations of drops 7-19 ~m were recorded by the FSSP. Needles, or spikes pro- truding out of frozen drops were often seen in the 2D- C images in regions of large drops. b. Treatment A seeding experiment was conducted as a test ofthe effectiveness of AgI NH4I NH4Cl04 at -60e. The seeding location was chosen based on measurements by the research aircraft, Fig. 1, which showed a region ofliquid water of 0.05 g m -3 at 10 km west of SHR. This was nearly the only region of liquid water observed during the ascent sounding and descent along the bar- rier by the research aircraft. From 1810 to 1818 the seeder aircraft released AgI NH4I NH4Cl04 at 0.4 g km -( along one 40-km-Iong seedline. The seedline was 10 km west of SHR and oriented nearly north-south, although average winds were from 2350. The seeder aircraft then left the area while the research aircraft tracked the seedline for 1.5 h making perpendicular passes through the seedline. c. Results of seeding Measurements by the research aircraft at the seeding location 10 min prior to seeding showed 2D-C con- centrations of 50 L -( . After the seeding material was dispensed the research aircraft made 16 penetrations of the seedline. The air parcel tracking capability of the aircraft was used for the first 11 penetrations. The average advection of an air parcel for these penetrations was 2420 at 18 m S-I. This average advection was used to determine the seedline age during each penetration by the research aircraft. These ages are shown at the bottom of Fig. 2 which displays the research aircraft measurements during the first five penetrations. The first penetration occurred 5 min and the last penetra- tion 92 min after seeding. A strong signal on the IN counter confirmed seedline interception for the first two penetrations at 5 and 12 min after seeding, Fig. 2a. On the third and fourth penetrations, 19 and 24 min, however, only a few IN were detected, Fig. 2b. Recall that the IN concentrations are calculated based only on the sampling rate and the number of IN counted, and that there is a 25-40 s delay between' intake of nuclei and detection. Also, because of the long residence time of nuclei in the chamber and the stochastic nature of ice crystal development, several minutes are required for the instrument to decay to background after sampling a high concentration ofIN. This explains the long tail after sampling of IN at 181930 and at 182520. No IN were measured on the next seven penetra- tions, 29-61 min after seeding. Particle concentrations near the seedline during these penetrations were high, and although increases in particle concentration were observed at the expected seedline interception times, they were not different from increases observed away from these locations. Examination of the 2D-C and 2D-P images indicated that the particles were a mixture of small particles, possibly water drops ( 1825), graupel, needles and frozen drops with protruding needles or spikes ( 183637), Fig. 3. Decreases in the FSSP droplet concentration and a shift of the droplet distribution to smaller sizes at the seedline interception times at 24 and 29 min indicate that evaporation was occurring there; suggesting a higher depletion rate of water in the seedline compared to surrounding regions. CSIR:O liq- uid water concentrations also decreased between 19 and 24 min; however, based on the ice nucleus mea- surements, the research aircraft was not sampling the seedline after the penetration at 12 min. After the eighth penetration the crew on the research aircraft decided to climb to attempt to intercept again the line ofIN. As the aircraft ascended ice particle con- centrations decreased. Definite increases in ICC were then observed simultaneously with seedline penetra- tions at 55 and 61 min ( not shown). These penetrations were at temperatures < -80e. Because the aircraft had climbed into higher winds its tracking information was invalid; however, clear signals on the ice nucleus coun- ter confirmed seedline penetration for the last five passes, 64-92 min, Fig. 4. Increases in particle con- centration were also observed during these five pene- trations at temperatures between -9 and -12.60C. Note that IN concentrations in the seedline had de- creased from a peak near 300 L -( soon after seeding to a peak near 30 L -( . Still considering the efficiency of the IN counter to be between 1 and 10% a significant number of nuclei were still available. In Fig. 4 the time of seedline interception is based on the peaks observed in ICe. In 3 of the 5 penetrations shown, the point indicated precedes the increase in IN by the delay time of the IN counter, 25-40 s, while in the penetrations at 64 and 77 min the 25-40 s delay indicates that IN were sensed on the edge of the ice crystal plume. In all five penetrations shown, the IN and ICC increase nearly simultaneously, providing reasonable evidence that the increases in ICC are the result of seeding. For these penetrations the advection speed of the seedline in- creased from 18 to 20 m s -1 . After the penetration at 92 min the seedline was near the crest and the down- wind edge of cloud. Given the age ofthe seedline and the width of the ice crystal plume, the overall dispersion rate was calculated to be 1.3 ill S-I.