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60 JOURNAL OF APPLIED METEOROLOGY VOLUME 17 photographs taken from the upper cloud physics aircraft. The water base appeared to be constantly rising. The ice appeared photographically at cloud base at about 15 min after seeding and was confirmed about 1 min later on a pass by the MRI cloud physics aircraft. The UW aircraft measurements indicated that the most intense portion of the snow shower occurred before 25 min after seeding. Particle concentra- tions for the lower two aircraft were taken from the PMS2 and PMS3 probes. Though the snow would definitely be melting, it is assumed that no appreciable fragmentation or coagulation was occurring. The concentrations quoted are the linear averages for the pass and, therefore, do not represent any particular location in Fig. 4. All interpolations and extrapolations away from the sampling levels are obviously crude. The temperatures plotted in Fig. 6 are from a combination of aircraft and rawinsonde data. Dew points measured by the upper two aircraft showed an extremely dry layer near cloud top. Turbulence measurements by an MRI probe on the upper aircraft showed values of 0.8 cmf S-l outside the cloud. Values averaged 5.1 inside the cloud on passes 2 and 3 and then dropped steadily to 2.1 on pass 8 and Were slightly less than 2 for most of the subsequent passes. Updrafts at the upper level reached an average of 6 m' S-l in / the western bubble on passes 2 and 3. On pass 4 the updraft values varied slightly around zero. Subsequent values were typically about 1 m S-l and therefore in the noise region of the instrumentation. The cloudbase aircraft found updrafts of about 2-3 m S-l during the early life of the experiment but little organization. -The aircraft soundings showed a dry adiabatic lapse rate below cloud to the surface. A generally constant mixing ratio, appropriate to the lowest cloud base, was found throughout most of the subcloudlayer. A lesser mixing ratio, appropriate to the final base, was found for several hundred meters below the initial base. Therefore, the cloud may have been initiated by surface convection, but it appeared to be maintained only by mid-level convection at its later stages. The cloud seeded with dry ice was the only cloud within sight ,that showed the presence of ice crystals visually except for the cumulonimbus clouds on the horizon. It was also the only cloud within 100 km to produce an echo on the 5.4 cm radar. That echo was weak and first was noticed at about the 0 dBZ level 10 min after seeding when ice crystal concentrations peaked over 100 .e-1 and the PMS3 instrument on the upper aircraft showed a very small number of particles (presumably aggregates) at the maximum size measur- able, 4.5 mm. The echo vanished (less than 0 dBZ) by 41 min after seeding 'Yhen crystal concentrations had decreased to less than 20 .e-1 and diameters were less than 1 mm at the upper aircraft level. The PMS2 probe on the upper aircraft showed an increasing crystal size after seeding. Part of this increase was due to aircraft descent to temperatures of faster crystal growth rate. The growth of the largest crystals is consistant with a growth rate initiated within .a few minutes of seeding and thereafter following the growth rates of Ryan et at. (1974). The 90 percentile size as measured by the PMS2 probe rose from 140 J.lm at 4.5 min after seeding (sampled at -9.50C) to 245 J.lm at 10.5 min after seeding (sampled at - 6.50C). Crystal sizes thereafter increased very slowly. The 90-percentile size of particles (presumably aggregates) measured by the PMS3 probe was about 1.8 mm from 10-24 min after seeding, while the median size reached only 0.75 mm. This is another indication of aggregation of high concentrations of ice crystals following seeding, pre- viously reported by Holroyd and Jiusto (1971). If one could account for the large number of crystals contained in aggregates, the dry ice effectiveness values cal- culated for this experiment would probably increase appreciably. The visual, radar and high ice crystal concentration characteristics of this cloud (and their timing) give strong indications that the ice crystals observed in the seeded cloud resulted from the seeding, especially when compared to all the other altocumulus elements present in the area. The crystal effectiveness estimate from this experiment alone ranges from 4 to 10 X 1011 crystals per gram of dry ice. These are probably conservative values due to crystal aggregation and undercounting by the ice crystal counter. e 6. Practicability of dry ice seeding Cloud seeding with dry ice offers several advantages over silver iodide provided that 1) it is practical to carry and drop adequate amounts from on-top cloud positions, and 2) on-top seeding is judged desira.ble for reasons not discussed in detail here. Some of the reasons for on-top seeding Can be found in St.-Amand and Elliott (1972), Summers et at. (1972) and Simpson (1976a,b). Of perhaps most significance is the greater visibility and ease of identification of the young, active elements of a developing cloud complex from on top rather than at cloud base. Advantages of dry ice over AgI have been mentioned recently by Smith (1974) and Simpson and Dennis (1974). Some of the more important possible advantages include the following: 1) Significant nucleation appears possible at warmer temperatures than with Agl. Using the pyrotechnic effectiveness values of Garvey (1975) it is evident that dry ice produces more crystals per gram, than AgI pyrotechnics at temperatures warmer than - 90C. There is some disagreement concerning the actual threshold temperature of dry ice pellets falling through a cloud (Eadie and Mee, 1963; Fukuta et at., 1971); however, significant nucleation is reported at - 20C for dry ice. The situation with AgI is more complex with the fraction of particles nucleating ice depending e e