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APRIL 1990 DESHLER, REYNOLDS AND HUGGINS 323 TABLE 5. Comparison of initial conditions in cases with seeding effects vscases..with no efIeCts. Cases with Cases with no A verages of seeding effects seeding effects Liquid water content (g m-3): Range 0.0 to 0.12 0.02 to 0.09 Mean 0.05 0.05 Number of cases 15 11 2D-C concentration (L-1): Range 4 to 78 2 to 130 Mean 23 81 Number of cases 14 11 2D-P concentration (L -I): Range I to 10 2 to 15 Mean 3 7 Number of cases 14 11 dBZ max: Range <MDS to 17 5 to 22 Mean 8 14 Number of cases 9 11 then evolved to include more careful monitoring of delivery of CO2 and results improved in 1984/1985. A chopper was installed on the aircraft for 1985/1986 to allow an even more direct delivery of CO2, It is not known if this was partly responsible for the poor success rate for that year. 2) AGI FLARES Prior to the winter of 1986/1987 seeding with AgI was in the form of 20 gm TB I type pyrotechnic flares dropped at a rate of one every 10 s (I km) to produce a I km deep curtain of seeding material. These flares were used for 15 seedlines on four days and the aircraft penetrated all of them. Seeding signatures were found in 4 (27 percent) of these 15 curtains. Two of the cur- tains with seeding signatures were tracked during two experiments in 1980 (Stewart and Marwitz 1982). During one experiment the effects of seeding were fol- lowed for 57 min in a nonprecipitating altostratus cloud with no natural ice. Ice nuclei counts above background were noted during these two experiments, but the signal was not nearly as strong as observed in the 1986 ex- periments. This was probably due to more careful op- eration of the counter during 1986. No ice nucleus counts were observed for the case on 26 February 1982, while the ice nucleus counter was not used for the 4 March 1985 case. Increases in ICC were not observed on 4 March 1985, even though there were relatively high amounts of SL W at the seedline. Although this is a limited sample for an assessment of the effectivity of the flares, there are problems associated with them. They are expensive. For the ten seedlines released on 4 March 1985 nearly $10 000 worth of flares was con- sumed. The ice nucleating activity of the flares is two to three orders of magnitude lower than AgI in an ace- tone solution, especially when the tracer cesium is added to the flares. The flares deteriorate with age, and the quality control is poor. In one test of the flares <50 percent of them burned for the 30 s required. 3) AGI IN ACETONE In 1986/87 AgI was released as a line source using a 3 percent solution of AgI NH41 NH4Cl04 burned in acetone. Interception of seedlines were noted by in- creased ice nucleus counts and increased ICC. There were seven seedlines initiated on three days using this mixture. The research aircraft penetrated all seven seedlines and detected seeding signatures in six (86 percent) of them. On 2 February 1980 one seedline was created by burning Agl in an acetone solution, but not using the AgI NH41 NH4C104 mixture. No seeding effects were noted from this line, although clear seeding signatures were noted when Agl flares were used earlier on this day (Stewart and Marwitz 1982). In 1986/87 the AgI produced seeding effects that could be tracked for over 45 min, while one seedline on 22 December 1986 was tracked for 90 min. The increased success with the AgI-acetone solution compared to the AgI flares was probably due to: 1) more dependable operation of the NCAR ice nucleus counter during 1986 and, therefore, better documen- tation of plume location; 2) a continuous source of ice nuclei rather than a curtain with gaps as may have occurred with AgI flares; and 3) higher activity of the AgI acetone solution compared to the AgI flares. Continued production of small ice crystals as nuclei are lifted to colder temperatures, and thus a continuous source of high ICC explains the increased longevity of the AgI-acetone seedlines compared to the CO2 cur- tains which nucleate a large number of crystals only at the seedline. The maximum age observed for a CO2 curtain was 36 min, but more commonly the longest times were 25-30 min. 4) ICE CR YST AL GROWTH The results noted in columns 7-10 in Table 6 will be discussed together. The ice crystal habits produced by seeding were consistent within the narrow temper- ature range over which these experiments were con- ducted. Plates and columnar habits dominated the ob- servations, but needles were rarely observed. Most early curtain interceptions ( < 10 min) showed pristine crys- tals while the later ones (> 20 min) had rimed particles. One case (26 January 1985) with very low liquid water and weak winds had unrimed crystals 33 min after seeding. There was a distinct lack of aggregation within the seeded curtains. This is in contrast to the obser- vations of Huggins and Rodi (1985), who found large aggregates after 10 min in convective clouds seeded with CO2, and the observations of Deshler et al. (1987), who found aggregates in snowfall from low-level stratus seeded with CO2.