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<br />APRIL 1990 <br /> <br />DESHLER, REYNOLDS AND HUGGINS <br /> <br />299 <br /> <br />The possibility that the increased ICC at the seedline <br />resulted from aircraft produced ice particles (APIP) is <br />unlikely. During the SCPP several specific experiments <br />were conducted to test for APIP from the seeder aircraft <br />and no evidence was found (Gordon and Marwitz <br />1986); although in other experiments the research air- <br />craft was found to generate APIP in 2 out of 37 pen- <br />etrations (Marwitz et al. 1986). During sampling of <br />this placebo seedline the research aircraft flew orthog- <br />onal to a line first flown by the seeder aircraft, and did <br />not overfly any of its own track during the penetrations <br />at 5, 8, 16, and 23 min. Yet the dispersion of the region <br />of increasing ICC was 5 to 7 m s -I, Table 2. If the <br />research aircraft was creating the ice crystals such high <br />dispersion rates could be expected only on passes in <br />which the aircraft overflew a previous track through <br />the seedline. This fact plus the abrupt change in the <br />ice crystal size distribution on the 28-min pass, a result <br />unexpected from APIP, are evidence that this was a <br />region of natural ice generation. <br />The placebo nature of this case is borne out by the <br />first six penetrations of P3. Neither ice nuclei nor <br />abrupt increases in ICC were measured at the penetra- <br />tion times. Although ICC were increasing at the seed- <br />line the dispersion of this ice region would be > 5 m <br />S-1 if it had originated at the seedline. This is much <br />higher than the 1-2 m S-l measured by several inves- <br />tigators (Hill 1980a; Stewart and Marwitz 1982). It is <br />likely that increasing ICC were associated with the <br /> <br /> <br />seedline because the aircraft pointer was set in a region <br />of high liquid water, a favorable region for ice crystal <br />growth, and an ice multiplication process became ac- <br />tive. The radar data during this period showed a wide <br />area of weak echo. The surface measurements collected <br />during this experiment will be discussed along with the <br />surface measurements from the second experiment on <br />this day. <br /> <br />3) FIXED TARGET EXPERIMENT 2, 1900-2000 UTe <br /> <br />Using the same seedline position as the first exper- <br />iment, the seeder aircraft released AgI along three 37 <br />km long seedlines (S 1-S3) between 1841 and 1857. <br />Since seeding was done at -60C-1he top of the low- <br />level liquid water cloud-this case presented an op- <br />portunity to test the capability of AgI NH4I NH4Cl04 <br />at this warm temperature. <br /> <br />(i) Aircraft measurements <br /> <br />Following delivery of the last seedline the research <br />aircraft tracked S3 for 35 min. A temporal plot of se- <br />lected aircraft data is shown in Fig. 7. The increase in <br />ice nuclei (IN) 25-40 s after the aircraft returns to S3 <br />indicates clearly that this cloud was treated with Agl. <br />Based on the aircraft pointer, S3 advected from 2080 <br />at 15.5 m S-I. Although the pointer was set on S3, the <br />aircraft flew orthogonal to the seedlines and S I and S2 <br />were also penetrated. This can be seen in both the mea- <br /> <br /> IlXJ.O <br />:. 50.0 <br />y <br />0 <br />N <br /> 0.0 <br /> 20.0 <br />E <br />"" <br />a 10.0 <br />Q. <br /><( <br /> 0.0 <br /> 30.0 <br />~ <br />~ 15.0 <br />z <br />1l <br /> 1).0 <br /> 1855 1900 1905 1910 1915 1920 1925 <br /> TII"E (UTe) <br /> <br /> <br />\930 1935 <br /> <br />FIG, 7. Overview of the aircraft data collected during the second fixed target experiment, 1855- <br />1935. Points of interception ofSI-S3 and the age (min) of each seedline are shown. The aircraft <br />pointer was set on S3. <br /> <br />__L <br />