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<br />1138 <br /> <br />JOURNAL OF APPLIED METEOROLOGY <br /> <br />VOLUME 27 <br /> <br />ments may be approximately applicable to mountains <br />of comparable dimensions under similar atmospheric <br />conditions. <br />It is also noteworthy that at least one ice particle <br />plume was detected in air that was saturated with re- <br />spect to ice but unsaturated with respect to water. The <br />previous history of these air parcels was not constantly <br />monitored, but it appeared that natural liquid droplets <br />were not being produced in the air exposed to the IN. <br />Either the IN from the AgI-NH4I complex can operate <br />as deposition nuclei, or the IN encountered water <br />droplets in a small (unnoticed) fog formed from the <br />combustion products of the seeding generator very near <br />the seeding site. <br /> <br />c, Concentrations <br /> <br />One final aspect of the ground-released AgI plume <br />is its concentration. It was not possible to measure the <br />concentration profile of the plume with the acoustical <br />ice nucleus counter because of the long time required <br />to flush the system. Furthermore, the ice particle plume <br />concentrations were dependent on the presence of su- <br />percooled liquid water or at least, ice-saturated con- <br />ditions and the rate of activation of the IN under those <br />conditions. The ice particles may not have been sam- <br />pled near their altitude of nucleation and growth, so <br />that ice particle concentrations may not represent IN <br />concentrations at the flight level. The total number of <br />IN detected per pass, however, did appear to be related <br />to IN plume concentration. In some cases the ice par- <br />ticle plume was only a few seconds of travel time wide <br />compared to the several minutes over which the IN <br />were counted. In general, the highest counts were found <br />near the generator and near the ground, as would be <br />expected. Low counts were found near the top of the <br />plume and farther downwind as the plume broadened <br />and became more dilute. Plume meanders in the ver- <br />tical created a large scatter in the total count measure- <br />ments. <br />The total IN counts per pass were partitioned ac- <br />cording to the locations at which the IN were first de- <br />tected. The vertical resolution for the data shown in <br />Table 2 was selected to be 0.2 km; the horizontal res- <br /> <br />olution was 2 km beginning at the seeding site. Shown <br />in each bin of Table 2 is the average of the total counts, <br />the number of contributing passes ranging from zero <br />(blank bin) to 20. The standard deviation of the values <br />was of similar size, sometimes larger, sometimes <br />smaller. The generator locations were near the lower <br />left corner of the table where the numbers are larger. <br />Data from all of the experiments illustrated in Fig. 4 <br />were used, totaling 20 I passes. The altitudes for the <br />Uncompahgre Plateau experiment were raised by 400 <br />m to make the crest altitude agree with the Grand Mesa <br />crest. The general thinning of the plume with altitude <br />and downwind distance can be seen within the scatter <br />of numbers. Further partitioning into lower and higher <br />altitude releases of AgI smoke did not reduce the <br />scatter. <br />The same data were further combined to include <br />only those passes from 2 to 14 km downwind of the <br />seeding site, or over the first crest for plumes traveling <br />over both western arms of the mesa. Using the same <br />vertical resolution but no horizontal resolution, a ver- <br />tical profile was determined, as shown in the rightmost <br />column of Table 2 and plotted in Fig. 5 for 161 passes. <br />Again, the standard deviations (not shown) were about <br />the size of the averages. A trend line that excludes the <br />upper and lower points (which have few contributing <br />measurements) is plotted on a logarithmic scale. That <br />line is expressed by <br /> <br />loglOC=-1.l19A+6.147, (1) <br /> <br />where C is the average total nuclei count per pass and <br />A is the altitude in km. A second, dotted line shows <br />the trend for the averages of total IN ("+" symbols) <br />for only the 45 passes with an ice particle plume. Con- <br />sidering the large scatter in the data, all that will be <br />claimed is that there is no obvious scavenging of AgI <br />in these ground seeding cases involving ice. Otherwise <br />the IN from passes with ice particles would have been <br />fewer than those in clear air. This is in contrast to the <br />results (mentioned below) for the aircraft-released AgI, <br />but no explanation is offered here. <br />The average ice particle concentrations ( determined <br />in the manner of Super et al. 1988) within the ice par- <br /> <br />TABLE 2. Averages of total AgI nuclei counts per pass, <br /> <br />4 <br /> <br />Range from seeding site (km) <br /> <br />Combined <br />2-14 <br /> <br />6 8 10 12 14 16 18 20 <br />92 <br />32 <br />III 92 29 22 71 <br />71 137 66 171 252 60 64 <br />184 194 107 13 32 55 <br />214 242 322 77 56 <br />668 203 208 47 262 <br /> 159 183 388 <br /> <br />Alt. <br />(km) 0 2 <br />4.4 <br />4.2 <br />4,0 <br />3,8 <br />3,6 8 87 <br />3.4 0 321 <br />3,2 504 246 <br />3,0 1150 <br />2,8 <br /> <br />76 <br />70 <br />26 <br />98 <br />185 <br /> <br />383 <br /> <br />92 <br />39 <br />82 <br />69 <br />162 <br />217 <br />675 <br />271 <br /> <br />