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<br />1140
<br />
<br />JOURNAL OF APPLIED METEOROLOGY
<br />
<br />VOLUME 27
<br />
<br />clouds. While this might not be expected due to dif-
<br />ferences in residence time and much higher cloud
<br />droplet concentrations in the acoustical counter, the
<br />counter agreed well with cloud simulation chamber
<br />tests reported by Langer and Garvey (1980). Thus, the
<br />IN estimates might be considered a first approximation.
<br />The estimates of effective IN concentrations from
<br />Table 3 for the typical temperature profile and for those
<br />30K colder and warmer are plotted in Fig. 6 for com-
<br />parison with actual average ice particle concentrations.
<br />The shape of the dashed lines is determined by the
<br />decreasing concentration of IN with altitude combined
<br />with the increasing effectiveness of the IN at colder
<br />temperatures. The lines join in the upper right at tem-
<br />peratures colder than -15 oC, where the AgI effective-
<br />ness is no longer a function of temperature. It is seen
<br />that the production of ice particles did not exhaust the
<br />supply of IN at the higher altitudes. It is also apparent
<br />that on warmer days, the plume must rise to near the
<br />3.8 km level before the IN can reach a temperature at
<br />which they can become effective in concentrations
<br />greater than 3 L -I . The typical plume, rising over 500
<br />m, as shown above, will therefore have a top cold
<br />enough for significant nucleation only part of the time
<br />on the Grand Mesa. But when it is effective, ice-particle
<br />concentrations of about 10 L -1 can be expected to re-
<br />sult at the seeding rates used. These results are similar
<br />to the observations of Super and Heimbach (1988) for
<br />the Bridger Range of Montana.
<br />
<br />7. Aircraft seeding experiments
<br />
<br />Silver iodide ice nuclei were released from the aircraft
<br />on several occasions (see Table 4). On some of these
<br />occasions the air was expected to remain cloud-free,
<br />
<br />while on others the IN were released in, or upwind of,
<br />supercooled clouds. The amount of turbulence, shear,
<br />stability, and convection varied from experiment to
<br />experiment. In spite of the difficulty of consistently
<br />flying through an invisible aircraft-released plume,
<br />whose expanding cylindrical shape was often distorted
<br />by shear and lifted or lowered by orographic flow, the
<br />AgI plumes were detected in all experiments. The
<br />number of ice particle plume intercepts and the average
<br />ice particle concentrations in those plumes are listed
<br />under the "All" columns in Table 4.
<br />
<br />a, Horizontal widths and spreading rates
<br />
<br />As mentioned previously, the width of the seeding
<br />plume was best measured when the IN generated an
<br />obViously higher than background concentration of ice
<br />particles. In the manner of Super and Boe (1988),
<br />plume edges were determined from a visual exami-
<br />nation of a graph of the buffer-by-buffer IPC plotted
<br />against time. In high ice particle concentrations, the
<br />sample rate was restricted to one buffer s -1, during
<br />which time the aircraft traveled about 0.1 km. At lower
<br />concentrations the spatial resolution was coarser, but
<br />no ice particle plume edges had uncertainty ~0.5 km.
<br />(Interior edges where the plume concentration some-
<br />times returned to background values were ignored.) It
<br />was usually obvious when the seeding effect occurred
<br />because the concentrations ranged from at least twice
<br />to sometimes more than ten times the background val-
<br />ues. Ice particle plumes oflow concentration were used
<br />only if the IN measurements showed the edge of an
<br />AgI plume in about the same location.
<br />A flight track for each experiment was calculated
<br />from the integration of heading and true air speed.
<br />
<br />TABLE 4, Aircraft seeding experiments,
<br />
<br /> Intercepts
<br /> Minutes from
<br /> seeding Ice Ice-cone, L-1
<br />Date Time AgI
<br />(1986) (MST) Total Crest All All Crest All Crest
<br />Gear:
<br />7 Feb 1619 22 2 0 0 0 0
<br />5 Mar 1052 173 13 0 0 0 0
<br />6 Mar 0946 141 13 0 0 0 0
<br />Cloudy:
<br />10 Mar 1519 40 24-40 4 5 3 9,0 12,8
<br />10 Mar 1621 52 26-46 5 5 3 15,0 10,1
<br />II Mar 1521 9 I I 0 39.0
<br />14 Mar 1056 40 34 6 I I 12,5 12,5
<br />18 Mar 1053 107 53-75 9 8 3 10,2 9,7
<br />18 Mar 1249 45 21-45 7 4 4 20,9 20,9
<br />18 Mar 1506 115 46-70 II 7 4 19,1 24,1
<br />18 Mar 1636 26 6-13 4 4 2 27,6 19,3
<br />19 Mar 1356 44 24-44 5 4 4 16,6 16,6
<br />19 Mar 1452 54 35-54 6 5 4 11.7 9,3
<br />Totals and averages 46 28 15,9 15.4
<br />
<br />Ice plume
<br />spreading
<br />Rate m S-I
<br />
<br />Pot, temp, (K)
<br />Ave, Span
<br />292 1
<br />306 3
<br />306 4
<br />300 5
<br />300 5
<br />300 2
<br />298 3
<br />295 4
<br />295 4
<br />296 4
<br />296 4
<br />295 4
<br />295 4
<br />
<br />0,8
<br />1.7
<br />
<br />0,3
<br />5,5
<br />3,5
<br />1.7
<br />4,2
<br />1.9
<br />0,9
<br />
<br />2,3
<br />
<br />
<br />
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