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<br />62
<br />
<br />JOURNAL OF APPLIED METEOROLOGY
<br />
<br />VOLUME 17
<br />
<br />and complete sublimation of the dry ice takes place
<br />within the volume.
<br />The number of turrets treatable with an aircraft
<br />payload of dry ice is shown in Fig. 7 for various com-
<br />binations of effectiveness and desired ice crystal
<br />concentrations. For example, if dry ice has an effective-
<br />ness of 1011 crystals per gram and an average ice crystal
<br />concentration of 100 .e-1 is desired (as for possible
<br />dynamic results), then curve B applies. From this
<br />curve it is seen that a 100 kg payload, if efficiently
<br />used, can seed 100 km3, or 4 large or 20 small turrets,
<br />where large and small turrets are somewhat arbitrarily
<br />defined as having volumes of 2S and S km3, respectively.
<br />Similarly, additional curves can be drawn for other
<br />effectiveness and concentration values. An effectiveness
<br />of 3 X 1011 crY5tals per grams in the above example will
<br />apply to a curve drawn about half way between curves
<br />Band C.
<br />Effectiveness values requiring the use of curves near
<br />A in Fig. 7 would indicate dry ice to be practicable
<br />only for small clouds. Effectiveness values near the
<br />8XlOll crystals per gram given by Fukuta et at. (1971)
<br />and crystal concentrations 100 .e-1 or less require the
<br />use of curves drawn near and to the right of curve C.
<br />Such values would make dry ice utilization very
<br />attractive.
<br />
<br />7. Conclusions
<br />
<br />The effectiveness of dry ice in producing ice crystals
<br />in supercooled clouds appears to be at least 2-S X 1011
<br />crystals per gram, though one well-documented experi-
<br />ment gave values twice as large. This may be a conserva-
<br />tive estimate yet to be adjusted upward when the
<br />absolute counting efficiency of the ice detectors becomes
<br />known and the numbers of crystals contained in
<br />aggregates can be estimated.
<br />The number of clouds that can be treated depends
<br />on the payload capacity, the cloud sizes, the desired
<br />crystal concentrations and the pellet sizes. As an
<br />example, Fig. 7 indicates that for the above effectiveness
<br />estimate and for a desired crystal concentration
<br />averaging 10 .e-\ about a hundred 2S km3 turrets could
<br />be seeded (apart from timing constraints) with a
<br />100 kg payload of small dry ice pellets. Ten such
<br />turrets could be seeded for 100 .e-1 concentrations
<br />instead. Except in clouds capable of suspending hail,
<br />pellet sizes larger than 7 mm diameter equivalent
<br />spheres will result in wasted dry ice faIling below the
<br />freezing level for seeding at the - lOoC level. The
<br />effectiveness values found in these experiments,
<br />together with the discussion of Section 6, indicate that
<br />dry ice is a practicable and attractive agent for on-top
<br />seeding of cumulus clouds.
<br />
<br />Acknowledgments. The above experiments are ob-
<br />viously the results of large team efforts on the parts of
<br />the Commonwealth Scientific and Industrial Research
<br />
<br />organization and the Bureau of Reclamation personnel
<br />and their contractors. The development of the various
<br />aircraft measurement systems by CSIRO, Convergence
<br />Systems, Inc., Meteorology Research, Inc., the Univer-
<br />sity of Washington, and Particle Measuring Systems
<br />has been vital to the success of these experiments.
<br />
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<br />
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