|
<br />906
<br />
<br />JOURNAL OF APPLIED METEOROLOGY
<br />
<br />VOLUME 30
<br />
<br />increases in ICC attributable to seeding may be
<br />10-20 I-I. An estimate of the ICC expected due to
<br />contact nucleation at this point is only 5 l-l, and in
<br />fact may be even lower since for this calculation a
<br />droplet concentration of 100 cm-3 was assumed,
<br />whereas the droplet concentration in the cloud after
<br />60 min was 10-15 cm -3. As FP observe, the small par-.
<br />ticles observed coincident with the ice nucleus regions .
<br />here probably nucleated, or at least began growing, only
<br />within the previous 5-15 min, so this lower droplet
<br />concentration may be more appropriate for calculation
<br />of the ice nucleus scavenging rate. This lowers the ICC
<br />resulting from contact nucleation to less than 1 l-I,
<br />much less than the ICC observed. Thus, we believe
<br />there are substantial reasons to doubt that the AgI-
<br />AgCl ice nuclei used here functions only by contact
<br />nucleation in the field. This is different than the con-
<br />clusions reached by DeMott et al. (1983) based on .
<br />laboratory measurements.
<br />We can only speculate concerning the nucleation
<br />mechanism for these ice crystals. Perhaps a turret of
<br />rising air coincident with the ice nuclei plume created"
<br />enough water supersaturation to allow the AgI-AgCl
<br />to act as condensation freezing nuclei; or perhaps there
<br />were embryonic crystals nucleated shortly after seeding
<br />due to forced condensation freezing (Finnegan and
<br />Pitter 1987), which only lately had experienced enough
<br />ice supersaturation to begin growing. The field mea-
<br />surements are not detailed enough to determine the
<br />exact conditions and rates of ice crystal nucleation
<br />
<br />within the seeded plume and thus determine the
<br />mechanism of ice crystal "nucleation. We do feel, how-
<br />ever, that these measurements combined with those of
<br />Deshler et al. ( 1990) are enough to suggest that these
<br />regions of enhanced ICC were generated by seeding
<br />and that more than one nucleating mechanism was
<br />operating in these cases. It remains for future experi-
<br />ments to better document the exact nucleating pro-
<br />cesses occumng.
<br />
<br />REFERENCES
<br />
<br />DeMott,f. J., W. G. Finnegan and L. O. Grant, 1983: An application
<br />of chemical kinetic theory and methodology to characterize the
<br />ice nucleating properties of aerosols used for weather modifi-
<br />cation. J. Appl. Meteor., 22, 1190-1203.
<br />Deshler, T., and D. W. Reynolds, 1990: The persistence of seeding
<br />effects in a winter orographic cloud seeded with silver iodide
<br />burned in acetone. J. Appl. Meteor., 29,477-488.
<br />-, - and A. W. Huggins, 1990: Physical response of winter
<br />orographic clouds over the Sierra Nevada to airborne seeding
<br />using dry ice or silver iodide. J. Appl. Meteor., 29, 288-330.
<br />Finnegan, W. G., and R. L. Pitter, 1987: Rapid ice nucleation by
<br />acetone-silver iodide generator aerosols. J. Wea. Mod., 20, 51-
<br />53.
<br />-, and -,1991: Comments on "The persistence of seeding
<br />effects in a winter orographic cloud seeded with silver iodide
<br />burned in acetone." J. Appl. Meteor., 30, 903-904.
<br />Hallett, J., and S. C. Mossop, 1974: Production of secondary ice
<br />particles during the riming process. Nature, 249, 26-28.
<br />Hill, G. E., 1980: Dispersion of airborne-released silver iodide in
<br />winter orographic clouds. J. Appl. Meteor., 19,978-985.
<br />Slinn, W. G. N., 1971: Time constants for cloud seeding and tracer
<br />experiments. J. Atmos. Sci., 28, 1509-1511.
<br />
|