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<br />2180
<br />
<br />JOURNAL OF THE ATMOSPHERIC SCIENCES
<br />
<br />VOLUME 35
<br />
<br />contribution mechanical fracturing can play in crystal
<br />concentration.
<br />
<br />a. Stratiform clouds
<br />
<br />Changes in crystal concentration for natural strati-
<br />form clouds due to mechanical fracturing appear to
<br />be limited to less than a factor of 10. A stratiform
<br />cloud is probably highly self-limiting in the process.
<br />If the cloud top is not sufficiently cold to generate
<br />enough crystals from natural ice nuclei and to utilize
<br />all of the condensate, the crystals will become rimed
<br />and begin to generate secondary particles by me-
<br />chanical fracturing. Since the growth time is limited
<br />in the slow updraft of a normally shallow' stratiform
<br />cloud, the generation of secondary particles should
<br />reach peak efficiency only near the base, and only
<br />a small fraction of the cloud will be affected by this
<br />mechanism.
<br />
<br />b. Isolated convective clouds
<br />
<br />Convective clouds contain several features which
<br />would at first appear to make them very efficient in
<br />generating secondary particles by mechanical fracturing
<br />compared to stratiform clouds. The turbulence in a
<br />convective cloud should increase the collision fre-
<br />quency over that of a stratiform cloud and the col-
<br />lisions should be more forceful. Convective clouds
<br />normally have higher liquid water contents which
<br />allow accretion and diffusion to proceed at an ac-
<br />celerated pace. Crystal sizes are normally larger and
<br />many convective cells contain large graupel.
<br />On the other hand, the updraft is sufficiently strong
<br />so that crystals do not reside in a favored growth
<br />region very long unless they are falling at the same
<br />speed as the updraft. In addition, most of the frag-
<br />ments are probably produced near the end of the
<br />life cycle of a convective cloud and sublimate before
<br />being reincorporated into an active cloud. The results
<br />of this study on secondary particle generation should
<br />be put into a convective cloud model before definite
<br />conclusions can be drawn, but the characteristic
<br />features of an isolated convective cloud seem to
<br />indicate that the generation of secondary particles by.
<br />mechanical fracturing has little effect on the main
<br />portion of an isolated convective cloud.
<br />
<br />c. Embedded convective clouds
<br />
<br />Embedded convective clouds should contain the
<br />same favorable features for secondary particle gen-
<br />eration that isolated convective cells contain, but
<br />should also be able to retain the fragments before
<br />sublimation in the surrounding environment. Frag-
<br />ments transported to the sides and top of an em-
<br />bedded convective cell will be continually mixed into
<br />the surrounding stratiform deck or into new cells.
<br />As the fragments grow, rime, and generate new frag-
<br />
<br />ments in turn, the background concentration of ice
<br />crystals in the cloud will rise above that expected
<br />from natural ice nuclei. As the concentration increases,
<br />the collision frequency increases. When a relatively
<br />high crystal concentration is reached the reduced
<br />liquid water content of the cloud limits further me-
<br />chanical fracturing. Crystal concentrations could reach
<br />100-1000 tim'es that expected from natural nuclei.
<br />
<br />Acknowledgments. This research was sponsored by
<br />the Atmospheric Sciences Section, National Science
<br />Foundation, under Grant GI-31460 and the State of
<br />Colorado, Weather Modification Rese'arch Funds.
<br />I would like to express my appreciation to Professor
<br />Lewis O. Grant for his guidance during the period
<br />of this study. The material presented in this paper is
<br />taken from a dissertation by the author submitted
<br />as partial fulfillment of the degree of Doctor of Phi-
<br />losophy at Colorado State University.
<br />
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