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<br />V. CONCEPTUAL MODELS <br /> <br />The characteristics of cumulus congestus clouds observed in the Miles <br />City area indicate that a firm evaluation within a reasonable 'length <br />of time is most likely from an experiment using ice-embryo seeding <br />for microphysical effects. This does not imply that dynamic effects <br />are not present; however, dynamic effects are expected to be negligible <br />with the light seeding rates used in HIPLEX-1. This assumption will <br />be verified in the physical evaluation that is an integral part of <br />the experiment. <br /> <br />The concept presumes that an increase in the average ice-crystal <br />concentration to about 10 per liter in that region of selected <br />growing cumulus congestus clouds where water is present and the <br />temperature is higher than -lD oC will lead to additional precipi- <br />tation and to an increase in the proportion of seeded clouds that <br />precipitate. The sequence of events (physical hypotheses) leading to <br />the changes in the precipitation characteristics of the clouds is <br />hypothesized to be as follows: <br /> <br />i <br /> <br />1. An average ice-crystal concentration of about 10 per liter is <br />produced in the supercooled water cloud at temperatures higher <br />than -10 .C. The initial ice-crystal concentration in the unmixed <br />seeding plume will be considerably higher to allow for the effects <br />of diffusion. The average ice-crystal concentration produced by <br />seeding in these warmer cloud regions is higher than found :in <br />un seeded clouds at comparable times after treatment.* <br /> <br />2. Ice crystals grow by diffusion to a size where riming occurs, <br />so that higher concentrations of rimed crystals appear in seeded <br />clouds at comparable times after treatment. The seeding-produced <br />crystals tend to develop as columns. Crystals found at these <br />temperatures in un seeded clouds at comparable times after treatment <br />tend to have habits characteristic of growth at lower temperatures. <br /> <br />3. Rimed ice crystals in the liquid portions of the cloud grow by <br />accretion to graupel on the order of 1 mm in diameter and in <br />concentrations of about 0.1 per liter, which then fall through the <br />cloud. Accretional crystal growth is accompanied by a decrease, <br />relative to the unseeded clouds at comparable times after treatment, <br /> <br />* "Treatment" refers to the release of seeding material from the seeding <br />aircraft (in which case the cloud is seeded) or the activation of a <br />placebo mechanism (resulting in an unseeded cloud). <br /> <br />8 <br />