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1766 JOURNAL OF APPLIED METEOROLOGY LEGEND ~'\ A..,~ ~'V- <(0 ..:o,,'V- \..~ ot~ CLOUD TOP 5486 m - .,....... 26JLm e 5182 m _ _..,. .......,'~!i.--_ B eBB B ./"' / 100JL';iii--B'-ij--B'---a--e---;;- .... ...-' ..... ..",. . .... 10 B E 6 -= l- I c.:> 4 W I VOLUME 17 ...... 0- I m m .... 1.0 - 2.5 mm 2.5-5.0 mm (DROP BREAKUP REGION) B DROP BREAKUP 2 CLOUD BAS E SALT RELEASED F'RST ECHO FIRST RAIN MOD ER ATE SHOWERS BEN EATH CLOU 0 OeSER v ED OBSERVED REACH'NG GROUND BASE 5182 m AT CLOUD BAS E 0 5 Iq /5 20 25 30 35 40 45 TIME (min) FIG. 9. Condensation-coalescence model computations of the growth pattern and trajectory of three different sized salt particles released beneath the base of a cloud whose physical characteristics are believed to be similar to a real cloud system that was stimulated to rain through salt seeding. See text for discussion. the Hirsch (1971) convective cloud model (Biswas and Dennis, 1972). These internal profiles (liquid water content, updraft, etc.) were then used as input data to the condensation-coalescence, single particle model. Salt seeding was simulated on this predicted steady-state cloud. Rather than assume the particle to be ejected when it reached within a certain distance from cloud top, condensation or evaporation was allowed to act throughout the process. The results are shown in Fig. 9. Following the reasoning set forth in the last sec- tion, it is assumed that the 10 .urn hygroscopic seed represents the natural state of existence of large cloud droplets (i.e., ",SO .urn) near cloud base. Notice that this size particle travels right to the top of the cloud where it evaporates. This is consistent with the fact that none of the unseeded portion of the cloud system ever precipitated. The larger hygroscopic seeds (26 and 100 .urn shown) were predicted to grow to 1 mm size around 5334 m about 11-15 min after release beneath the cloud. The .first radar echo was observed at 5182 m 13 min after seeding began. The larger particles con- tinued to grow and reached a balance level around the 5 km level (4953 m). Drop breakup began about 19-21 min after seeding began, and due to the steady- state nature of the simulated cloud, continued on indefinitely. The first rain was observed at cloud base by the seeding aircraft's observ.er 22 min following the start of seeding. Up to the point where the first echo was observed there is good agreement between the model results and actual cloud observations. However, since no drop competition or negative buoyancy and divergence effects due to waterloading are accounted for, the interpretation of results gets more qualitative con- cerning a drop breakup situation. The formation of a drop breakup or waterloading zone as predicted by the model seems to indicate that a rainshower would be expected from the real cloud. The model does not predict the amount of precipitation to occur from seeding, but it can give a good indication of whether or not initiation or enhancement of precipitation would be expected. 6. Comparison with similar s~udies In the Introduction it was mentioned that Takeuchi (1975) and Rokicki and Young (1978) carried out similar modeling investigations using one-dimensional, steady-state, parcel microphysical models. Takeuchi's model computes the growth of an array of particles . in an ascending parcel. It computes the growth of 26 categories of precipitation particles (13 liquid and 13 crystal) using continuous coalescence equations as were used in the present study. Particles are removed by sedimentation, but are not considered in sub- sequent parcels which may be rising in the cloud. Collecti'on processes in the model used by Rokicki and Young are treated quasi-stochastically and sedi- mentation is not computed. As would be expected some of the results of the present investigation can be compared with related ones from these two studies. All three studies agree that seeding with large drops (or hygroscopic seeds which grow to large drops at cloud base) produces precipitation more efficiently than AgI in all cases with cloud base temperatures > lOoC. Furthermore, the present study and that of Rokicki and Young found this to be true for cloud- base temperatures > Ooe. The reason why Takeuchi found AgI seeding to be effective at the warmer cloud-base temperatures may be due to his assump- tion that the effect AgI seeding had was to freeze the large particles (40-80 .urn) that occurred naturally " .~ \.