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DECEMBER 1978 GERARD E. KLAZURA AND CLEMENT J. TODD 1759 proceeds by coalescence alone. A drop-freezing tem- perature of -150C was selected. When a drop warmer than -150C grows to 5 mm, it breaks up into a 2.5 mm drop and many other fragments. Growth then con- tinues on the 2.5 mm drop but not on the fragments. Computations for a given trajectory are terminated under the following circumstances: 1) when the particle travels to within 0.2 km of the top of the cloud (as- sumed to be 16 km), 2) when the particle falls below cloud base (if liquid particle), 3) when the particle falls below melting level (if ice particle) or 4) if the particle remains in-cloud for 40 min. Inputs to the model are cloud-base height, relative humidity, liquid water content and updraft profiles; sounding (height, pressure, mixing ratio and tempera- . ture); and initial sizes and physical characteristics (molecular weight, density and van't Hoff factor) of the particles. The model output produces .both trajectories and growth patterns for particles of various initial sizes. An example of the output of the model is illustrated in Fig. 1. The cloud conditions in this example range from what would be expected in a warm, moist air mass (Figs. la-H) to a much colder, drier air mass (Figs. Ii and 1j). These graphs show the growth pattern and trajectory (height vs time), of particles with dif- ferent initial sizes. In addition to the height and time axis, one other ordinate is illustrated. It is used in conjunction with the height axis to show the liquid water content profile. The trajectory curves are con- structed of dotted, dashed or solid lines depending on what size category the particle is in. The curve labeled UP represents the trajectory of a parcel that would ascend exactly at the speed of the updraft. 3. Design of modeling experiment Multiple computer runs were made for cloud con- ditions characteristic of temperate and warm regions. Moist adiabatic conditions were assumed in-cloud and adiabatic values of liquid water content were used. The relative humidity was 100% at cloud base with a supersaturation of 0.1% existing at 100 m above 'cloud base all the way to cloud top. Relative hu- midities below cloud base were computed using a dry adiabatic temperature lapse rate with a constant mixing ratio. For simplicity, constant updrafts ranging from 1 to 25 m S-1 were used. Initial hygroscopic particle sizes ranging from 5 to 400 ~m were intro- duced at 0.5 km for cloud bases of 1, 2 and 3 km and in-cloud saturation adiabats (Om) of 16 and 23.20C. Some of the input data may not reflect accurate cloud conditions. The assumptions of steady updraft, adiabatic values of total liquid water content and non-competing precipitation particles can be validly questioned. Their use is justified by the nature of the twofold objective of the investigation. The first goal was to paint a broad picture of the effects of hygroc '-:ji~i~ij~f:~~~~~m~;tl~i~';~~~~~i~t;~,~~~S~~W;;i~~~~~~~'e+?IL-~-\-r~_,1~~~._,,/ scopic seeding under different cloud conditions and to form some general conclusions from it. Second, it was desired to illustrate how such a simple model can be used to gain a better understanding of complex pre- cipitation processes, and how it might also be used on a real-time basis as a tool in decision making. In addition there is no universal agreement on some of these matters. For instance,. some investigations have shown that liquid water contents in cumulus clouds are generally less than moist adiabatic (e.g., Warner, 1955; Draginis, 1958; Hirsch and Schock, 1968), whereas others have found moist adiabatic cores to exist in cumulus congestus clouds (e.g., Heymsfield et at., 1978; Ackerman, 1974). Twomey (1976) and Rokicki and Young (1978) conclude that it is the rare pockets (1% of total cloud volume or less) of adiabatic or near-adiabatic liquid water con- tent which dominate the formation of large drops. Based on his model runs, Nelson (1971) has found that high local liquid water contents, even if main- tained for only 1-2 min, completely dominate the coalescence history of the rest of the cloud. Competition between hygroscopic seeding particles and natural condensation nuclei should be minimal due to the relatively low density of seeding particu- lates which are anticipated to be required (on the order of 1 (-1). Biswas and Dennis (1972) theorize that the raindrop size distribution below a seeded cloud does not differ greatly from that in rain below an unseeded cloud. The drop distribution evolves during fall, with some drop collisions leading to coalescence and others leading to drop breakup (e.g., Spengler, 1972). These drop impactions reveal a self- regulating mechanism in nature that enables collisions to influence both the initial growth and the deter- mination of final size for large drops. This mechanism may preclude any significant competition between hygroscopically induced drops. Rokicki and Young (1978) conclude that unlike silver iodide seeding, there is no danger of oversee ding using hygroscopic seeds. 4. Results a. Case studies Referring back to Fig. ,1, it can be seen that ini- tially the hygroscopically initiated hydrometeo!."s rise at almost the same velocity as the updraft, but fall rapidly as they grow to larger sizes. The particles in Fig. 1a (1 m S-1 updraft) rise only 0.5-1.0 km into the cloud and fall out the base 21-28 min later as 0.6 to 1.5 mm diameter drops. It is interesting to note that the smallest particles (5 ~m initially), which rose highest and spent the most time in cloud, ap- peared at cloud base as the largest drops. The same pattern is apparent in Fig. 1b (2 m S-1 updraft), except here the in-cloud time of the particles is decreased by about 4 min and the drops are larger (2-4 mm diameter). In this case, the hydrometeors .. , .'''';~ hi", ~~~:",4;,"~:;;i.;2~.~",,,*,,......~.~ '~'r""';,;;;j;, /'