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<br />. <br /> <br />e <br /> <br />11 <br />The effects of ice particle concentration on the probability <br />of occurrence of aggregates and crystal type on the size of <br />aggregates was shown b~l Hobbs et ale (1974a) for ice particles <br />collected in clouds and at ground stations in the Cascade <br />Mountains. At temp,eratures < -lSoC and ice particle <br />concentrations < 0.1 an-3, aggregates were unlikely to form. For <br />temperatures > _SoC and particle concentrations > 1 an-3 there was <br />> 50% chance for aggregates to form. Further, the size spectra of <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />aggregates depended on their canponent crystals. <br /> <br />The larger the <br /> <br />.e <br /> <br />component crystals, the larger the aggregate. <br /> <br />Size spectra were <br /> <br />independent of the precipitation rate and air temperature. <br /> <br />. <br /> <br />2.2.2 Aggregation models <br /> <br />.e <br /> <br />One of the earliest numerical models of snowflake aggregation <br /> <br />(Austin and Krauss, 1968) assumed an ensemble of ice crystals with <br /> <br /> <br />uniform size and shape. Aggregation was initiated through "random" <br /> <br /> <br />collisions due to erratic fall paths and electrical attractions. <br /> <br />Aggregation proceeded by further "random" collisions and also <br /> <br /> <br />through "ordered" collisions as snowflakes overtook crystals or <br /> <br /> <br />other aggregates with smaller fall velocities. The model was used <br /> <br />to study the assumptions regarding the initial size, number and <br /> <br />type of crystals, and the frequency of "random" collisions to <br /> <br />produce realistic size s~ectra. <br /> <br />. <br /> <br />. <br /> <br />e <br /> <br />. <br /> <br />. <br />