Evidence for Changes in Microphysical Structure and Cloud Drafts Following Agl Seeding

William L Woodley, Daniel Rosenfeld

Abstract


A cloud physics aircraft was used in Texas during the period from 6 through 27 September 1998 to learn more about how silver iodide (AgI) seeding affects the microphysical structure and draft circulations within supercooled clouds. This was done in the context of the cold-cloud conceptual seeding model, which has been under development for many years. These investigations are part of a long-term master plan to provide a strong physical basis for glaciogenic seeding, as practiced now both experimentally and operationally in both Texas and Thailand. A historical overview of past studies of relevance and an examination of the conceptual model are presented prior to discussion of the new results. Twenty-one cloud physics units (14 Seeded and 7 Non-Seeded) were obtained during the 1998 studies. Composites of supercooled cloud liquid water, 2DC concentrations and cloud drafts were constructed for the seeded and non-seeded cases relative to the center of the initial cloud pass for each case. Composite differences were then obtained as a function of the treatment decision. The composite differences at the time of the treatment pass just before release of the nucleant were small except for the region +5 to +10 sec where the updraft for the S cases exceeded that of the NS cases. The S vs. NS differences were small in the period 60 to 180 sec after the treatment pass within +/- 10 sec of the center of the pass composite. This suggests that the overall effect of the seeding was small within 3 min of the initial seeding. By 181 to 420 sec, however, the seeded clouds near the center of the composite had stronger updrafts with higher 2DC counts and less cloud water than the non-seeded clouds. These findings are in accordance with the conceptual model, which calls for invigorated updrafts during the transition from supercooled water to ice. This pattern persisted for the period 421 to 660 sec and then decayed for passes > 660 sec (11 min) after the initial seeding. These results are supportive of the conceptual model that is guiding the Thai and Texas cold-cloud AgI experimentation, suggesting the main characteristic of seeded clouds is the existence and growth of ice particles in rising air that is being depleted of its cloud water by the growth of the ice.

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