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<br />;. <br /> <br />from a Plateau top microwave radiometer and from aircraft sensors. Temperatures of SL W cloud reached <br />by the AgI were usually only mildly supercooled, and estimated AgI ice nucleus concentrations were quite <br />small. Aircraft measurements of ice particle concentrations and gage observations of snowfall indicated <br />that any snowfall increases caused by seeding were at limited rates and occurred only when seeded clouds <br />were colder than -9 oC at aircraft sampling altitudes. - <br /> <br />2.3 Summary of SLW Findings <br /> <br />In summary, excess SL W often existed over the Plateau during at least portions of storm passages. During <br />the early 1991 and early 1994 major field programs, SL W existed during post-frontal southwest flow and <br />typically disappeared after frontal passage, as soon as Plateau top winds shifted to northwest. The SL W <br />was confined to ~ shallow layer less than 1,000 m thick (often considerably less) above the Plateau where <br />temperatures were typically mildly supercooled. Maximum SL W amounts were over the windward slope, <br />in the orographic uplift zone, and amounts rapidly decreased during passage over the Plateau. <br /> <br />This general portrayal of SL W availability implies that effective seeding must create significant ice <br />particle concentrations in the SL W cloud very soon after condensate is produced during transport of moist <br />air up the Plateau's windward slope. Otherwise, growth times will be too limited for much snowfall <br />production before the SL W is depleted. Accomplishment of ground-based seeding under these <br />circumstances presents a significant challenge, especially with AgI, because the condensate is fonned at <br />only slightly supercooled temperatures. This challenge_ is not limited to the Plateau, as much of the Rocky <br />Mountain region has similar shallow, slightly supercooled liquid water zones as reported by a number of <br />authors. <br /> <br />, <br /> <br />l <br />! <br />I <br />I <br />I <br />I <br /> <br /> <br />! <br />I <br />I <br />I <br />I <br />1 <br /> <br />9 <br /> <br />