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<br />. . <br /> <br />30 L-1 and snowfall by 0.25 mm h-I. Higher propane release rates may be appropriate for these slightly <br />supercooled temperatures. <br /> <br />A third case study experiment with AgI seeding, conducted just after the March 5 propane seeding, did not <br />enhance IPC during temperatures near -1.5 oC at the HAS and -3.6 oC at the TAR. It was too warm for <br />the contact-freezing mechanism to operate efficiently, and apparently the forced condensation-freezing <br />mechanism was also ineffective in spite of AgI release within supercooled liquid cloud. <br /> <br />The second experiment of March 11 th had a HAS propane release into a cloud temperature of 0.9 0 C with <br />TAR temperature of-3.0 oC. This experiment suggests the seeding resulted in about 15 L-1 IPC <br />enhancement at the TAR with perhap~ 0.1 mm h.1 snowfall. <br /> <br />These March 5th and March 11 th results both argue for improved field documentation of propane <br />effectiveness, especially at temperatures warmer than -5 oC where AgI is believed ineffective. Higher <br />propane release rates seem appropriate for the slightly supercooled temperatures (-1 to -2 OC) experienced <br />at the release site during these two experiments. It may be that the propane rate used, 6000 g h-t, is <br />appropriate for colder cloud temperatures. Unfortunately, no successful propane experiments were <br />conducted when HAS temperatures were in the -2 to -5 oC range. <br /> <br />There is no doubt that propane seeding can increase snowfall attemperatures warmer than -5 oC when <br />excess SL W is available. The questions yet to be answered are what release rate is required to produce <br />significant snowfall and is such a release rate economical? The potential importance of being able toseed <br />slightly supercooled cloud, found in abundance especially in California but throughout the intermountain <br />West as well, is too great to ignore these questions. <br /> <br />. <br /> <br />~. <br /> <br />Two experiments conducted on January 5th suggested that the forced condensation-freezing mechanism <br />may have increased both IPC and snowfall when HAS temperatures were between -3.0 to -3.7 oCand the <br />TAR was near -5.5 oC. Ifverified, such warm temperature AgI nucleation would have important <br />ramifications. But additional evidence is needed to ensure that these preliminary suggestions are not <br />simply the result of natural variability. There is little other evidence from this series of 24 experiments on <br />9 different days that high altitude AgI seeding was effective in IPC enhancement when Plateau top <br />temperatures were warmer than -6 oC. <br /> <br />These experiments have provided valuable information to partially address seeding effectiveness. Data <br />from the subsequent season are now being analyzed. Additional seeding experiments should be conducted <br />on the Plateau between the HAS and the TAR. These experiments are simple and economical, and <br />targeting ofthe TAR site is routine with southwesterly flow at the HAS. Future experiments of this type <br />should concentrate on periods with no more than very light natural snowfall. Detecting microphysical <br />changes caused by seeding during higher natural snowfall rates is difficult because of the variability of <br />such snowfall. <br /> <br />. ~ <br /> <br />80 <br />