Laserfiche WebLink
<br />unknown credibility. As would be expected, Secs. 5,6 and 7 contain some redundancy to <br />information already covered in less detail in Secs. 2, 3 and 4. Section 8 is a list of all references <br />cited within this report including those in Appendices A, B and C. <br /> <br />Several selected articles are briefly summarized in Appendices A and B for those readers with <br />a still deeper interest in the details of SL W availability and AgI seeding considerations. No <br />analogous appendix exists for propane seeding because that topic is well covered in recent and <br />readily available publications cited in Sec. 7. Appendix C is included to discuss recent analyses <br />of stability and wind observations collected over the Wasatch Plateau of central Utah during early <br />1991 and early 1994 major field programs. The results of those analyses are briefly summarized <br />in Sec. 3, dealing with valley-released AgI seeding. <br /> <br />2. Summary of Findings Regarding SL W Cloud <br /> <br />The major features of winter SLW cloud over Colorado and nearby Utah mountains are <br />summarized as follows: <br /> <br />1) Supercooled liquid water cloud frequently exists over windward slopes and crestlines of <br />mountains. It is estimated that SL W is present during approximately one out of five winter hours <br />for the Park Range and Grand Mesa of Colorado, and the Wasatch Plateau of Utah. The Park <br />Range value was 24% and it is a primary barrier, not shadowed from approaching westerly flow <br />storms by nearby upwind ranges. Both the Grand Mesa and Wasatch Plateau had values of 17% <br />for significant ( >= 0.05 mm) vertically-integrated SL W, and each has a nearby upwind barrier, <br />although significantly lower for the Grand Mesa. None of the datasets were oflong duration and <br />certainly cannot be considered as "climatologies." Thus, the estimate of 1/5 of all hours having <br />meaningful SL W present at mountain top altitudes should be regarded as a first approximation. It <br />may be that frequencies are lower for the secondary, shadowed ranges of central Colorado. <br />Hindman (1986) suggested that, when SL W cloud alone, snowfall alone or both existed at several <br />mountain top locations, SL W was more frequent over primary than secondary ranges. The SL W <br />frequencies for hours with cloud and/or snowfall present were given but not the frequencies for <br />all hours. <br /> <br />The overall results of the Colorado and Utah studies suggest that approximately 300 to 600 <br />seedable hours can be expected during a typical 5 month winter with the lower value estimated <br />for secondary ranges. As with snowfall rates, SL W amounts are highly skewed with many hours <br />having limited values and relatively few hours having abundant values. Even if seeding produced <br />average snowfall rates of only 0.01 inch h-I when SLW was present, beneficial snowpack <br />increases would certainly result. Evidence exists that seasonal excess SL W flux is equivalent to a <br />large fraction of the natural seasonal snowfall over all mountain ranges studied. The "bottom <br />line" finding is that excess SL W availability is sufficient for presumed successful seeding to <br />provide seasonal snowfall increases of at least 10%. <br /> <br />2) Highest frequencies and amounts of SL W cloud are routinely found in a zone just above <br />the windward slopes and crests of mountain barriers. The SL W condensate is produced by forced <br />uplift of moist air over the barriers. This sometimes triggers embedded convection which <br />produces additional SL W. This primary zone of SL W cloud is often concentrated in a shallow <br />layer, usually within about 3000 ft of the terrain (Rauber and Grant 1986, Sassen and Zhao 1993), <br />although the layer may be thicker, especially when embedded convection is present (Super 1995). <br />Fortunately, at least the lower portion of the primary SLW zone can be treated with ground <br /> <br />4 <br />