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<br />. . <br /> <br />CONCLUSION <br /> <br />~ . <br /> <br />Development of an ice sensing system using capacitive icing probes shows promise in that such a system <br />may prove to be an inexpensive and reliable alternative to icing probes currently in use. Preliminary tests <br />has given favorable results and have encouraged further capacitive probe/ice sensing system development. <br />Development is now focused on probe shape and ice/melt water shedding capabilities. <br /> <br />8.25. Heimbach, J. A., W.D. Hall and A.B. Super, 1997: Modeling and observations ofvaIley-released <br />silver iodide during a stable winter storm over the Wasatch Plateau of Utah. J. Weather Modification, <br />29,33-41. <br /> <br />ABSTRACT <br /> <br />An experiment was conducted over Utah's Wasatch Plateau which, due to stable conditions, resulted in <br />unexpected transport of silver iodide (AgI) and sulfur hexafluoride (SF 6) tracer gas from valley floor and <br />canyon mouth locations. The AgI ice nuclei (IN) and gas were transported vertically more than 1 km <br />above the valley release sites in spite of the existence of a valley-based inversion beneath a saturated <br />pseudo adiabatic layer. The Plateau top was well-target by IN at the surface; however, few IN were found <br />. at aircraft sampling levels. The SF6 was poorly targeted, with most of the surface penetrations by a <br />mobile sampler being in the valley or in a canyon accessible by highway north of the release point. There <br />were only three small airborne penetrations of the SF6. <br /> <br />The four-dimensional mesoscale model of Clark and associates was applied to this case. The model <br />showed generally a good agreement with the field observations and offered some insight into the manner <br />in which the tracer material was transported over the Plateau. The model suggests that the AgI was <br />initially transported from the surface through the inversion by the gravity wave mechanism, whereas the <br />SF 6 likely had a limited initial vertical impetus through anabatic motion. <br /> <br />DISCUSSION <br /> <br />The transport of AgI released from three valley sites was examined for an experiment conducted on <br />7 February 1994. Sulfur hexafluoride was also released from the mouth of a canyon located about 1.5 km <br />east of the northern-most AgI generator. The observations indicate transport of the AgI over the Plateau <br />despite its being released in a stable surface layer with weak and sometimes easterly winds. <br /> <br />, . <br /> <br />At both the RRS and the TAR Site, concentrations of 100 IN L-1 were commonly sampled with the <br />acoustical IN counter, effective at -20 oe. The temperatures at these sites were approximately -6 oe. The <br />IN effectiveness of the generators and solution used is 2.5 orders of magnitude less at -6 oC than at -20 oe <br />(DeMott et al. 1995), giving an effective concentration of only 0.24 IN L-~ at the Plateau-top sites. There <br />were, however, periods having concentrations of several thousand per liter effective at -20 oC suggesting <br />effective. concentrations on the order of 5 L -I. <br /> <br />No IN or SF 6 plumes were detected by the aircraft on the west track which could sample no lower than <br />300 m above ground level. The only significant IN plume detected aloft was over the east track in the <br />vicinity ofthe TAR Site up to 3.5 km MSL (minimum IFR elevation). For the acoustic IN counter, the <br />indicated spread of an IN encounter is due to actual spread plus a temporal component from mixing within <br />the cloud chamber (Heimbach et al. 1977). For this reason, estimating the peak concentration of IN at a <br /> <br />77 <br /> <br /> <br />