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<br />. <br /> <br />.- <br /> <br />water depths greater than or equal to 0.05 mm) data sample indicates an SL W cloud frequency of <br />occurrence of about 73% of the time the lidar was operating, although only 51 % of the lidar shots detected <br />SL W clouds because of the frequent presence of range limiting attenuation from snowfall. A significant <br />development from this study is the application of a new autonomous computer algorithm for identifying <br />SL W clouds from single normalized lidar shots, using only the height derivative of the returned signal and <br />the minimum linear depolarization ratio. In terms of operational AgI cloud-seeding practices, we <br />conclude that it is mainly the upper portions of SL W clouds with relatively wann cloud-base temperatures <br />(2: -7 oc) that display potential for yielding increased snowfall, since only these clouds regularly produced <br />significant C2: 0.15 mm) radiometric liquid water depths. <br /> <br />CONCLUSIONS <br /> <br />... <br /> <br />The University of Utah Mobile Polarization Lidar has participated in several Utah-NOAA cooperative <br />weather modification field campaigns with the primary goal of identifying the heights of SL W clouds <br />embedded in the winter storms over the Tushar Mountains. With this knowledge and a large amount of <br />supporting in situ and remote sensing data, previous basic research using case study analyses have <br />illuminated important connections between orography, 5L W clouds, and precipitation (Sassen et aI., 1986, <br />1990). In this study, we have examined the climatological properties of lidar-detected SL W clouds for the <br />applied research objective of assessing the general seedability (using AgI-compounds) of Southern Utah <br />winter mountains storms and have concluded that there appears to be only a limited "window" for success <br />involving mainly the upper portions of the relatively warm (greater than -7 oC cloud base temperature) <br />SL W clouds containing significant C2: 0.15 mm) L W depths. In other words, cloud layers with inferred <br />cloud-top temperatures of colder than about -12 0 C in this region appear to be efficient users of <br />orographically generated 5L W. <br /> <br />We have found that the total of liquid-dominated and mixed-phase SL W clouds detected by the lidar with <br />the radiometrically detected (LW2: 0.05 mm) liquid clouds comprises approximately 75% of the lidar <br />dataset. The SL \V cloud-base heights during snowfall were most frequently observed within about 0.5 km <br />of the 2.57 km midbarrier field site. In comparison with the considerably more limited study from the <br />1983 field campaign (Sassen. 1985), these statistics differ somewhat. The previous study found a bimodal <br />distribution of SL W cloud-base heights with peaks at 3.0 and 4.5 km, and the combined lidar and <br />radiometer SL W frequency was 96%. However, the 1983 Tushar Mountain dataset, collected from a <br />western mountain-based site (1.89km) included only nonprecipitating or lightly precipitating storm stages. <br />This probably accounts for the number of layers previously noted at approximately 4.5 km, presumably in <br />the form of prefrontal altocumulus layers that did not produce snowfall. The SLW cloud height maximum <br />at 3.0 km, on the other hand. corresponds to an upwind part of the dominant orographic SLW cloud type <br />noted here to prevail at the midbarrier. Similarly, although the combined 96% SLW frequency from 1983 <br />data may reflect a bias from the absence of snowfall attention effects, it is possible in view of threshold <br />L W depth uncertainties that a frequency of as high as 90% could have been present during the 1985 and <br />1987 studies (i.e., using a L W> 0 threshold.) <br /> <br />I <br />,"" <br />I <br /> <br />One finding from the current study that deserves elaboration relates to the angular L W depth distributions <br />given in fig 8. In agreement with a similar analysis in Sassen et al. (1990), which indicated a rather <br />intimate association of nearby topographical features with the SL W distribution in just above-ground-level <br />orographic clouds, our averaged spatial L W distributions tend to display localized concentrations where <br />low-level flow is forced to ascend over steep topographical features aligned perpendicular to the wind <br />direction. Thu's, in a fairly linear, isolated barrier displaying complex terrain, as with the Tushar <br />Mountains, SL W concentrations could be found at many locations, depending on the wind direction. <br />According to this view, the concept of cross-barrier wind Speed as a cloud seeding criterion has limited. <br /> <br />.--. <br />. <br /> <br />45 <br /> <br />