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<br />Arizona winter storms illustrated by Super and Holroyd (1989) which tended to go through alternate stages <br />that were either efficient or inefficient. This may be common throughout the West. For example, Hobbs <br />(1975a) indicated that clouds over the Cascades were generally in two categories, "those in which ice <br />particle concentrations never exceeded 0.1 liter-i, and those in which the maximum concentrations were <br />no less than about 10 liter-i irrespective of temperature." It seems probable that well designed physical <br />experiments will show snowfall enhancement from naturally inefficient clouds, no enhancement from very <br />efficient clouds, and only suggestive evidence of snowfall increases from moderately efficient clouds. <br /> <br />~ <br /> <br />ci 'Q? <br />I <br />........... <br />I <br />U <br />Z <br /> <br />'~ <br /> <br /><Xl <br />ow <br />'I- <br />0<( <br />0::: <br /> <br />Il) <br /> <br /> <br /> <br />, <br />0.. <br />U <br />W <br />00::: <br />00.. <br />o <br /> <br />r-"'. <br />~ <br />~ <br />'--" <br />o <br />0:::' <br />w~ <br />I- <br /><( <br />~ <br /> <br />all) <br />- . <br />:::>0 <br />a <br />-' <br /> <br /> <br /> <br /> <br />o <br />00 <br /> <br />6 <br /> <br />12 <br />TIME (MST) <br /> <br />18 <br /> <br />24 <br /> <br />Figure 4-1. - Precipitation rate on, and vertically integrated liquid water above. the Wasatch Plateau on March 5, 1991. <br /> <br />Figure 4-1 demonstrates some of the complexities involved in attempting to conduct physical experiments <br />caused by natural storm variability. The figure shows the SL W observed by the microwave radiometer <br />on the upwind side of the Wasatch Plateau (bottom panel) and the precipitation rate measured on the <br />downwind side (top panel) during part of the wettest storm of the 1991 season. While some valley AgI <br />generators were on, several had problems apparently due to high winds, and little AgI was detected on <br />top the plateau until after 1300 hours (all times m.s.t.). Precipitation rates varied from less than 0.01 in <br />per hour to as high as 0.16 in per hour. (The latter value corresponded to a shift in wind direction and <br />large decrease in windspeed). To put the precipitation and SL W values in perspective, the SL W flux will <br />be estimated so it can be compared with precipitation. <br /> <br />35 <br />