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<br />considered. A knowledge of this ratio is therefore <br />vital to the definition of modification poten tial in cold <br />orographic clouds. <br /> <br />The study of Grant (1968) suggests that <br />occasionally the ice crystal to ice nuciei ratio may <br />attain a value of about ten at Climax in the temperature <br />range where dendritic crystals would be expected to <br />form in the upper portion of the cloud system. If the <br />results of Grant are incorporated into the model, the <br />overall picture of requirements for modification is <br />hardly changed. From Figure 3 it is seen that a <br />value of ten for the ratio at 500 mb temperatures <br />about -l5C would reduce, but not eliminate the <br />modification potential for most conditions displayed. <br />The natural snowfall process at Climax and Wolf <br />Creek Pass, Colorado, is now investigated for <br />further evidence pertinent to a solution of this <br />question. <br /> <br />f. - "-Reiatio~shipof mean daily snowfall to cloud <br />top temperature <br />Mean daily snowfall was computed as <br />a function of th e f100 mh temperature for two winter <br />seasons at Wolf Creek Pass and for five years at <br />Climax, Colorado. Only non-seeded precipitation <br />was used in this study. The precipitation data <br />recorded at Wolf Creek Summit and Wolf Creek West <br />stations were pooled to increase the available sample. <br />The mean daily snowfall was computed utilizing a <br />running mean over a two-degree temperature interval. <br />Figure 5 shows the results of this study. <br /> <br />~ ~6r ~ 13 <br />ii6r g ,6 <br />~~ 15 <br />S..J4 iJi 4 <br />~ ~ r <br />~ ~2 Q2r <br />~~ ~ [ <br />o -10 <br /> <br />WOLFCREEK <br /> <br /> <br />\--- <br />( <br /> <br />-14 <br /> <br />-18 -22 -2s -30 -34 <br />500 MB TEMPERATURE (OCl <br /> <br />-is <br /> <br />w <br />F~ <br />6~6 <br />~~ <br /> <br />zw4 <br /> <br />~~ <br />..J~2 <br />~g <br />!zol'i <br />~~o <br /> <br />~ <br />~ ,6r <br />I 4 <br />~ ,2 <br />g <br />~ 0 -10 <br />::;: <br /> <br />ell MAX <br /> <br />- - '__ _ ~~TENTlAL CONDENSATE <br /> <br />-, <br /> <br />~ <br />./ <br /> <br />-14 <br /> <br />-16 <br /> <br />-22 -26 <br /> <br />-30 - 34 <br /> <br />-38 <br /> <br />5:XlMB TEMPERATURE ("C) <br /> <br />Figure 5. --Mean daily snowfall related to clo:J.d top <br />temperatures (500 mb) at Climax and Wolf Creek <br />Pass, Colorado. Snowfall is computed using a run- <br />ni.ng rnean over a t\'~.'o-degree terr.Lperature interval. <br /> <br />'I <br /> <br />A peak in the mean daily snowfall is <br />evident in the 500 mb temperature range from -22C <br />to -24C at Wolf Creek Pass with amounts decreasing <br />as cloud top temperatures become colder. This <br />decrease is mainly due to the reduction in the <br />potential condensate of the cloud system. This is <br />illustrated by the dashed lines in Figure 5. These <br />lines show the amount of condensate that would be <br />produced by a parcel moving upward through a <br />700-500 mb saturated layer. The peak in the mean <br />daily snowfall at -22C to -24C appears to reflect a <br />500 mb temperature mode where on the average the <br />available effective ice nuclei and the cloud water <br />supplied combine to maximize the precipitation. <br /> <br />The decrease in mean daily snowfall <br />as 500 mb temperatures become warmer than -22C is <br />interesting. The mean daily snowfall decreases <br />steadily from - 22C to -l8C in spite of an increas e in <br />potential condensate for these cloud systems. This <br />suggests the natural precipitation process is becom- <br />ing more inefficient through this temperature range, <br />probably due to a growing deficit of effective ice <br />nuclei in the cloud system. These observations are <br />consistent with the trends suggested by the model <br />shown in Figure 3. For estimated Wolf Creek Pass <br />conditions a 500 mb temperature mode that maximizes <br />the natural precipitation is found at -23C through <br />-25C. Growing deficits or excesses of effective ice <br />nuclei are indicated at temperatures above and below <br />this range respectively. <br /> <br />The marked increase of mean daily <br />snowfall at Wolf Creek Pass for 500 mb temperatures <br />around -14C to -15C is striking. This pronounced <br />peak in amounts probably reflects a temperature <br />mode where dendritic crystals form in the upper <br />portion of the cloud system. This could result in <br />fracturing of the dendritic crystals and an ice <br />multiplication process. This apparently occurs <br />occasionally at Climax (Grant, 1968). This increase <br />in the ice crystal to ice nuclei ratio from about one <br />to ten or more would increase the efficiency of cloud <br />water removal and result in natural precipitation <br />increases. <br /> <br />It is interesting that the mean daily <br />snowfall appears to decrease again at the warmest <br />cloud top temperatures contained in the sample <br />(-llC to -13C). However, these events are near the <br />tail of the distribution and have limited sample sizes. <br /> <br />Figure 5 also shows the mean daily <br />snowfall at Climax. This curve was generated <br />similarly to the Wolf Creek Pass diagram. The <br />mean daily snowfall at Climax indicates a 500 mb <br />temperature mode that maximizes the natural <br />precipitation from -20C through -22C. This is about <br />2C warmer than found for the Wolf Creek Pass area. <br /> <br />The snowfall trend at Climax follows <br />closely the potential condensate in the 700-500 mb <br />layer from -34C up to -20C. From -20C to -17C <br />the mean daily snowfall decreases abruptly in spite <br /> <br />of the rise in potential condensate. A minor secon- <br />dary peak is in evidence at about -14C followed by <br /> <br />10 <br />