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Show SECTION 6. 0 DISCUSSION OF RESULTS 6. 1 Relationship to Climax Results Grant, et. al. (1968) have reported target area precipitation increases of > 100% in the winter orographic precipitation modification experiment near Climax, Colorado, for 500 mb temperatures of :2: -20°C. For 500 mb temperature s -20°C, slight precipitation decreases due to seeding were also noted (10-20%). Their primary reasons for stratifying data by 500 mb temperatures were ( 1) that cloud tops are frequently found near 500 mb, and (2) that there exists a serious deficiency of naturally active ice nuclei for temperatures :2: -20 °C (resulting in inefficient natural precipitation processes), whereas for temperatures s .... 24°c, sufficient numbers of natural ice nuclei become active to allow natural precipitation processes to be efficient. Thus, for cloud minimum temperatures -20°C artificial seeding should result in precipitation increases, while the opposite should be true when cloud temperatures fall to s -20°c. Park Range 1968-69 season data were stratified similarly into warm and cold cloud top categories in two different ways. The first of these stratifying routines divided the precipitation rate samples into categories of (1) cloud minimum temperature -20°C, and (2) cloud minimum temperature s .- 20°C. The cloud minimum temperature was estimated from Mt. Harris temperature and humidity profiles rather than by making any assumption of standard altitude of cloud tops. The second stratifying method was identical to that of Grant, et. al. (1968), except that 550 mb (rather than 500 mb) was used as standard cloud top, since the Park Range target altitude is near 700 mb compared to 650 mb for the Climax target area. The 550 mb temperature categories were (1) warm(~ -23°C) and seedable, and (2) cold (< -23°C) and unseedable. When a non-parametric statistical rank test was applied to the pooled seeded and unseeded precipitation rate samples, stratified according to the above temperature categories, an increase of > 100% due to seeding was indicated at Rabbit Ears Pass for the warm category of indicated cloud top temperature -20°C, with the increase significant at the 3% level. A decrease of 24% was noted for cloud tops < -20°C but with less statistical significance. For the 550 mb temperature stratifications, increases of approximately 50% were noted at Rabbit Ears Pass for the warm (~ -23°C) category, with a 45% decrease for the cold category. Significance levels were 15 to 20%. Without the above temperature stratification, no significant seeding effect was apparent in the precipitation rate sample. 330 |
