| OCR Text |
Show For the maximum possible effect (during the warmest snow-producing conditions) then, it would be desirable to seed between 14, 000 and 18, 000 ft. MSL. Diffusion test results indicate these altitudes are seldom reached with ground-based generators in the Park Range area, however. Table 17, constructed from 1967 analysis periods using Buffalo Pass total precipitation and thermograph average temperature, indicates a strong tendency for the heavier amounts of snow to occur with Buffalo Pass (near 700 mb altitude) temperatures of 2 -lOC. The percent efficiency of the natural precipitation process (criterion 3) is dependent upon the number of natural ice-nuclei which become active within the super-cooled cloud column. The number of active ice nuclei, in turn, is dependent upon the coldest temperature within the cloud (usually at cloud top). A study of five seasons of 700 mb and 500 mb temperature changes at Grand Junction, Colorado (Figure 214), was made for all cases with$ 8°C temperature-dewpoint difference. A similar study was made for Salt Lake City using 5°C (Figure 215). It was found that 12-hour absolute tempe_rature changes both at 700 mb and 500 mb were ::;; 3°C in 77% of the samples at Grand Junction and in 70% at Salt Lake. If cloud tops are assumed to be near 500 mb, and the 12 hour change is assumed to extrapolate linearly to _a 24hour change, then within a 24-hour period there is frequently a change of natural active ice nucleus concentration of 1 / 2 to 1-1 / 2 orders of magnitude. The study of variability of indicated cloud tops from 165 Mt. Harris soundings implies an even greater variability in cloud top ice nucleus concentration due to large, rapid departure of cloud tops from the 500 mb level. From the Mt. Harris soundings taken during operational periods over the past three seasons, estimated cloud tops were between 10, 500 ft. MSL (690 mb) and 17,500 ft. MSL (515 mb) on 80% of the soundings, with the mode near 13,000 ft. (610 mb) (see Figure 216). Cloud top temperature was 2 -20C in 61 % of the cases. The optimum dendrite crystal growth temperature of -15C averaged near 12,300 ft. MSL, and in 69% of the cases it fell between 10, 000 ft. and 14, 000 ft. MSL (see Figure 217). Using -24C as the critical 600 mb (~ 13, 000 ft. MSL) temperature below which seeding effects become negative, five years of synoptic scale 700 mb temperature data (restricted to cases of temperature - dewpoint differences of 7C ) were used to compute the % of seedable cases (Table 18). The assumption of moist adiabatic lapse rate from 700 mb to 600 mb was used to e stimate 600 mb temperature. Conditions of non-seedability due to cold cloud tops are indicated to be quite rare (see Figure 216). Such occurrences usually are associated with a deep zone of frontal convergence (or post-frontal over-running) when cloud tops extend to above the 500 mb level. The importance of conditions to which the seeding agent is exposed in its travel 316 |
