OCR Text |
Show temperatures to be reading). (D) -20°C or < -20°C (regardless of the 500 mb temperature Stratification by 550 mb Temperature Since the Park Range crest is located at approximately the 7 00 mb level, while Chalk Mountain in the Climax Project (Grant, et al, 1968) is near 650 mb, Park Range data were also stratified into 550 mb temperature classes (rather -20°C and< -23°C. than 500 mb) Using only stratifications A and B, no statistically significant seeding effects were found, although there was an area of positive effect indicated in the southern and western portions of the target area (i. e. for westnorthwest and northwest flow conditions). However, when cloud top temperature stratification -20°C) was added, the indicated positive effect was strengthened (type C for to the point that statistically significant results were obtained. The indicated percentage change in precipitation due to seeding, along with the level of significance, is shown for each station's optical snow rate sensor data in Figures 201 and 202. Results using the stratifications A, B, and D are shown in Figures 203 and 204. The patterns of seeding effect revealed in Figures 201-204 are physically reasonable when viewed in the light of known valley temperature inversion characteristics. That is, for flow from south of west (thus placing the northern portion of the target area in the expected area of seeding effect), the valley inversion (extending frequently to near or just slightly above Mt. Harris) is more frequently present than when the flow is north of west. This reduces the reliability of the position and timing of computed area of seeding effect (by increasing the probability that seeding material was either trapped in the inversion, or meandered slowly in the very light wind flow just above the inversion). Thus, seeding effects should be more clearly evident for cases of northwest flow (and therefore in the southern portion of the target area). This verifies. A possible interpretation of the very large indicated percentage increases in the valley west of the ridge (at Lufkin), is that seeding caused snow to occur further west than would have been the case naturally. The percentage increase for the warm temperature categories under stratifications A, B, and C and A, B, and D for Rabbit Ears Pass (the most frequently serviced target area snow rate sensor) is quite comparable to that obtained by Grant, et al (1968) for similar temperature categories at Chalk Mountain. The indicated decrease in the cold categories is somewhat larger than they found. The important question of possible overall seeding effect when optimizing 295 |