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j 4f Z Cf)(1j ~~2 <1 u' Cl~ a 0 ~ 20 ",I 10) ~ ,----, 1: (b) : ~A;:RCENT AGE Of" TOTAL SNOWFALL I ' [-----1 I FREQUENCY OF OCCURRENCE t'5 ~ 15 ~ w CL 10 W > !;i iLl 5 0:: 94 16 -2 0 2 4 6 8 10 12 MOiST STABILITY INDEX {~tba}("C) 14 Figure 50. --(a) Mean Daily Snowfall at Wolf Creek Summit as a function of a moist stability index. (b) Distribution of total snowfall and total occurrences as a function of a moist stability index computed over a 2 degree class intervaL orographic effect at Climax appears l~ss , pronounced than at Wolf Creek Summit. Snowfall at Climax occurs from cloud systems that average slightly colder and drier than those observed at Wolf Creek Pass. The most interesting contrast between the Clirrlax and Wolf Creek Pass daily snowfall climatology is the lack of the marked peak in snowfall at around _140 C to _150 C. The very heavy daily snowfalls observed at Wolf Creek Pass (1. 25 water equivalent or more) are nearly non-existent in the Climax area. The occurrence of daily snowfall on Wolf Creek Pass appears to be controlled by the presence of disturbances in the prevailing westerlies during the winter season. The quantity of snowfall per day, however, is mainly controlled by a combination of the orographic features of the area, available moisture supply, and synoptic scale upward motions. There are strong indications that th~ natural snowfall process becomes relatively inefficient for cloud top temperatures warmer than _220 C except for around _140 C to _150 C where there is some evidence of possible ice crystal multiplication proC'esses. It appears that the wind speed, wind, direction, moisture supply, cloud system temperatures, and synoptic scale upward motions combine to produce rather infrequent but very heavy daily snowfalls. b. Upper Air Climatology Related to Excess Snowfalls at Wolf Creek Pass An upper air climatology of excessive daily snowfall at Wolf Creek Pass was i determined for the 4-year period 1964-68. Daily \ snowfalls containing 1. 20 inches: water equivalent or more at Wolf Creek West and/o-J Wolf Creek Summit recording gages were chosen fo~ the study. Seeded and non-seeded events were investigated separately and then combined into a single $ample. Table XIX shows the results of this investigation. I A large orographic influence in the production of these heavy snow episodes is readily apparent. The 700 mb wind speed averages about 16.5 mps (32 knots) for the seeded and unseeded heavy snow events, while the 50b mb wind speed averages nearly 28 mps (55 knots). The strong orographic effect is also demoni;ltrated in the pre- vailing wind direction during these heavy snow events. The 700 mb wind direction associated with these excessive snows ranges from 1800 to 2600 , averaging 2200 for the unseede4 cases and 2390 for the seeded events. An azimuth perpendicular to the San Juan Mountain barrier at Wolf Creek Pass lies at an angle of 2250. Therefore, all events have 700 mb wind direction.: within 450 of the normal to the barrier. I I I I I I I I I -r ;~ ~). \ ! The 500 mb wind clirection exhibits similar characteristics. It ranges from 1900 to 2800 during all events averaging 2300 for the unseeded cases and 2370 for the seeded episodes. The turning of the:wind with height indicates that excessive snowfall is generally associated with neutral or warm advection in the 700-500 mb layer. Table XIX shows only one case where slight cold advection was occurring in this layer. The average warm advection computed for seeded and non-seeded cases amounts to approximately 20 C per 12 hour's and 30 C per 12 hours, respectively. ~: ~ 60