WMOD00540 - Laserfiche WebLink

Home Browse Search

Bulletin American Metf:orological Society In 1980 the preliminary criteria were modified to include excess snowpack accumulation, rain-induced winter flood- ing, severe weather, and other special circumstances. The ex- cess snowpack water equivalent refers to excesses above a pre- determined threshold value that changes daily through the winter months. The thresholds were established using histor- ical data to determine at what point one should anticipate weather-related problems. Flooding refers to anomalous in- flows or releases at Folsom Reservoir, which is on the Ameri- can River near the Sierra Nevada foothills. Severe weather includes hailstorms, lightning, and the like. Special circum- stances are conditions that might be hazardous to the public or project personnel, such as avalanche warnings for the Sierra Nevada, snowfalls at low elevations, and high water in Lake Tahoe threatening property owners. Specifics of these criteria are documented each year in the SCPP Operations Plan. Suspensions have had a serious impact on seeding opera- tions, mostly because of the wet conditions during 1981/82 through early 1983/84 (Table 3). Table 4 ranks each of the last eight years of annual precipitation at Blue Canyon, Cali- fornia, against all years from 1899/1900 to 1983/84; it indi- cates that the precipitation regime in the Sierra Nevada dur- ing the SCPP years has been anomalous. 7. Seeding studies j Despite the frequent suspensions, some seeding experiments have been conducted. They have generated a considerable amount of knowledge about the physical processes involved in natural and augmented precipitation development. Table 5 lists all seeding operations conducted on SCPP since its inception. During the 1977/78 winter season, both ground and aerial releases of AgI and tracers wen: performed to identify condi- tions leading to the optimum transport and diffusion of these materials. Although performed during fair weather for flight-safety reasons, the aircraft surveys using ice-nucleus counters and millipore filters showed that seeding material from ground generators was not reaching sufficient heights for adequate nucleation nor did the material diffuse ade- quately to treat large volumes of cloud. During stormy con- ditions, upward transport might be better; however, target- ing the ground-generator plumes would be more difficult, especially with a barrier jet present. Therefore, it was decided to use aerial seeding to ensure that an adequate supply of seeding material was delivered to the appropriate locations at the desired times. In 1978/79, calibration seeding trials were conducted in which particular clouds or cloud groups were treated with specified amounts of either dry ice or AgI. Both stable oro- graphic clouds and isolated convective clouds were treated. These seeding trials continued through the 1979/80 winter season. As mentioned earlier, 1980/81 had no field operation. a. Floating-target experiment The general conclusion from the trials through 1979 was that the convective clouds which often develop in post-frontal sit- uations appeared to have a greater seeding potential than any 519 TABLE 5. SCPP seeding operations. No seeding was done in the winters of 1976/77 and 1977/78. Date Cloud Type Seed Type 31 Jan 1979 14 Feb 16 Feb 18 Feb 20 Feb 28 Feb 29 Feb 15 Mar 17 Mar 18 Mar 21 Mar 22 Mar 08 Jan 1980 10 Jan 02 Feb 14 Feb 27 Feb 03 Mar 18 Jan 1982 19 Jan 28 Jan 09 Feb 26 Feb 02 Mar 03 Mar 10 Mar 15 Mar 16 Mar 17 Mar 27 Mar 30 Mar 18 Jan 1983 19 Jan 05 Feb 12 Feb 18 Feb 09 Feb 1984 I3 Feb 15 Feb 16 Feb 21 Feb 24 Feb I3 Mar 14 Mar 15 Mar 31 Mar 26 Jan 1985 28 Jan 02 Feb 07 Feb 02 Mar 04 Mar 07 Mar II Mar 24 Mar 26 Mar 27 Mar 28 Mar CO, CO, CO, Agl CO, Agl, CO, randomized Agl, CO, randomized Agl, CO, Agl randomized AgI randomized CO" AgI AgI AgI Placebo, randomized AgI AgI AgI AgI, CO" randomized CO, CO, Placebo, randomized AgI CO" AgI CO, randomized CO, randomized AgI CO, randomized Placebo, randomized CO, CO, CO" AgI CO, CO, randomized CO, CO, CO, randomized CO, CO, CO, CO, CO, CO, CO, CO, Placebo, randomized CO, CO, CO, randomized CO, randomized Placebo, randomized CO, AgI randomized CO" AgI CO, Placebo,. randomized Placebo, randomized CO" randomized CO, Stratus Cony. cells Cony. cells Band Orographic Orographic Orographic Orographic Convective complex Convective complex Cony. cells Orographic Stratus Conv. cells Orographic Orographic Orographic Orographic Cony. cells Cony. cells Cony. cells Orographic Orographic Cony. cells Cony. cells Orographic Cony. cells Cony. cells Orographic Cony. cells Orographic Orographic Cony. cells Orographic Orographic Cony. cells Cony. cells Orographic Orographic Cony. cells Orographic Orographic Orographic Orographic Cony. cells Orographic Orographic Orographic Orographic Orographic Cony. cells Orographic Cony. cells Cony. cells Orographic Orographic Cony. cells Cony. cells other cloud type observed in the SCPP. This conclusion was reached because 1) in-cloud microphysical evidence shows more water and less ice in this cloud type, 2) surface observa~. tions of crystal type and degree of riming indicate a si'milar trend, and 3) data in hand showed rapid dispersion of seeding materials and uncomplicated fall-out trajectories compared to the other clouds. It should be noted that this same conclu-