Laserfiche WebLink
<br />Success in cloud seeding requires substantial knowledge of the physical processes in natural <br />clouds and how seeding materials change those processes to augment precipitation. There have <br />been two major research projects related to cloud seeding in California. The larger effort was the <br />Sierra Cooperative Pilot Project (SCPP), which \vas conducted by Reclamation and the states of <br />California and and Nevada between 1977 and 1987. The SCppB focused on physical <br />mechanisms affecting Sierra Nevada clouds, so that sound cloud seeding technologies could be <br />developed. Ground-based and airborne silver iodide seeding was done, along with the release of <br />tracer materials to assess the transport and ditTusion (T &D) of seeded plumes (Section C3a <br />below). Major findings were: Sierra Nevada storms onen have rapidly changing phases that <br />affect secdability; a low-level barrier jet stream frequently complicates T&D and targeting of <br />seeding materials: clouds are frequently efficient natural snowfall producers because of a <br />process kno......n as ice multiplication: and most of the SLW that is needed for seeding to be <br />elTectivc is within 3000 feet of the ground, at temperatures warmer than _lOoCB.34:3s. <br /> <br />Thc second project, the Lake Oroville Runoff Enhancement l}roject (LOREP), was performed in <br />the northem Sierra near Beckwourth, C A, from 1991-1994. The LOREP was the first project in <br />the USA to use LP gas as the seeding agent. The choice of LP was based on findings of SL W <br />existence at relativc1y warm temperatures, since LP can be more effective at those temperatures <br />(Section C I). Seeding plumes were successfully tracked using tracer gases, and ice crystals <br />within plumes were also studied. The LOREP \....as suspended al1er three years (short of the <br />intended five years) because T&D caused problems in targeting seeded ice crystals, necessitating <br />a design change. lne shorter duration also precluded statistically significant results from the <br />randomized part of the seeding experiment. There has been no LP experimentation in the Sierras <br />since the LOREP. Nevertheless. the existence of signilicant SLW when temperatures were <br />wamler than _.foe was confirmed to occur about 80% of the time2'l, <br /> <br />Several review articles3o;J1;38 have stated that achie\'ing adequate T&D jhr seeding S[W regions <br />is probab~)' Ihe most diJficull problem facing winter orographic cloud seeding. This was <br />recognizcd as a still-fundamental problem in a more rccent review articleJ9 and it remains an <br />issue in California's operational programs, although chemical tracer experiments and plume <br />dispersion models have improved understanding. First. seeding materials must be transported in <br />adequate concentrations to cloud regions with sutlicient Sl.W and proper temperatures. If that is <br />achieved, the materials must then generate ice crystals in su!1icient concentrations in regions <br />where the crystals can grow and fall Ollt, producing significant snow precipitation in the desired <br />target area. If any processes in this physical chain of events are not satisfied, the seeding \vill not <br />significantly increase precipitation in the target. Tracer experiments have been conducted by <br />PG&E. SCE. and the Desert Research Institute (DR!). lnese experiments have revealed some of <br />the complexities of targeting seeding materials, given thc complicatcd wind fields that occur <br />within the Sierra Nevada (Figure 5). Local wind steering by valleys and ridges. Oovo' blockages <br />by mountain peaks. and other dynamic meteorological elTects can shift seeding material and <br />effects to areas outside the target. Sometimes the shifts can be toward control areas, adversely <br />affecting evaluation eOorts. Use of high-altitude ground seeding devices. at least halfway up the <br /> <br />14 <br />