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1294 SHALLOW OROGRAPHIC CLOUD 5.0 Natural LW ZONE Nucleation .05-.1gm-3 ("~~~ ,;;< L' .1 '~,-:' ( ., WE" I 'I . .-.CONVECTION ~. I ~ 4.0 ~ III Q 3.0 ::> .... 5 C( 2.0 -130C [-16] 1.0 DISTANCE FIG. 7. Conceptualized drawing of a shallow orographic cloud and the vertical and horizontal distribution of liquid water. Temperature scale on left is for Sierra Nevada with colder intermountain regions in brackets. Radiometer positions shown are indicative of locations utilized in various programs referred to in figure I. 3. Tapping cloud water for snow augmentation L The basic concepts of winter orographic cloud modification as put forth by Ludlam (1955) still hold true today. As mentioned in the WMO statement of seed ability , what is being modified in winter clouds is a rate process. That is, clouds contain in- sufficient nuclei, nucleation occurs too late, or nucleation is too slow to convert the available cloud condensate to precip- itation-size particles before the condensate is lost to the lee of the barrier. Glaciogenic seeding is an attempt to convert the cloud condensate (small cloud droplets with low terminal ve- locities) to ice earlier and allow growth to precipitation-size particles (larger particles with higher terminal velocities) before the particles pass over the crest. This seeding strategy is known within the weather modification community as a trajectory- lowering hypothesis. In order to implement this concept, seed- ing material must be introduced at the proper time and place and in the proper quantity to initiate the precipitation process efficiently. Only two primary modes of delivery are available on an operational basis. These are aerial releases and ground releases. Each have their advantages and disadvantages. For aerial seeding, one can be assured that the seeding ma- terial, be it dry ice (seeding rates of 0.2 to 10 kg . m-I) or silver iodide (one 20 gm flare every 5 to 10 s or 100-150 gm . hel from an acetone burner), is delivered to the cloud at the proper location and within the SLW. However, aerial de- livery is compromised by cost, aircraft safety factors, and the limited horizontal dispersion (downwind spread) of a given seedline. Figure 8 shows the simple advection of a set of aerial- released seed lines using a horizontal dispersion of 1 m . s - I as has been observed in seeding plumes (Deshler et al. 1988). Note that a substantial volumt: of cloud is left untreated.4 The combination of silver iodide in an acetone solution com- plexed with either sodium or ammonium iodide (seeding rates of 20-30 gm/hr) is the standard method of ground delivery. Advantages are ease of operation, economy, and continuous seeding for long hours under any weather situation. However, it does require the seeding material to reach clouds at temper- Vol. 69, No. 11, November 1988 atures below - 50C and in concentrations of 10-100 L - I .5 Mechanical turbulence induced by wind flowing over com- plex terrain is the largest contributor to the transport and dis- persion of ground released material. Mechanical turbulence tends 1:0 deepen the boundary layer over such terrain. A rather consistent set of results is emerging, showing that mechanical mixing, even in neutral-to-stable environments, can induce 100_ 200 of lateral plume spread and 0.2 to 0.3 m . S-1 (500-1000 m rise) induced vertical motion within 20 Ian of the source. Obviously convection, with its roots in the boundary layer, will increase vertical dispersion. However, the dispersion is limited to narrow vertical columns (chimneys) (Stith et al. 1986). These results suggest that an appropriate seeding strategy is for gen- erators to be placed 3-5 km apart, located in exposed high elevation zones within 500-1000 m of the - 50C level, and positioned no further than 20-30 km from the target area. It is interesting to note that the three seeding programs discussed in the introduction that have shown statistically significant re- sults all had generator locations meeting the criteria above. It is appropriate to discuss briefly some recent findings on nucleation rates of silver iodide materials because these findings have a significant bearing on results from the statistical pro- grams that show seeding effects occurring relatively close to the generators. Fukuta and Schaller (1982), DeMott et al. (1983), Blumenstein et al. (1987), and Finnegan and Pitter (1988) have reported that in the presence of high supersaturations, silver iodidt: nucleates much more rapidly and with higher yields per gram of material. Finnegan and Pitter suggest that supersatur- ations are induced by the combustion of acetone and propane yielding a large local supply of water vapor. At temperatures below - 50C, this induces a forced condensation-freezing mechanism and rapid nucleation within 10-30 m of a generator. This result provides some physical basis for the documented seeding results from both the Bridger Range (Super and Heim- bach 1988) and for the PG&E Almanor program (Mooney and Lunn 1969) that reported seeding-induced precipitation increase within 4-15 km of the generators. 4. Treatment effects Several studies (including some of the physical studies dis- cussed in this paper) have attempted to document through phys- ical observations the effects of seeding in winter stratus and stratocumulus clouds over mountains and elsewhere. The re- sults of these studies show some consistency. In the presence 4 Weickman (1974) was the first to observe that vertical curtains of ice crystals growing at different rates (therefore having different fall velocities) could be spread downwind much more rapidly (2-3 m . s -I) in the presence of vertical speed shear. This observation was confinned by Stewart and Marwitz (1982) and Super and Boe (1988). However, even with vertical speed shear the amount of cloud volume affected by one aircraft is rather restricted. , An enormous amount of work has gone into documenting the trans- port and dispersion of ground-released material. Including tracking silver iodide and other tracers such as sulfur hexafluoride (SF.), (Smith and Heffernan 1954; Crozier and Seeley 1955; Super 1974; Super and Heimbach 1988; Holroyd et al. 1988). These studies and others have documented such features as "dead" layers and barrier jets (Parish 1982), which both contribute to air moving around rather than over the barrier.