Weather Mod Critical Issues Report

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- I I I I I I I I I I I I I I I I I I I Orographic clouds in the mountainous western states are often associated with passing synoptic scale storm systems. Wind flow over a mountain barrier causes the orographic lift to produce the cloud. Other types of clouds associated with frontal boundaries, convergence bands, and convective instability are also present during these storm systems, thus the orographic cloud scenario is often complicated by the dynamics of the storm system (changing winds, temperatures, and moisture). It has been recognized for many years that achieving adequate transport and dispersion (T&D) of the commonly used ground-released silver iodide seeding agent is a key problem in seeding winter orographic clouds (Rangno, 1986; Reynolds, 1988; Super, 1990; Warburton et aI., 1995a,b). Failure to documentthat clouds are actually being seeded continues to seriously hamper the development of this promising technology. If SL W clouds upwind and over mountain barriers are routinely seeded to produce appropriate concentrations of seeding ice crystals, exceeding 10 to 20 per liter of cloudy air, snowfall increases can be anticipated in the presence or absence of natural snowfall. It has been repeatedly demonstrated with physical observations that sufficiently high concentrations of seeding agent, effective at prevailing SL W cloud temperatures, will produce snowfall when natural snowfall rates are negligible. Seeded snowfall rates are usually light, on the order of I mmlhr or less, consistent with median natural snowfall rates in the intennountain West (Super and Ho]royd, ] 997), Weather modification scientists are well aware that Agl effectiveness is strongly dependent upon cloud temperature. Little physical (as opposed to statistical) evidence exists that Ag] seeding has produced meaningful snowfall when treated SL W cloud temperatures were wanner than -8ee to _gee except for the special case of forced condensation-freezing where seeding crystals may form near _6ee near the generators. But even in such special cases the crystals are carried to higher, colder SL W cloud regions. In order to be effective, the seeding material must be routinely transported into sufficiently cold SL W cloud and dispersed through large volumes of cloud, in sufficiently high concentrations. Both calculations and observations have shown that concentrations of effective artificial ice nuclei must exceed at least 10 per liter for detectable snowfall increases at the surface (Super and Boe, ] 988; Super 1994; Holroyd and Super, ] 998). The classic paper by Ludlam (1955) suggested that ]0 to 100 seeding crystals per liter would be needed within cloud. Higher concentrations may be required for moderate seeded snowfall enhancement. For example, Super and Holroyd (1997) presented clear physical evidence of an Agl-seeded snowfall increase of 0.04 inches per hour (1.02 mm per hour) with an associated seeded ice crystal concentration of about] 40 per liter, Median hourly snowfall rates are typically about half that rate at high elevations in the intermountain West, 3.2.2 Static seeding of winter orographic clouds There are strong statistical suggestions of seeding effects from at least two randomized programs, the Lake Almanor Experiment (Mooney and Lunn, 1969) and the Bridger Range Experiment (BRE) as reported by Super and Heimbach (1983) and Super 16