Abstract: The current status of winter orographic cloud seeding technology was reviewed to assist in lhe consideration of a snowfall enhancement program for the Black Hills of South Dakota. However, this material is relevant to the intermountain west in general. The emphasis is on the physical evidence that key processes occur in the hypothesized chain of events from release of seeding agent to snow reaching the ground. The range of opinions concerning the technology is briefly noted. Some problems related to reliable production of silver iodide are discussed. Evidence is reviewed from a number of locations showing that excess supercooled liquid water often exists during portions of winter storms, though usually in limited amounts. Much of the liquid water is found near the windward slope and crest of the mountains. The main problem with seeding winter orographic clouds is the transport and associated dispersion of the silver iodide between its release and the supercooled water zone. An associated problem is ensuring that the seeding agent reaches a sufficiently cold region of liquid cloud in an appropriate concentration. While adequate transport and dispersion can be difficult with airborne seeding, it is a special problem with low-level ground generators. Evidence is cited showing that ground seeding projects have frequently failed to enhance the silver content of the target area snowfall. It is recommended that the silver content be monitored in all winter orographic projects which use silver iodide as a minimum first step in documenting whether or not the seeding agent was transported to the target. Where practical, additional testing of the physical hypothesis should be accomplished to improve validation of winter orographic cloud seeding.

Abstract: A HIPLEX- A cloud case has been used to describe the effects of ice-phase cloud seeding on a cold base cloud. The cast was used at the First International Cloud Modeling Workshop/Conference held in 1985 in Irsee, Federal Republic of Germany, to provide modelers with a well-observed cloud-seeded case. Solid dry ice pellets (CO2) were dropped into the cloud at about the −10°C level. Aircraft and radar observations of the cloud were obtained over approximately 40 min of its life history. Seeding produced clear ice seeding signatures in this continental type cloud. Graupel formed within 10 min and rain was observed at the +10°C level 13 min after seeding. About 9.4 kT of rain was estimated to fall from the cloud. The modeling results also show enhanced ice in the seeded cloud and can be compared with an unseeded model cloud (not available in nature). The precipitation path from cloud ice to snow to graupel/hail to rain from the melted graupel is evident. The Bergeron process was important for snow in...

Abstract: A wintertime ground-based cloud seeding experiment conducted as part of the 1989 Utah/NOAA cooperative weather modification program is described. The results from one experiment on 3 February 1989 are presented. Meteorological conditions led to the development of orographic clouds over the Tushar Mtns of southern Utah which appeared to be nearly ideal for seeding operations. Radiometrically measured liquid water was abundant in the vicinity of seeding generators and the water appeared to be sufficiently supercooled to enable nucleation by silver iodide. The experiment entailed pulsed releases of silver iodide from high altitude generators located on upwind ridges of the Tushar Mtns. A Ka-band radar, aspirated PMS 2D-C probe, and manual microphysics observations were used to monitor precipitation 10-13 km downwind of the seeding generators. Snow samples were also collected periodically and analyzed for silver content. The overall results were disconcerting in that two estimated periods of effect showed no enhanced silver content, and no clear microphysical or radar seeding signatures due to large background variability produced by a propagating mesoscale cloud feature, and natural snow characteristics which resembled the expected characteristics due to seeding. A third period of effect had apparent microphysical and radar signatures, but also lacked the presence of silver in the snow. Targeting of the single downwind ground target apparently failed in this case due to inadequate wind documentation in the cloud layer, remote-generator malfunction, or fallout of the seeded plume upwind of the target.

Abstract: An assessment of five years of warm-season cloud seeding over the watershed of San Angelo, Texas is presented. The goals of the seedings were the replenishment of surface reservoirs, channel dams and surface aquifers and increased precipitation over the residential areas to reduce demand for municipal water. It was recognized that increased rainfall also would benefit the farming and ranching communities. During the program a total of 125 kgm of AgI were expended during the course of 2,315 separate seeding events at cloud top or at cloud base. Most of the seedings took place within 30 n.mi. of San Angelo as intended, primarily to the west and southwest of the City. Assessment of the effect of seeding made use of target-control regressions that had been derived from historical rainfall records. Historical monthly precipitation data were accumulated for long-term rainfall stations within the target and outside to the west and to the south. The period of record was 1960 through 1984 inclusive. Six control stations and nine target stations were used in the analysis. The analysis, suggests a positive effect of seeding (i.e. more rainfall) in each of the five years. The probability of this happening by chance may be a low as 3%. An overall effect of seeding of about +17% for the target for all years of operation is indicated. Sensitivity testing supports the interpretation that seeding was responsible for a sizeable portion of this apparent increase in rainfall.

Abstract: A McIDAS (Man-computer Interactive Data Access System) workstation was employed to monitor the development, movement, and eventual decay of convective clouds over target areas totalling 366,000 km^2 in western North Dakota. Information relevant to cloud seeding strategies for both rainfall enhancement and hail suppression was relayed to the field meteorologists at their radar sites in near real-time, where the operational decisions were made. Both the utility and the timeliness of the information were evaluated. Shortcomings are also discussed.

Abstract: This paper updates a report to the World Meteorological Organization (WMO) in 1981 made by a "panel of experts" in cloud modeling, chaired by the author. The primary uses of cloud models are reviewed and two tables constructed, one which details the types and uses of cloud models in weather modification and references to the models, and a second table describing the computational tasks involved in running the models. Some outstanding problems are mentioned and reasons given for hope regarding the numerical simulations of cloud seeding effects.

Abstract: Data from two Reclamation-sponsored randomized cloud seeding experiments near the Black Hills of South Dakota are combined with data on size distributions of showers to estimate the potential for modifying summertime precipitation. The analysis suggests that increases amounting to roughly 12 percent of the natural precipitation are possible through a combination of microphysical and dynamic effects. Convective clouds around 6 km deep are judged to be the most promising targets for seeding to increase summer rainfall.