WMOD00572 - Laserfiche WebLink

Home Browse Search

Printed January 30, 1990 (WMO, 1981). Under this hypothesis, seeding with ice-forming agents produces additional hail embryos, which compete with the natural ones for the available supercooled water. Substantial seeding rates, amounting to at least several hundred grams of silver iodide per hour, are normally used. The supposed result is more numerous but smaller hailstones, of which many, if not all, would melt before reaching the ground. The best known randomized experiments set up to test the effectiveness of silver iodide seeding for hail suppression were the National Hail Research Experiment (NHRE) in the United States, Grossversuch IV in Switzerland, and the Alberta hail experiment in Canada. NHRE and the Alberta project employed aircraft for seeding; Grossversuch IV used ground-to..air rockets developed to seed hailstorms in the U.S.S.R. All three projects were inconclusive when analyzed according to their original statistical designs (Crow et al.,1979; Federer et aI., 1986; Goyer and Renick, 1980). However, the Alberta experiment, which had a randomized crossover design, showed evidence of a suppression effect on all operational days that was not limited to the area designated for seeding on a given day (Goyer and Renick, 1980). There are other pieces of favorable evidence. Miller et aI. (1975) found tentative indications in crop-insurance data (but not in hail-pad data) from a randomized experiment in North Dakota that silver iodide seeding from aircraft suppressed crop-damaging hail. A recent randomized experiment in Greece showed reductions in hail impacts on hail pads by some amount between 40 and 90 percent (Rudolph et aI., 1989). An analysis of an extensive, multi-year operational program in North Dakota using crop-hail insurance data suggested a 40-percent suppression effect (Smith et a!., 1987). All three of these projects involved silver iodide seeding from aircraft. A long-term project in southwestern France using silver iodide generators on the ground has also yielded indications of a suppression effect of around 40 percent (Dessens, 1986). The agencies operating extensive programs in the U.S.S.R., which rely mainly on seeding with ground-to-air rockets, have reported "efficiency coefficients" ranging from 44 10 100 percent, with a preponded'te around 80 to 95 percent (Burtsev, 1980). '" /' Although operational hail suppression programs have continued in about 30 countries (WMO, 1981), research programs have lagged during the past few years, partly because of loss of funding and partly because of a lack of hypotheses that are amenable to testing in the field. Some idea of the difficulties involved in testing advanced hail suppression concepts can be gained from a paper by English (1987) on the last seeding trials carried out in Alberta. These trials involved seeding of "feeder clouds" around existing hailstorms to produce additional hail embryos. These embryos were supposed to be carried inward as the feeder clouds merged into the main cloud mass. Tracking them posed a challenge for the aircraft crews involved. A modest revival of thunderstorm research took place in North Dakota in 1989, when that state hosted the North Dakota Thunderstorm Project (NDTP). The NDTP was organized around the existing operational program of the North Dakota Atmospheric Resource Board. Several Government agencies, the National Center for Atmospheric Research, and university groups participated in the NDTP. Project objectives included determining whether seeding agents could reach the targeted portions of the clouds and verifying that seeding produces increases in ice crystal concentrations sufficient to influence significantly the precipitation process. Publication of results is expected over the next year or so. 10