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 Similar coverage results were found by chemical analyses in Grossversuch IV (Lacaux et aI., 1985). Two cells on one day showed 7% and 25% coverage and two cells on two other days had seeding coverage of 100% and residence times, in cloud colder than _50C, of 500 to 700 seconds. Another concern about hail suppression is its impact on rainfall. Because hailstorms often occur in semi-arid regions where rainfall is limited, Changnon (1977) estimated that in general, the destructive effects of hail damage are often outweighed by the positive benefits of rainfall from those storms. This is, of course, not true for certain high-risk crops such as tobacco, grapes, or certain vegetables, Modeling studies like Nelson (1979) and Farley and Orville (1982) suggested that rainfall and hailfall are positively correlated so that reductions of hail fall coincide with reductions in rainfall. Later modeling studies with better microphysics, carried out by Farley (1987) and Farley et al. (2004b), showed less hail and more rain in the seeded cases. In addition, an evaluation of rainfall from an operational hail suppression program in Alberta, Canada by Krauss and Santos (2004) suggested that seeding to reduce hail damage also resulted in an increase in rain volume by a factor of2.2. Consequently, the effects of hail suppression on rainfall needs further study and measurement on research and operational projects. At present the design of a randomized hail suppression experiment involving response variables measured at the ground (with the objective of substantiating a hail suppression effect) appears to be impractical, but should be a research goal. The required size of the instrumented target area and/or duration of such an experiment are prohibitively expensive. Moreover, funding agencies are very cautious about committing their resources to supporting a program of more than 5 years duration. Randomized experiments such as Grossversuch IV were designed with the intent to discern a seeding signal in a 5-year period based on the optimistic expectation of a 60% reduction in kinetic energy offalling hail (Federer et aI., 1986). Note that Mesinger and Mesinger's (1992) evaluation of the 40-year long hail suppression program in Yugoslavia suggested only a 15-20% reduction in hail frequency. Thus a funding agency would have to be committed to supporting a randomized hail suppression experiment for 10 years or more! Scientific understanding sufficient to sharpen the focus of such an experiment, for example by forecasting the response variables, or to increase the efficacy of the seeding treatments, should precede any efforts to implement a randomized experiment. This also argues for numerical storm models that simulate realistic hailstone spectra for use in refining hail suppression concepts, a step that is well under way, but that needs stronger support. These concepts, figure, and discussion represent the present state of hail suppression science. The stated concepts have been and are being used to guide operational hail suppression projects, and should help focus future research experiments on hailstorms and hail suppression. Much of the material is used in the American Society of Civil Engineers' Standard Practice for the Design and Operation of Hail Suppression Projects. One of the prime lessons for future operational hail suppression projects that has been learned from past projects is that the most effective seeding is done on the smaller, younger feeder cells 14