Weather Mod Critical Issues Report

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- I I I I I I With a new conceptual model suggesting seeding for dynamic effect can also produce a substantial increase in rainfall without causing a sizeable increase in the maximum height of the seeded cloud (Rosenfeld and Woodley, 1993), further research in Texas in the 1990s documented the physical processes operative within the vigorous supercooled convective towers at the time of treatment with glaciogenic material. In addition to finding that the internal cloud structure is strongly dependent upon cloud base temperature, evidence was produced strongly suggesting seeding works well in clouds having an abundance of supercooled water, especially where such water in a vigorous, supercooled updraft region is available for artificial nucleants having a greater cross- sectional area for accretion of cloud water (Rosenfeld and Woodley, 1997), It was also observed that the time to reduce the maximum amount of cloud water in seeded convective towers to half of its initial value was lessened by some 2 to 3 minutes from that in the unseeded cases, I I I I I I I I I I I I I Additional exploratory research in the Montana HIPLEX (1975-1980), and later in North Dakota (1987-1993) further examined the precipitation processes in cumuliform clouds. In the North Dakota work, many of the new technologies cited in the NRC report were applied. In addition to the tracer techniques cited in the brief review of the NOAA Atmospheric Modification Program (AMP) elsewhere in this response, the North Dakota researchers used dual-channel microwave radiometers, in situ cloud microphysical measurements, including within hailstorms themselves (Detwiler et aI., I 994a, b), and numerical cloud models. Some of the modeling was done in real-time, for predictive purposes, much else was done post hoc, to gain a better understanding of the observations made, and to allow further improvements to the models. These modeling efforts are also discussed elsewhere within this response, Our point here is two-fold, First, contrary to the implications of the NRC report, there has been quality research in conjunctions with ongoing operational programs published in the refereed literature. Secondly, the research ceased only when federal involvement at a significant scale ended. We wholeheartedly endorse the NRC recommendation that a renewed long-term research effort be undertaken, and agree that a number of critical issues remain to be fully answered. We also maintain that coupling physical experiments with ongoing operational programs for exploratory experiments would be a productive, cost-effective approach to answering many ofthe questions posed in the NRC report. We acknowledge, however, that only conducting randomization apart from existing operational programs will afford the strength of statistical design necessary for confirmatory experiments, 24