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~ Presented at the Ninth International Cloud Physics Conference Tallinn, U.S.S.R. August 25, 1984 SCIENTIFIC WEATHER MODIFICATION EXPERIMENTATION IN THE UNITED STATES Bernard A. Silverman Bureau of Reclamation Denver, Colorado U.S.A. 1. Introduction This paper discusses recent scientific weather modification research in the United States. Four large precipitation enhancement research programs are briefly reviewed: the recently completed HIPLEX-l (Montana High Plains Experiment) and FACE-2 (confirmatory Florida Area Cumulus Experiment) experi- ments, the ongoing SCPP (Sierra Cooperative Pilot Project) program and the planned CREST (Colorado River Enhanced Snowpack Test) demonstration program. In addition, highlights of selected activities in three areas of related research are presented: 1) dual-channel microwave radiometer applications, 2) laboratory studies of the ice nucleating properties of silver iodide complexes, and 3) eva- luation of experiments by monitoring the expected chain of physical events following seeding. Finally, the probable nature of future scientific weather modification experimentation in the United States is discussed. 2. HIPLEX-l HIPLEX was aimed at establishing the physical basis for enhancing beneficial growing season precipitation from convective clouds on the High Plains of the United States. HIPLEX-1 (Bureau of Reclamation, 1979) was a randomized, double- blind exploratory experiment to test the static mode seeding concept for summer convective clouds over the Montana High Plains. The experimental unit of HIPLEX-1 was semi-isolated cumulus congestus clouds which develop precipitation naturally through the Bergeron mechanism, that is ice crystal growth by vapor diffusion followed by riming into graupel. Many of these clouds do not rain to the ground at all. The objective of the static seeding was to provide an opti- mum concentration of ice particles for the available liquid water, about 10 per liter, and initiate the precipitation process earlier and lower in altitude in the developing clouds than would occur naturally. It was hypothesized that the efficiency of the natural precipitation process would be improved by the seeding and thereby lead to both additional precipitation and to an increase in the pro- portion of clouds that precipitate. Detailed criteria and procedures for selecting and classifying test cases, for performing the seeding and for the collection of observations to be used in calculating response variables were prescribed. Based on preliminary explora- 1 I.