The Second Israeli Randomized Cloud Seeding Experiment: Evaluation of the Results

Abstract

The Second Israeli Randomized Cloud Seeding Experiment was conducted during the six rainfall seasons November?April 1969?75. Its primary purpose was to examine the possibilities of enhancing rainfall in the catchment area of Lake Kinneret which serves as the principal reservoir of the Israel National Water Carrier. The present report constitutes the final statistical evaluation of this experiment. It has been preceded by a series of preliminary reports, some of which have dealt with specific subject areas. The study shows positive overall results, of 13 and 18% increases of precipitation under seeding in the total North and Catchment areas, respectively. The corresponding significance levels are 2.8 and 1.7%. These findings may be attributed to the following combination of reasons: 1) The clouds are cold (winter) continental cumuliform clouds, associated mostly with cold fronts and post-frontal bands, and have microstructures which seem to suggest that the rain-forming processes most frequently operate through the growth of ice crystals, followed by the more rapid growth of riming by graupel particles rather than through the all-water processes of collision-coalescence. 2) The water budget of the prevailing cumuliform clouds (average base temperature of 8°C) is such that the addition of moderate concentrations of ice crystals probably increases the precipitation efficiency of these clouds. 3) The distributions of cloud-top heights and temperatures indicate a very high predominance of cumuliform clouds, with tops in the range of ?15 to ?20°C. Such clouds have been found to be most amenable to seeding for rainfall enhancement. The above findings are supported by both experimental and theoretical evidence. The detailed statistical tables indicate that the overall increases in rainfall can also be tested with regards to their physical plausibility. Thus, it will be shown that (i) the attempt to target the seeding effects, on the basis of diffusion studies of the seeding agents, to a predesignated area, seems to have been successful; (ii) increases in daily rainfall under seeding are very large when the mode of the cloud top temperature distribution is in the range ?15 to ?21°C?under such conditions, increases of 46% at an 0.5% significance level (a 90% confidence interval for the increase of 13?113%) were detected; and (iii) stratification of the data according to daily precipitation in the control area shows that the days with large precipitation contributed much less to the end result. The effect of seeding is most pronounced and consistent on most of the days (82%) when the daily rainfall is ≤15 mm. The mean rainfall was 8.1 mm. It is suggested that the repeatability of significant and positive effects on rainfall under seeding, as demonstrated by the various statistical analyses, constitutes a measure of confirmation of the similar results obtained in the Israeli Experiment 1. The physical plausibility of these results adds further support to the statistically strong indications of a positive effect on rainfall.