The Second Israeli Randomized Cloud Seeding Experiment: Evaluation Results
A. Gagin and J. Neumann
Department of Atmospheric Sciences,
The Hebrew University of Jerusalem, Israel
(Manuscript received August 4, 1980, in final form June 17, 1981)
The Second Israeli Randomized Cloud Seeding Experiment was conducted during the six rainfall seasons November–April 1969–75. (image right showing location of Israel, click image for detail, from samueljscott.wordpress.com)
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 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.