Reassessment of rain enhancement experiments and operations in Israel including synoptic considerations

A re-analysis of the results from cloud seeding in northern Israel is described. The analysis covers the period of the randomized Israel II experiment (1969–1975) and the subsequent period (1975 to the present) where operational seeding is being conducted. The evaluation is carried out using the double ratio method, as was done in the past. We analyzed the precipitation data in the north of Israel before and during the seeding period, stratified the data based on synoptic conditions and compared the results with an area to the south that had been unseeded during the seeded days in the north. The results show that during Israel II the rain enhancement in the target area in the north of Israel was about 12%, similar to the results reported previously. However, these results have two major problems: 1) During Israel II the rainfall ratio between the inland areas (target) and the coast (control) was unusually higher in comparison with the unseeded periods prior to the initiation of the experiment and also, in comparison with the seeded period following Israel II. 2) Comparison of the double ratio in the north during the Israel II experiment was found to be slightly lower than that in an equivalent unseeded area to the south on the same days (1.15 vs. 1.13). This implies that the high double ratio values in the north may not be a consequence of successful cloud seeding but of preferred synoptic conditions during seeding in which inland rainfall is relatively higher, i.e. deep cyclones over the E. Mediterranean.The analysis also shows that during Israel II the frequency of deep lows that accompanied the rainy days was higher on seeded days than during unseeded ones with stronger westerly winds. This can explain why on seeded days, rain clouds penetrated more efficiently towards inland areas resulting in higher rainfall ratios of target (inland mountainous)/control (coastal strip) on seeded days both inside and outside the seeding project area. Furthermore, the double ratio obtained for the whole study period (1.00) also strengthens the point that the high double ratio value during Israel II experiment is not the result of cloud seeding.