Seeding effectiveness - The interaction of desert dust and the southern margins of rain cloud systems in Israel

Statistical analyses suggest that cloud seeding has caused a net increase of rainfall only in northern Israel. These analyses also identify the reported desert dust as a detrimental factor for the seeding effectiveness. This paper deals with the question of what role the interaction of desert dust and the dynamic properties of the clouds plays in the determination of divergent seeding effects in Israel. This question is investigated through analyses of the cloud seeding effectiveness in northern Israel (Israel-2 experiment and the operational seeding) stratified into days when the southern margins of the rain cloud system (SMR) locations were in the north or in the south and into "dust" and "no-dust" days. The results indicate that the SMR plays an important role on dust days, where a seeding effect of 11% is indicated on days with the SMR in the south, and an effect of -11% is indicated on days with the SMR in the north. On no-dust days positive effects were indicated regardless of the location of the SMR. These results are consistent with the following observations. • The strongest interaction of desert dust with rain clouds in the north occurs on dust days when the SMR is in the north. • When the SMR moves to the south, much of the desert dust is washed down by the intervening rain between the north and the SMR.