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Ronit Nirel and Daniel Rosenfeld

Abstract

During the period 1961–75, two cloud seeding experiments were carried out in Israel. The first Israeli experiment had a two-target crossover design. The results indicated a positive seeding effect of 15%, significant at 0.9%. The second experiment indicated an enhancement of 13%, significant at 2.8%, in rain amounts in the northern part of Israel. Since November 1975, seeding in the north has been operational.

The objective of this paper is to develop a method for estimating the seeding effect of the operational seeding. The proposed method is based on a historical logarithmic model of target precipitation on control precipitation.

It is argued that the validity of the proposed approach stems from the stability of the relation between target and control precipitation over time, and the robustness of the meteorological system in the region.

The seeding effect in the operational period 1976–90, on the annual rainfall, is estimated by a 6% increase in rain amounts, with a 95% confidence interval of (1.01, 1.12). An analysis of the sensitivity of the effect estimate to the choice of period shows a persistent indication of a positive seeding effect. Possible explanations for the reduction in seeding effect, in comparison to the Israel-2 experiment, are discussed.

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Ronit Nirel and Uri Dayan

Abstract

The ratio of sulfur dioxide to nitrogen oxides (RSN = SO2/NOx) is one indicator of air pollution sources. The role of this ratio in source attribution is illustrated here for the Ashdod area, located in the southern coastal plain of Israel. The main sources of pollution in the area are the tall stacks of the Eshkol power plant, the stacks of oil refineries, and areal sources (stationary and mobile). The factors that affect RSN are studied using four regression models: a binary regression tree in original scale, a tree in logarithmic scale, a data partition produced by a combination of the two trees, and a linear regression model. All models have similar relative prediction error, with the combined partition best highlighting the sources of variability in RSN: (a) very low values (interquartile range of [0.12, 0.48]) are associated with traffic, (b) low values ([0.43, 1.00]) are attributed to the power plant and to daytime emissions of local industry, (c) medium values ([0.74, 1.90]) are associated with local industry emissions during cooler hours of the day and refinery emissions mainly on slow wind episodes, and (d) high values ([1.07, 4.30]) are attributed to refinery emissions during moderate to fast wind episodes. Analysis of the number of episodes of increased concentrations indicates that, during 1996 and 1997, about 42% of SO2 episodes are attributable to the power plant and 33% to the refineries. Increased-NOx episodes are mainly contributed by traffic (91%) and power plant (4.5%) emissions.

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Daniel Rosenfeld and Ronit Nirel

Abstract

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 an 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.

  1. The strongest interaction of desert dust with rain clouds in the north occurs on dust days when the, SMR is in the north.

  2. 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.

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