Separation between Cloud-Seeding and Air-Pollution Effects

Abstract

Enhancement of precipitation by cloud-seeding operations has been reported in many studies around the world in the last several decades. On the other hand, suppression of rain and snow by urban and industrial air pollution recently has been documented and quantified. Here it is shown that the two effects are the opposite sides of the same coin, demonstrating the sensitivity of clouds to anthropogenic aerosols of different kinds. This is done by analyzing the rainfall amounts in northern Israel during the last 53 years and explaining the changes there as the combined opposite effects of precipitation suppression by air pollution and enhancement by glaciogenic cloud seeding. Time series based on precipitation from rain gauges were analyzed for seeded and nonseeded days and periods in the experimental control and the target areas. The response variable is Ro, the orographic enhancement factor, which is the ratio of gauge-measured rainfall in inland hilly areas (500?1000 m) to the rainfall at the upwind coasts and plains. The results show that for the whole period of 1950?2002 the Ro of the hilly areas decreased by 15%. In the early nonseeded period (1950?60) Ro was found to be higher than the nonseeded days of the following period, which was the randomized experimental period (1961?74). This result apparently shows the effect of the increasing pollution. Factor Ro had an identical decreasing trend during the seeded days of the experimental period and through the subsequent fully operationally seeded period (1975?2002). However, the trend line of Ro was shifted upward by 12%?14% for the seeded rain time series in comparison with the unseeded time series. Thus, the opposite effects of air pollution and seeding appear to have nearly canceled each other in recent years, leading to the false impression that cloud seeding is no longer effective. However, the findings here suggest that if the operational seeding were to stop, Ro would decrease further by about 12%?14%. The sensitivity of Ro to both seeding and pollution effects was greatest in the areas with the greatest natural orographic enhancement factor and was practically nonexistent in areas in which Ro is near unity. This result suggests that the orographic clouds are the most sensitive to air pollution as well as to cloud-seeding effects on clouds and precipitation, in agreement with the large susceptibility of precipitation from such short-living shallow clouds to aerosols. Based on previous studies and on the results of this paper, it is suggested that the proposed mechanism is the most likely explanation to the observations, and no alternative explanations such as long-term trends in the cross-mountain moisture flux were found probable. It is certain that additional research is required.