Monday, 10 January 2005: 11:00 AM
Separation between cloud seeding and air pollution effects
Enhancement of precipitation due to 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 due to urban and industrial air pollution recently has been documented and quantified. Here it is shown that both effects are the opposite sides of the same coin, demonstrating the sensitivity of clouds to human 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 due to air pollution and enhancement due to glaciogenic cloud seeding. Time series based on precipitation from rain gauges were analyzed for seeded and non seeded 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 non-seeded period (1950 – 1960) Ro was found to be higher than the non-seeded days of the following period, which was the randomized experimental period (1961-1974). This apparently shows the effect of the increasing pollution. 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% for the seeded rain time series compared to 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%. The sensitivity of Ro to both seeding and pollution effects was greatest in the areas with the greatest natural orographic enhancement factor and practically non-existent in areas where Ro is near unity. This suggests that the orographic clouds are the most sensitive to air pollution and cloud seeding effects on clouds and precipitation, in agreement with the large susceptibility of precipitation from such short living shallow clouds to aerosols.