Silver enrichment in rain from clouds in Israel under different seeding and cloud microstructure conditions

This study addresses the question of targeting and activity of AgI in seeded clouds. The silver content in the rain was found to be enriched during seeded conditions, especially when the cloud tops was composed of mixed phase, as detected by satellite microphysical retrievals. This is consistent with the hypothesis that silver would be active mostly in such clouds, but not in clouds that are already naturally glaciated. This study is being conducted in the framework of the Israeli rain enhancement program.

Cloud seeding in Israel is being preformed since 1961 with three statistical experiments and operational seeding in the north of Israel since 1975. Presently the fourth cloud seeding experiment in being planned (Israel 4). As a part of the preparatory studies, the chemical composition of the rain samples is being analyzed by ICP-MS for 23 different metals. The rain is sampled with fine resolution (each 1 mm) in four locations in northern Israel, three being in the catchments area of the Sea of Galilee, which is the target area. In addition the micro-physical properties of the clouds are taken from the geostationary satellite METOSAT-9 (cloud top temperature, cloud phase: water, mixed phase or ice, and cloud top particle effective radius). The Ag concentrations are normalized to Al concentrations in order to get the seeding signal as an Ag enrichment factor. This enrichment factor was examined with the satellite time series of retrieved cloud top microstructure in order to explain the variability in silver enrichment.

The results revealed seeding signals (Ag enrichment) in the target area. High enrichment was observed in rain from mixed phase clouds, whereas in most cases of rain from fully glaciated top clods enrichment was not detected, but in sum cases smaller enrichment was still seen in deep convective clouds with fully glaciated tops.

The combination of the Ag enrichment and cloud microstructure data sets can explain when and why the seeding material reach their target clouds and take active part in the precipitation forming processes. This will help improving the efficiency of cloud seeding in Israel.