1.2
Microstructure of convective and orographic clouds in Israeli and their suitability for seeding

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Monday, 24 January 2011: 11:15 AM
Microstructure of convective and orographic clouds in Israeli and their suitability for seeding
605/610 (Washington State Convention Center)
Daniel Rosenfeld, The Hebrew University of Jerusalem, Jerusalem, Israel; and E. Freud

The Israeli cloud seeding experiments have been initiated in 1961 based on the concept of glaciogenic seeding of microphysically continental convective clouds. Previous findings showing that the cloud micostructure is not as continental and that they apparently develop ice multiplication casted doubts on that physical foundation of the Israeli experiments.  Recent cloud physics aircraft measurements reveal that a major factor in the precipitation forming processes of the convective clouds in Israel is natural hygroscopic seeding by sea spray. Rain clouds occur in Israel during winter storms over the eastern Mediterranean Sea, where the strong westerly winds raise the sea spray and move the clouds from sea inland. Large concentrations of sea spray aerosols are found in the air and find their way into the clouds, causing very large tail of large cloud drops already at cloud base. These clouds were observed develop precipitation quickly, and are indeed not likely to be susceptible to rain enhancement by cloud seeding.

Orographic clouds that form over the Galilee hills and especially over the Golan Heights farther inland ingest less sea spray, as evident by the smaller tail of large drops in the cloud drop size distributions. Supercooled water is found over a wide horizontal extent and contains more than 1 g m-3. Reanalysis of the Israeli experiments is consistent with the hypothesis that seeding enhanced mainly the orographic component of the precipitation.

This leads the design of Israeli-4 experiment to focus on the orographic clouds, which are the main contributors of precipitation in the catchment area of the Sea of Galilee.