Comments on “A New Look at the Israeli Cloud Seeding Experiments”

Rangno and Hobbs (1995, hereafter RH95) note that for years many meteorologists believed that Israel I and Israel II were the only experiments providing confirmatory evidence of the ability of cloud seeding to stimulate rainfall and that Israeli winter clouds were seedable due to some unique characteristics, namely, a continental drop size distribution, with an attendant lack of a coalescence process, and a scarcity of natural ice crystals at temperatures above 2158C or so. RH95 provide convincing evidence that Israeli winter clouds sometimes produce drizzle or rain through coalescence and that significant ice particle concentrations exist in some of them at temperatures as high as 2108C. RH95 therefore conclude that they are ‘‘not as conducive to rainfall enhancement by artificial seeding as previously believed.’’ In doing so, RH95 mimic, albeit in a reverse sense, the scientists whose work they are criticizing. They judge the seedability of clouds in Israel with reference to a single, very simple concept of seedability and then allow that judgment to sway their interpretation of the Israeli statistical results. The early descriptions of seedable orographic clouds (e.g., Mason 1971) concerned stratified orographic clouds with low cloud water concentrations and precipitation growth solely by the deposition of water vapor on ice crystals. They are inadequate for orographic clouds, including those in Israel, that contain embedded convective cells. The presence of ice particles in a convective cloud does not guarantee absence of seedability. Substantial concentrations of supercooled cloud water can exist in the presence of ice particle concentrations of 10 or even 100 L21, provided that air is ascending to provide a fresh supply of condensate. The critical concept is one of production versus consumption of supercooled cloud liquid. Elementary calculations and many modeling results have shown that even very large amounts of graupel, snow, and smaller cloud ice particles cannot deplete