The search for the optimal size of hygroscopic seeding particles

Ronen Lahav, Daniel Rosenfeld*

*Corresponding author for this work

Research output: Contribution to conferencePaperpeer-review

Abstract

It is well known that large concentrations of small aerosols, such as produced by smoke and air pollution, can suppress coalescence and precipitation. Our observations have shown that when giant salt CCN (e.g., sea spray) restore the precipitation from such polluted clouds (Rosenfeld et al., 2002). Aircraft and satellite microphysical observations in Israel and adjacent areas show that hygroscopic seeding of clouds occurs naturally by sea spray, but the effect diminishes with distance inland. Cloud simulations and observations show that hygroscopic seeding is most effective with 2-4 □m diameter particles. This is larger than can be produced currently with hygroscopic flares, but smaller than can be produced with other methods, such as spray seeding. The calculations also show that the mass that needs to be dispersed for such "optimal" seeding is larger than can be delivered by current flare technology, but starts to be feasible with spray seeding, and most practical with powder seeding.The results from a couple of salt spray experiments in Israel showed early formation of large raindrops and somewhat wider cloud drop size distribution in the seeded clouds, in accordance with the theoretical expectations. The small concentrations of the seeding-induced raindrops suggest that the concentrations of seeded particles were too small. We realized that the spray system produced particles that are much larger than the optimal size, Increasing the number concentrations can be achieved by getting closer to the optimal size for hygroscopic seeding, using smaller particles in the form of salt powder. Field tests in Israel have specified the size distribution of the manufactured powder, which was found to be in the desired range specified by the models. Preliminary results with powder salt seeding in Israel and more recently in Texas have shown that the seeded clouds were affected by the salt powder, producing high drizzle concentration and rain drops in the seeded cloud.

Original languageEnglish
Pages2867-2872
Number of pages6
StatePublished - 2005
Event85th AMS Annual Meeting, American Meteorological Society - Combined Preprints - San Diego, CA, United States
Duration: 9 Jan 200513 Jan 2005

Conference

Conference85th AMS Annual Meeting, American Meteorological Society - Combined Preprints
Country/TerritoryUnited States
CitySan Diego, CA
Period9/01/0513/01/05

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