Tuesday, 11 January 2005: 9:00 AM
The search for the optimal size of hygroscopic seeding particles
It is well known that large concentrations of small aerosols can suppress coalescence and precipitation. Our observations have shown that when giant salt CCN are ingested into such clouds they restore the precipitation. The most common source of these aerosols is sea spray, especially during high wind conditions. In situ aircraft measurements of the cloud microstructure show that clouds over sea and the Israeli coastline contain some drizzle and isolated warm raindrops, which become rarer with the distance inland. Coincident satellite measurements show that the cloud droplet effective radius becomes smaller for a given cloud depth with distance inland The large apparent positive impact of the sea spray on the precipitation prompted us to imitate nature and do it artificially. Experiments under the framework of the Israeli rain enhancement program of artificial addition of sea spray to clouds inland have shown that warm rain has returned to the clouds, which behave as natural clouds closer to the coastline. This new technique of hygroscopic seeding is done by spraying concentrated brine from the Dead Sea in a very high pressure from an agricultural sprayer. The various aerosol compositions were used in 1-D and 2-D cloud models which simulate very accurately the nucleation and coalescence processes. These observations and model simulations suggest that there is a large potential for precipitation enhancement for a cloud seeding method that could modify the microstructure of clouds having little natural coalescence in order to imitate clouds with active coalescence processes. Based on the model result we have found that the optimal size for that should be 2-3 ým. The current hygroscopic spray seeding system produces much larger than optimal particle sizes and dispersion of the near-optimal size. With the other widely used technology of hygroscopic flares, it is not yet technologically feasible to disperse 2 to 3-ým hygroscopic particles at the necessary seeding rates. Therefore, we developed a new seeding method, based on new micro-powder principle, which will generate even more CCN of optimum sizes. Preliminary results of in-situ measurements from the last winter in Israel were very encouraging. Now The next step is planned for the near future in Texas using SF6 gas for identification of the seeded volume. Those results will be presented in the coming AMS conference.