83rd Annual

Wednesday, 12 February 2003: 5:00 PM
Natural and artificial rain enhancement by sea spray
Ronen Lahav, Hebrew University, Jerusalem, Israel; and D. Rosenfeld
Poster PDF (217.9 kB)
Large concentrations of small aerosols were observed recently to suppress coalescence and precipitation. On the other hand, even newer 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.

The typical air mass that is associated with rain clouds in Israel form in polluted air mass downwind of Europe, which crosses the East Mediterranean in winter storms, that add to it sea spray. The air ingests additional local air pollution with its further transport inland. In situ aircraft measurements of the cloud microstructure show that over sea and the Israeli coastline contain some drizzle and isolated warm raindrops, which become more rare 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 Tropical Rainfall Measuring Mission Precipitation Radar confirms the diminution of precipitation associated with the reduction of the droplet effective radius.

The large apparent positive impact of the sea spray on the precipitation prompted us to imitate nature and do it artificially. Experiments 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 experiment is done in the framework of the Israeli rain enhancement program.

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