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2.5.07 The rainmakers of Israel – cloud seeding increases rainfall

Despite many years of research across the globe, attempts to increase rainfall in arid regions through human intervention have enjoyed little success. However, significant progress has now been made in a joint research venture between German and Israeli scientists: their simulations show that precipitation increases when clouds are seeded with suitable minute particles.

Israel is one of the world’s driest countries, yet water is a key economic factor for the state: Israel has a highly productive agricultural industry and exports both fruit and vegetables. Around 70% of the fresh water consumed by the country is used for agricultural irrigation, but high water requirements have caused a continuing decline of groundwater levels. The river Jordan is gradually being reduced to a trickle since 85% of its water is used to supply the population and agricultural areas. This in turn has had a direct impact on the water level of the Dead Sea (into which the Jordan empties), which has fallen by more than 20 metres over the last 70 years – a trend that is showing no sign of abating.

German-Israeli water technology co-operation

Israel – and the rest of the Eastern Mediterranean – would benefit greatly from increased rainfall, and this is exactly what scientists are working on. Supported by the BMBF, the Institute of Earth Sciences of the Hebrew University in Jerusalem and the Institute of Meteorology and Climate Research (IMK) of the Karlsruhe Institute of Technology (KIT) have given the “rainmakers” a significant boost in the form of their joint research project entitled “Numerical investigations on the effect of aerosol particles on the precipitation dynamics of clouds in the Israeli coastal region” (period of study: 2004 to 2008), which was conducted as part of the German-Israeli water technology co-operation. Computer-simulated calculations have shown that artificial “cloud seeding” off the Israeli coast delays rainfall such that inland precipitation is increased in the event of a westerly wind. Optimum results are achieved when (sea/table) salt particles of a specific size are dispersed into the clouds.

Satellite image of the region (source: visibleearth.nasa.gov)

Satellite image of the region (source: visibleearth.nasa.gov)
Satellite image of the region (source: visibleearth.nasa.gov)
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The focus of the project was on aerosol particles. These tiny suspended particles are everywhere in the air and are required for the formation of cloud droplets and the production of rain: when air is oversaturated with water vapour, the aerosol particles act as condensation nuclei to which the water vapour attaches, thus creating drops. The number and size of the drops depends on the composition of the condensation nuclei. For example, the air in coastal areas contains few but very large salt particles – large drops thus form in the clouds in small numbers. The air over the mainland, on the other hand, contains many aerosol particles and the cloud droplets are relatively small.

The project team discovered that manually released aerosols also have a significant influence on the temporal development, spatial distribution and volume of the resulting precipitation. For many years, researchers have been observing a decline in the rainfall over the Judean Highlands. Their calculations suggest that this development can be attributed to an increase in man-made suspended particles (e.g. soot, dust).

Simulation models employed

Calculated temporal development of total inland rainfall after seeding with hygroscopic particles. Solid line: control calculation without seeding; dashed line: simulation with seeding using medium-sized particles

Calculated temporal development of total inland rainfall after seeding with hygroscopic particles. Solid line: control calculation without seeding; dashed line: simulation with seeding using medium-sized particles
Calculated temporal development of total inland rainfall after seeding with hygroscopic particles. Solid line: control calculation without seeding; dashed line: simulation with seeding using medium-sized particles
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Calculated temporal development of total rainfall with seeding of deep convective clouds using hygroscopic particles of optimum size

Calculated temporal development of total rainfall with seeding of deep convective clouds using hygroscopic particles of optimum size
Calculated temporal development of total rainfall with seeding of deep convective clouds using hygroscopic particles of optimum size
 enlargezoom

To determine the impact of the aerosol effect on precipitation formation, the scientists utilised two different numerical simulation models: the complex computer model “Spectral Bin Cloud Microphysical Module” (SBM) of the Hebrew University and the enhanced program “Two- Moment Parameterization” (TMP) of the Karlsruhe IMK. The latter represents a rough (yet still sufficiently accurate) means of reducing the overall computing time. The German-Israeli team started by testing the ability of the two models to predict natural weather processes – both functioned very well and provided similar results. They then calculated means of actively influencing rainfall. The results showed that artificial seeding of the clouds forming off the coast successfully delays the formation of precipitation. The rate of rain development is slowed and the clouds blown east by the westerly wind then rain over the mainland, generally near the coast (though sometimes up to 50 km inland).

Particle size decisive

The size of the particles is of central importance to the precipitation development. Large hygroscopic particles accelerate the formation of rain, while smaller particles slow the development and often reduce the volume. The scientists used computer simulations to establish the particle size that would most effectively raise the level of rainfall achieved with cloud seeding: the optimum size – depending on cloud height and other meteorological parameters – is between 1.8 and 2.5 micrometres. Using hygroscopic particles of this size, the inland precipitation volume can be increased by 20 to 25%, while overall rainfall is marginally reduced.

To compare the model calculations with the actual natural effects of cloud seeding, a number of practical tests were performed in Israel, whereby aircraft were used to disperse salt particles of the calculated optimum size into the clouds; the observed results largely matched the theoretical calculations.

Karlsruhe Institute of Technology (KIT)
Institute of Meteorology and Climate Research (IMK)

Prof. Dr. Klaus Dieter Beheng
Kaiserstraße 12
76128 Karlsruhe, Germany
Tel.: +49(0)7 21/60 84 35 95
Fax: +49(0)7 21/60 84 61 02
E-mail: klaus.beheng@kit.edu
Internet: www.imk-tro.kit.edu/english/index.php
Funding reference: 02WT0536
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