Aerosols and Precipitation Enhancement

Earth is the only planet known in the solar system to support life. Life on Earth is critically dependent upon the cycling of water back and forth among the various reservoirs (ocean, land, and atmosphere) in the Earth system, which is referred to as the hydrological cycle. Both natural and human-induced climate variations manifest themselves in the global water cycle. In fact, the most obvious signals to humans of climate change in the Earth system will likely be changes in the hydrological cycle, particularly regional precipitation regimes, and the exacerbation of extreme hydrologic events, such as floods, droughts and hurricanes. The hydrological cycle also affects and interacts with other components of the climate system. Consequently, any significant perturbations to the hydrological cycle, whether they be caused by aerosols or in other ways, are of paramount concern.

In the hydrological cycle water molecules evaporate from the oceans, seas, rivers, soils and plants and return to earth mainly by precipitation. The lifetime of a water molecule has been estimated to be about 9 days. Consequently, changes in cloud properties due to aerosols can modify the lifetime only slightly and thus modify only slightly the global amounts of rainfall. However, such small changes in cloud and precipitation properties may result in spatial and temporal redistribution of rainfall. This is not a small matter, because such changes could mean drought in some regions (especially in arid regions) or flooding in others.

Clouds are known to play a major role in climate through their direct interactions with solar radiation. In addition, aerosols serving as cloud condensation nuclei (CCN) and ice nuclei (IN), determine cloud microphysics, the formation of precipitating particles and cloud radiative properties, particularly cloud albedo and emission. As a consequence, these properties influence the local radiation budget, atmospheric temperatures, land-surface and ocean temperatures. Aerosols can therefore affect regional cloud properties and may affect precipitation amounts. Precipitation from clouds is the only mechanism that replenishes ground water and completes the hydrological cycle. Changes in either the amounts and/or the spatial and temporal distribution of rainfall will have dramatic impacts on climate and on society. Increases or decreases in rainfall in one region could affect rainfall downwind. Similarly, changes in rainfall distribution will strongly affect semi arid regions that are in dire need of water.

There is growing evidence that human activities (e.g. anthropogenic aerosols) can alter atmospheric processes such as the hydrologic cycle on scales ranging from local precipitation patterns to global climate. Research and documentation of anthropogenic effects on precipitation processes strengthen the physical basis for deliberate attempts (weather modification through cloud seeding) to alter clouds with the goal of enhancing precipitation or mitigating severe weather. The potential for such man-made increases in precipitation or mitigating severe weather (weather modification via cloud seeding) is strongly dependent on the natural microphysics and dynamics of the clouds that are to be seeded. HAP scientists are involved with a variety of projects around the world related to aerosol-cloud interactions and weather modification (cloud seeding), including current efforts in Australia, Saudi Arabia, West Africa, Turkey and the state of Wyoming. RALs role has been to scientifically evaluate the potential for cloud seeding, as well as conducting basic research on the impact of seeding aerosols on cloud and precipitation processes through field measurements and modeling.

Hydrometeorological Applications Projects

Primary Contacts

• BRUINTJES, Roelof | PROJ SCIENTIST III | ph: 8909 | email: roelof
• BREED, Dan | PROJ SCIENTIST II | ph: 8933 | email: breed
• KUCERA, Paul | PROJ SCIENTIST II | ph: 2807 | email: pkucera