Aerosol – Cloud-Precipitation Interactions and the impacts on rainfall enhancement experiments via cloud seeding

Biomass burning, deserts and human activities are releasing large amounts of aerosols into the atmosphere. These particles enhance scattering and absorption of solar radiation. They also produce brighter clouds and impact the development of precipitation in clouds around the world. These in turn lead to large reductions in the amount of solar irradiance reaching the Earth's surface, a corresponding increase in solar heating of the atmosphere, changes in atmospheric temperature structure, potential suppression of rainfall, and less efficient removal of pollutants. These aerosol affects can lead to changes in the hydrological cycle, which connects directly to the availability of fresh water that could result in a major environmental issue.

The effects of aerosols on cloud microphysical and dynamical processes and precipitation were also highlighted by the results from experiments to enhance rainfall by hygroscopic seeding. Evidence exists that: 1) the particles produced by hygroscopic flares do indeed broaden the droplet spectrum and 2) seem to increase the lifetime of precipitating convective clouds possibly by changing the downdraft/updraft structures.

The aerosol effects on precipitation development, however, can have different effects on the precipitation development in clouds depending on the thermodynamic structure of the atmosphere and the cloud base temperatures. Some intriguing similarities and differences of the effects of aerosols on precipitation development have recently been observed in different experiments around the world that have direct impacts on cloud seeding experiments. These results and the special applications to satellite and radar measurements and future needs will be highlighted in the presentation.