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1. Improve Understanding of the Atmosphere, Earth System, and Sun

NCAR Strategic Priority 2: Investigating the Interactions of the Atmosphere, the Broader Earth System, and Human Society

NCAR Strategic Priority 3: Improving Prediction of Weather, Climate, and Other Atmospheric Phenomena

NCAR Strategic Priority 4: Community Modeling

NCAR Strategic Priority 2: Investigating the Interactions of the Atmosphere, the Broader Earth System, and Human Society

Aerosols and Precipitation – Feasibility Studies on Rainfall Enhancement

rainfall Saudi ArabiaMap of Saudi Arabia showing the distribution of annual rainfall (mm).

Over the past several years, the collection and analysis of data on aerosols, clouds and storms has been an important element of rainfall enhancement programs RAL conducts throughout the world. While the primary concern of all these efforts is precipitation, the effects of aerosols as agents of significant climatic perturbations, particularly with respect to precipitation, have received increasing attention over the past decade or so.  Understanding the potential for an indirect aerosol effect, which involves changes in cloud microphysical processes, has become an additional factor to consider in documenting cloud and precipitation characteristics in regions proposed for weather modification activities.   

FY2007 Accomplishments:

West Africa
A rainfall enhancement assessment study was conducted in Mali, West Africa.  Aerosol and cloud microphysical measurements collected during the field program combined with NASA satellite and NRL aerosol model forecasts show that even though a major source of aerosols is related to dust transport from the Sahara, the Saharan dust is mostly confined to the northern areas of Mali (even during monsoon conditions) with only an occasional penetration of dust further south. These studies also showed that local variations in aerosols embedded on the background levels may play an important role in the effects of aerosols on clouds.  Storm trends have also been analyzed, improving our understanding of where, when, and how storms develop in this region. Future studies, planned for FY08, will examine a more complete data set to evaluate these characteristics in the context of the whole rainy season.

Saudi Arabia
Using three weather radars and two research aircraft over a six-month field effort, RAL scientists collected data that substantially aid our understanding of the climatology of Saudi Arabia, as well as the character of precipitating storms in the area. Dryness is the prevailing climatic characteristic of Saudi Arabia except in the Asir region, which receives annual rainfall >400-500 mm due to its unique geographical configuration and the local mountains.  Rainfall in most of Saudi Arabia is <200 mm, highly irregular (i.e., large natural variability), and hence is not dependable.  The geographic distribution of annual rainfall across Saudi Arabia is shown in the figure to the right.  Analyses of the field data will continue into FY08, and more extensive field studies of air chemistry, aerosol, cloud microphysics, precipitation, and storm characteristics in the region will be conducted.

FY2008 Plans:

West Africa
The next step in the 2007 Mali study is the analysis of the collected aircraft and radar data to better determine the natural aerosol and precipitation characteristics in Mali clouds, and the effect of cloud seeding on these processes and vice versa.  In addition, more training sessions for scientists and technicians in Mali and in neighboring Burkina Faso, are planned for FY08.  Work will continue to sustain and extend the upgrades to observational networks in West Africa that were begun in FY07.

Saudi Arabia
The 2006-2007 study on aerosols, clouds, precipitation systems, and the feasibility of cloud seeding in north-central Saudi Arabia was the first step toward establishing scientific contacts, developing the infrastructure, and collecting data aimed at understanding these processes.  Continuing the studies in the same area, expanding them to other areas of Saudi Arabia, and building samples of seeded and non-seeded storms are planned for FY08.

Istanbul, Turkey
A new program to assess the feasibility of rainfall enhancement in Turkey will begin in FY08.  Airborne data are to be collected during three months of the winter, their peak season for precipitation, to study the distribution of aerosols, cloud physical properties, and the development of precipitation. 

Queensland, Australia
RAL has launched a new effort in Queensland to scientifically investigate when--and how well--cloud seeding works to enhance rainfall from convective clouds. For the first time in any rainfall enhancement experiment, dual-polarization, dual-wavelength and advanced weather radars will all be implemented in the field.  In combination with the airborne measurements, these measurement systems make it possible to trace the physical chain of events from the natural or seeded small particles to droplet and ice crystal growth, subsequent precipitation development in clouds, and ultimately rain on the ground in both natural and seeding clouds.  In addition, this will be the first time in decades that such a range of scientific expertise (e.g., several Australian institutions and universities, South African colleagues, and NCAR scientists and engineers) has been brought together in one program of this type.