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Aerosols and Precipitation

For more than a decade RAL has conducted scientific studies to determine the feasibility of enhancing precipitation by seeding clouds.  This work has been conducted on behalf of governments in Mexico, the Middle East, Southeast Asia, and Africa and has involved collaborations with colleagues at US and foreign universities, US and state government agencies, scientists and engineers in the host countries. While the primary focus of these efforts has been precipitation enhancement, the effects of aerosols as agents of significant climatic perturbations, particularly with respect to precipitation, have received increasing attention over the past several years. 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. 

FY06 Accomplishments:

Field and modeling studies over the past few years have clarified how meteorological conditions (e.g., large-scale dynamics, vertical thermal structure, local meso-scale circulations) combined with aerosol characteristics modulate the precipitation process.  Observations recently collected in Sulawesi, Indonesia show that precipitation develops quite efficiently in this region via the warm rain process (i.e., collision and coalescence).  Even clouds affected by high concentrations of aerosols from a local smelter plant were relatively efficient in developing rain, although somewhat delayed, suggesting that seeding with hygroscopic flares (as was proposed) would have little effect in this area.  Seeding to enhance precipitation from winter storm systems focuses on different precipitation processes.  The efficacy of enhancing the ice process in southern Italian storms was examined using numerical model simulations after flight restrictions thwarted attempts to seed at optimal altitudes. The results from several model simulations indicate that seeding material needs to be directly injected into regions of substantial amounts of supercooled liquid water temperatures between -5° and -15°C, as was dictated in the Operations Plan.  Little vertical mixing occurs in these winter frontal stratiform-type cloud systems, but when properly targeted, silver iodide (AgI) seeding can produce significant amounts of cloud ice and precipitation. For example, Figure 1 shows a dramatic increase in cloud ice between the control simulation and the simulation with seeding, and an enhancement in snow mass (not shown), which eventually leads to increased precipitation accumulation on the ground.   Similar numerical modeling work supported the development of the experimental design of a randomized seeding experiment in Wyoming, which seeks to evaluate the efficacy of AgI seeding for snowpack enhancement.  Additional dispersion modeling was carried out to verify the placement of ground-based AgI generators.  Observations were also collected to assist in establishing the design.  For example, the frequency and length of periods of supercooled liquid water helped identify optimal time increments for treatment (seed or no-seed) units.

FY07 Plans:

A precipitation enhancement feasibility study is underway in the southern region of Mali, West Africa with data collection expected to be completed in mid-October. Data will be analyzed to determine the natural aerosol and precipitation characteristics of clouds, and the effect of cloud seeding on these processes and vice versa. Preliminary atmospheric modeling studies of the formation of clouds and rain will be conducted to evaluate and assess the potential for cloud seeding.  Additional training sessions for Mali scientific and technical staff will be conducted, focusing on the use of the TITAN/CIDD software systems. A major new study on aerosols, precipitation systems, and the feasibility of cloud seeding during the winter months is planned for FY07 in north-central Saudi Arabia.  The field program will involve the use of three radars and four aircraft; background investigations will be conducted and a randomized seeding experiment begun.  And finally, the first season of randomized cloud seeding for the Wyoming Weather Modification Program will be conducted from November through March.  Seeding is planned for five successive winter seasons to determine whether it is successful over a broad range of winter conditions.

 

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