Some Past Research on Cloud Seeding Aerosols and a Future Outlook (Invited Presentation)

In this presentation, I will attempt to distill some of the historical research that was done in seeking to design and characterize artificially-generated ice nucleating particles (cloud seeding aerosols) for use by the operational weather modification community. I will use primarily as an example studies that were conducted at the Colorado State University Cloud Simulation and Aerosol Laboratory, the former de facto calibration facility for cloud seeding generators of all types. Starting during the late 1970’s, largely under the guidance of William Finnegan, research sought to move beyond the practical production of seeding "agents" and the definition of their effectiveness on a common basis (enumerating ice crystals formed upon injection into an isothermal cloud chamber), to understanding their ice nucleation behaviors through looking at the kinetics of ice formation. Subsequently, experiments were designed using an expansion chamber and, relatively new to the scene, portable flow-through devices to explore the ice nucleation mechanisms of different particles types. I will review this research and describe how it led to development of "designer" ice nucleating particle systems. The work elevated recognition that chemically different aerosol systems could lead to faster or slower ice formation in clouds, and did lead to some implementation, but how results and modeling exercises translated to the real world remained as questions. A practical consequence was a desire of the operational sector to seek seeding systems that both produced the most ice crystals per mass of material at a given temperature, and did so at the fastest rate possible. However, research decreased through loss of federal support and perhaps (personal opinion) a growing reluctance of the scientific community to engage in an enterprise that they viewed as having unresolved basic research issues. From the standpoint of an ice nucleation scientist, these included unresolved understanding and documentation of natural ice nucleation processes and their influence on cloud microphysics. Facilities that were difficult to support were abandoned as energies were directed to these basic issues, and at present there exists no facility for standardized testing or devoted research on seeding aerosols.

I will conclude with some suggestions for possible new directions for such research that can utilize new understanding and capabilities for defining ice nucleation processes. It should be possible to build better systems for releasing ice nucleating particles, perhaps to do so in more environmentally-friendly manners using alternate materials, and to quantitatively document both the action of atmospherically-released particles and the natural ice nucleating particle populations one seeks to augment. Advances that have occurred in fundamental areas of ice nucleation, atmospheric detection of natural INP populations, materials research, cloud microphysical measurements, and atmospheric modeling systems should be translatable to efforts to modify clouds for precipitation enhancement and/or hail suppression and to evaluate methods for doing so. There is evidence that this is occurring.