(Executive Summary of WMP Report 35, Foote and Bruintjes, 2000)

The Workshop on Hygroscopic Seeding: Experimental Results, Physical Processes, and Research Needs was held in Mazatlan, Mexico under the sponsorship of the World Meteorological Organziation, the State of Durango, Mexico, and the U.S. National Center for Atmospheric Research. It's purpose was to review three recent rain enhancement projects (South Africa, Thailand, and Mexico) utilizing hygroscopic seeding techniques that had claimed positive rainfall increases from seeding. An important part of the workshop was also to assess the current state of knowledge in the field of rain formation, and to consider the physical effects that seeding with hygroscopic particles might induce. Armed with this knowledge, the particitipants were asked to propose what the next steps should be in furthering our understanding of the physical processes involved, and possibly in moving ahead with this technology.

The three experiments were conducted in South Africa, Thailand, and Mexico, and all involved randomized seeding trials. It was recognized that substantial resources had been assembled for these experiments and that care was taken to develop the necessary specialized manpower. The goverments and institutions behind this work were congratulated for their foresight in supporting these exciting scientific and technological efforts.

The workshop participants found the recent seeding results to be highly interesting as well as intriguing. The common elements of the three randomized experiments were (1) seeding with hygroscopic flares, (2) evaluation using the time-resolved estimate of storm rainfall based on radar measurements in conjunction with an objective software package for tracking individual storms (different software was used for each experiment), and (3) statistically significant increases in radar-estimated rainfall.

The Mexico experiment was designed to replicate, if possible, the South Africa experiment, and employed end-burning flares to dispense the seeding agent. The results were found to be remarkably consistent. Increases in target cloud radar-estimated rainfall were deduced in the period 30-60 minutes after seeding in both cases (a somewhat later time period than might have been expected).

The warm clouds studied in Thailand were seeded by the direct injection of salt particles, and the results held further suprises. The analysis revealed an increase in radar-estimated rainfall, as in the other two experiments. However, the increase did not show up until 1-4 hours after seeding, rather than being in the first hour, as in South Africa and Mexico. Furthermore, the result was shown to be a result not of increased rain from the clouds actually seeded, but rather from the secondary clouds intitiated by the seeded clouds.

The reported results dealt with single-cloud experiments. It was agreed that an important next step would be to demonstrate the cost-beneficial increases in rain could be acheived on an area-wide basis. This had yet to be done.

While the randomized seeding results were viewed as very exciting, the workshop participants concluded that the chain of physical events that might be responsible is not well understood. It is generally accepted that this second pillar of scientific understanding is needed to reinforce the statistical results before such results can be fully accepted.

Research topics thought to be of the highest priority in resolving the scientific questions were discussed. These included details of the microphysical responses to seeding, and a coupled cloud-dynamical response that seemed to be taking place. It was throught that the latter might be critical to the rain enhancement effect. In Mexico and South Africa, for example, it was shown that the seeded clouds lived longer than the control clouds. In Thailand, as noted earlier, the seeded effect was found only in the secondary clouds. Such results were not anticipated in the original seeding hypothesis.

The possibility was raised that the apparent seeding result might be due to seeding-induced drop size changes that would affect the radar estimation of rainfall. Arguments were made on both sides, and suggestions for addressing the question were considered.

The workshop participants concluded that while the recent experiments were quite encouraging, there were nonetheless a number of cautionary points that needed to be kept in mind. These included the absense of an experiment showing an area-wide effect, the lack of physical understanding, and the use of radar alone to estimate rainfall, as noted above. It was emphasized also that the results from the three experiments could not automatically be transferred to a new geographic area, since the background areosol is thought to be very important in this process. It was also urged that education and training be considered a fundemental part of the activity of any institution considering involvement in rain enhancement.

The recent experiments, if accepted, lead beyond the classical result in cloud physics linking cloud condensation nuclei and droplet spectra at cloud base to the efficiency of rain (for example, the probability that a cloud of a given depth will produce rain). Rather, these experiments suggest that CCN affect the total rainfall from a cloud and apparently also the longevity of the cloud. This would have important practical implications not only for water resource issues, but also for such things as global change (for example, a regional change in CCN might easily accompany a mean change in temperature) and quantitative precipitation forecasting.

The final and perhaps the strongest conclusion of the workshop was that the experimental results were sufficiently exciting, and the topic sufficiently important, that a new international initiative should be lauched to understand the physical processes taking place. It was recommended that a major cooperative field experiment employing modern instrumentation be planned and carried out in the near future.

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