Microphysical signatures of hygroscopic seeding with 2-5 micron salt powder using aircraft and sf6 tracer

Model simulations show that the optimal diameter of hygroscopic seeding particles is 2 – 5 microns. These particles are just large enough to become embryos of rain drops, while small enough to be able to be dispersed in large number concentrations from a seeder aircraft with a fixed and limited carrying capacity. Smaller particles would be too small to serve as raindrop embryos, whereas the creation of larger drops by the competition effect (used to be ascribed as the microphysical effect of hygroscopic flares) is too weak compared with the embryo effect.

This was tested in a cloud seeding experiment in West Texas, where the salt powder was dispersed along with SF6 gas tracer. A monitoring aircraft with cloud physics instruments and an SF6 detector measured the seeded and ambient neighbouring clouds. The seeded cloud volume was tagged by the SF6, so that the composition of the seeded volume could be distinguished from the not-seeded cloud volume within the same cloud. This provided a powerful tool to identify the seeding effect. The main results are:

1. The seeded cloud volume was a small fraction of the cloud near its base, and increased with altitude and age of the cloud.

2. The seeding did not appear to reduce noticeably the cloud drop number concentration, i.e., the competition effect on nucleation at cloud base was not noticeably operative.

3. The seeding produced a tail of large cloud drops above cloud base, which grew to small raindrops (i.e., a few tenths of a mm diameter) at 1 – 2 km above base.

Explicit cloud simulations of the seeding effects replicated the observed seeding microphysical signatures and suggested that a net gain of about 30% of rainfall would be obtained form the seeded clouds. The simulations were done with the Hebrew University cloud model by Prof. Alexander Khain.

This first of its kind experiment was concentrated on the warm rain processes. When going to levels > 2 km above cloud base large raindrops that formed could not be uniquely ascribed to the seeding effect because it was no longer possible to assume that they moved or formed in the seeded cloud volume.

Using this methodology to investigate the effects on the mixed phase precipitation requires operating in young vigorous clouds with high performance cloud physics aircraft. A sponsor is being sought for continuation of these experiments, which were supported by the Weather Damage Mitigation Program (WDMP) with funds passed through the U.S. Bureau of Reclamation. The WDMP, which was the last surviving support for weather modification research in the United States has since been terminated.